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This article is about the part of a plant. For other uses, see Root (disambiguation).
Primary and secondary roots in a cotton plant
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil.This is not always the case, however, since a root can also be aerial (growing above the ground)
or aerating (growing up above the ground or especially above water). Furthermore, a stemnormally occurring below ground is not exceptional either (see rhizome). So, it is better to defineroot as a part of a plant body that bears no leaves, and therefore also lacks nodes. There are alsoimportant internal structural differences between stems and roots.
The first root that comes from a plant is called the radicle. The four major functions of roots are1) absorption of water and inorganic nutrients, 2) anchoring of the plant body to the ground and
3) storage of food and nutrients and 4) to prevent soil erosion. In response to the concentration of
nutrients, roots also synthesise cytokinin, which acts as a signal as to how fast the shoots cangrow. Roots often function in storage of food and nutrients. The roots of most vascular plant
species enter into symbiosis with certain fungi to form mycorrhizas, and a large range of other
organisms including bacteria also closely associate with roots.
Contents
[hide]
1 Anatomy 2 Root growth
3 Types of roots
o 3.1 Specialized roots
4 Rooting depths
5 Rooting Depth Records
6 Root architecture
7 Evolutionary history
8 Economic importance
9 See also
10 Notes
11 References
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12 External links
[edit] Anatomy
When dissected, the arrangement of the cells in a root is root hair, epidermis, epiblem, cortex,
endodermis, pericycle and lastly the vascular tissue in the centre of a root to transport the waterabsorbed by the root to other places of the plant.
[edit] Root growth
Root systems of prairie plants
Early root growth is one of the functions of the apical meristem located near the tip of the root.
The meristem cells more or less continuously divide, producing more meristem, root cap cells
(these are sacrificed to protect the meristem), and undifferentiated root cells. The latter become
the primary tissues of the root, first undergoing elongation, a process that pushes the root tip
forward in the growing medium. Gradually these cells differentiate and mature into specializedcells of the root tissues.
Roots will generally grow in any direction where the correct environment of air, mineral
nutrients and water exists to meet the plant's needs. Roots will not grow in dry soil. Over time,given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. Atgermination, roots grow downward due to gravitropism, the growth mechanism of plants that
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also causes the shoot to grow upward. In some plants (such as ivy), the "root" actually clings to
walls and structures.
Growth from apical meristems is known as primary growth, which encompasses all elongation.
Secondary growth encompasses all growth in diameter, a major component of woody plant
tissues and many nonwoody plants. For example, storage roots of sweet potato have secondarygrowth but are not woody. Secondary growth occurs at the lateral meristems, namely the
vascular cambium and cork cambium. The former forms secondary xylem and secondaryphloem, while the latter forms the periderm.
In plants with secondary growth, the vascular cambium, originating between the xylem and thephloem, forms a cylinder of tissue along the stem and root. The vascular cambium forms new
cells on both the inside and outside of the cambium cylinder, with those on the inside forming
secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary
xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result,tissues beyond the secondary phloem (including the epidermis and cortex, in many cases) tend to
be pushed outward and are eventually "sloughed off" (shed).
At this point, the cork cambium begins to form the periderm, consisting of protective cork cells
containing suberin. In roots, the cork cambium originates in the pericycle, a component of the
vascular cylinder.
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels aredead at maturity but are responsible for most water transport through the vascular tissue in stems
and roots.
[edit] Types of roots
This section does not cite any references or sources. Please help improve this section
by adding citations to reliable sources. Unsourced material may be challenged andremoved. (March 2010)
A true root system consists of a primary root and secondary roots (or lateral roots).
the diffuse root system: the primary root is not dominant; the whole root system is fibrous
and branches in all directions. Most common in monocots. The main function of the
fibrous root is to anchor the plant.
[edit] Specialized roots
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Aerating roots of a mangrove
The growing tip of a fine root
The stilt roots of Socratea exorrhiza
The roots, or parts of roots, of many plant species have become specialized to serve adaptivepurposes besides the two primary functions described in the introduction.
Adventitious roots arise out-of-sequence from the more usual root formation of branches
of a primary root, and instead originate from the stem, branches, leaves, or old woody
roots. They commonly occur in monocots and pteridophytes, but also in many dicots,
such as clover (Trifolium), ivy ( Hedera), strawberry (Fragaria) and willow (Salix). Mostaerial roots and stilt roots are adventitious. In some conifers adventitious roots can form
the largest part of the root system.
Aerating roots (or knee root or knee or pneumatophores or Cypress knee): rootsrising above the ground, especially above water such as in some mangrove genera
( Avicennia , Sonneratia). In some plants like Avicennia the erect roots have a large
number of breathing pores for exchange of gases.
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Aerial roots: roots entirely above the ground, such as in ivy ( Hedera) or in epiphytic
orchids. They function as prop roots, as in maize or anchor roots or as the trunk instrangler fig.
Contractile roots: they pull bulbs or corms of monocots, such as hyacinth and lily, and
some taproots, such as dandelion, deeper in the soil through expanding radially and
contracting longitudinally. They have a wrinkled surface. Coarse roots: Roots that have undergone secondary thickening and have a woody
structure. These roots have some ability to absorb water and nutrients, but their main
function is transport and to provide a structure to connect the smaller diameter, fine rootsto the rest of the plant.
Fine roots: Primary roots usually <2 mm diameter that have the function of water and
nutrient uptake. They are often heavily branched and support mycorrhizas. These rootsmay be short lived, but are replaced by the plant in an ongoing process of root 'turnover'.
Haustorial roots: roots of parasitic plants that can absorb water and nutrients from
another plant, such as in mistletoe (Viscum album) and dodder.
Propagative roots: roots that form adventitious buds that develop into aboveground
shoots, termed suckers, which form new plants, as in Canada thistle, cherry and manyothers.
Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that developunder low phosphate or low iron conditions in Proteaceae and some plants from the
following families Betulaceae, Casuarinaceae, Elaeagnaceae, Moraceae, Fabaceae and
Myricaceae.
Stilt roots: these are adventitious support roots, common among mangroves. They grow
down from lateral branches, branching in the soil.
Storage roots: these roots are modified for storage of food or water, such as carrots and
beets. They include some taproots and tuberous roots.
Structural roots: large roots that have undergone considerable secondary thickening and
provide mechanical support to woody plants and trees.
Surface roots: These proliferate close below the soil surface, exploiting water and easily
available nutrients. Where conditions are close to optimum in the surface layers of soil,the growth of surface roots is encouraged and they commonly become the dominant
roots.
Tuberous roots: A portion of a root swells for food or water storage, e.g. sweet potato. Atype of storage root distinct from taproot.
[edit] Rooting depths
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Cross section of a mango tree
The distribution of vascular plant roots within soil depends on plant form, the spatial and
temporal availability of water and nutrients, and the physical properties of the soil. The deepest
roots are generally found in deserts and temperate coniferous forests; the shallowest in tundra,boreal forest and temperate grasslands. The deepest observed living root, at least 60 m below the
ground surface, was observed during the excavation of an open-pit mine in Arizona, USA. Someroots can grow as deep as the tree is high. The majority of roots on most plants are however
found relatively close to the surface where nutrient availability and aeration are more favourable
for growth. Rooting depth may be physically restricted by rock or compacted soil close belowthe surface, or by anaerobic soil conditions.
[edit] Rooting Depth Records
Species Location Maximum rooting depth (m) References[1][2]
Boscia albitrunca Kalahari desert 68 Jennings (1974)
Juniperus monosperma Colorado Plateau 61 Cannon (1960)
Eucalyptus sp. Australian forest 61 Jennings (1971)
Acacia erioloba Kalahari desert 60 Jennings (1974)
Prosopis juliflora Arizona desert 53.3 Phillips (1963)
[edit] Root architecture
Tree roots at Cliffs of the Neuse State Park
The pattern of development of a root system is termed root architecture, and is important inproviding a plant with a secure supply of nutrients and water as well as anchorage and support.
The architecture of a root system can be considered in a similar way to above-ground
architecture of a plant — i.e. in terms of the size, branching and distribution of the componentparts. In roots, the architecture of fine roots and coarse roots can both be described by variation
in topology and distribution of biomass within and between roots. Having a balanced architecture
allows fine roots to exploit soil efficiently around a plant, but the plastic nature of root growth
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allows the plant to then concentrate its resources where nutrients and water are more easily
available. A balanced coarse root architecture, with roots distributed relatively evenly around thestem base, is necessary to provide support to larger plants and trees.
Tree roots normally grow outward to about three times the branch spread. Only half of a tree's
root system occurs between the trunk and the circumference of its canopy. Roots on one side of atree normally supply the foliage on that same side of the tree. Thus when roots on one side of a
tree are injured, the branches and leaves on that same side of the tree may die or wilt. For sometrees however, such as the maple family, the effect of a root injury may show itself anywhere in
the tree canopy.
[edit] Evolutionary history
Further information: Evolution of roots
The fossil record of roots – or rather, infilled voids where roots rotted after death – spans back to
the late Silurian,[3] but their identification is difficult, because casts and molds of roots are sosimilar in appearance to animal burrows – although they can be discriminated on the basis of arange of features.[4]
[edit] Economic importance
Roots can also protect the environment by holding the soil to prevent soil erosion
The term root crops refers to any edible underground plant structure, but many root crops areactually stems, such as potato tubers. Edible roots include cassava, sweet potato, beet, carrot,
rutabaga, turnip, parsnip, radish, yam and horseradish. Spices obtained from roots include
sassafras, angelica, sarsaparilla and licorice.
Sugar beet is an important source of sugar. Yam roots are a source of estrogen compounds used
in birth control pills. The fish poison and insecticide rotenone is obtained from roots of Lonchocarpus spp. Important medicines from roots are ginseng, aconite, ipecac, gentian and
reserpine. Several legumes that have nitrogen-fixing root nodules are used as green manure
crops, which provide nitrogen fertilizer for other crops when plowed under. Specialized baldcypress roots, termed knees, are sold as souvenirs, lamp bases and carved into folk art. Native
Americans used the flexible roots of white spruce for basketry.
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Tree roots can heave and destroy concrete sidewalks and crush or clog buried pipes. The aerial
roots of strangler fig have damaged ancient Mayan temples in Central America and the temple of Angkor Wat in Cambodia.
Vegetative propagation of plants via cuttings depends on adventitious root formation. Hundreds
of millions of plants are propagated via cuttings annually including chrysanthemum, poinsettia, carnation, ornamental shrubs and many houseplants.
Roots can also protect the environment by holding the soil to prevent soil erosion. This is
especially important in areas such as sand dunes.
Roots on onion bulbs
[edit] See also
Plant stem
From Wikipedia, the free encyclopedia
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Stem showing internode and nodes plus leaf petioles
A stem is one of two main structural axes of a vascular plant. The stem is normally divided intonodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence
(flowers), cones or other stems etc. The internodes distance one node from another. The term
shoots is often confused with stems; shoots generally refer to new fresh plant growth and doesinclude stems but also to other structures like leaves or flowers. The other main structural axis of
plants is the root. In most plants stems are located above the soil surface but some plants have
underground stems. A stem develops buds and shoots and usually grows above the ground.Inside the stem, materials move up and down the tissues of the transport system.
Stems have four main functions which are:[1]
Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light
and provide a place for the plant to keep its flowers and fruits.
Transport of fluids between the roots and the shoots in the xylem and phloem.
Storage of nutrients.
The production of new living tissue. The normal life span of plant cells is one to three years.
Stems have cells called meristems that annually generate new living tissue.
Contents
[hide]
1 Specialized terms for stems
2 Stem structure
o 2.1 Dicot stems
o 2.2 Monocot stems
o 2.3 Gymnosperm stems
o 2.4 Fern stems
3 Relation to xenobiotics
4 Economic importance
5 References
Specialized terms for stems
Stems are often specialized for storage, asexual reproduction, protection or photosynthesis,including the following:
Acaulescent - used to describe stems in plants that appear to be stemless. Actually these stems
are just extremely short, the leaves appearing to rise directly out of the ground, e.g. some Viola
species.
Arborescent - tree like with woody stems normally with a single trunk.
Bud - an embryonic shoot with immature stem tip.
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Bulb - a short vertical underground stem with fleshy storage leaves attached, e.g. onion,
daffodil, tulip. Bulbs often function in reproduction by splitting to form new bulbs or producing
small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be
considered as leaves because the leaves make up the greater part.
Caespitose - when stems grow in a tangled mass or clump or in low growing mats.
Cladode (including phylloclade) - a flattened stem that appears more-or-less leaf like and is
specialized for photosynthesis,[2] e.g. cactus pads.
Climbing - stems that cling or wrap around other plants or structures.
Corm - a short enlarged underground, storage stem, e.g. taro, crocus, gladiolus.
Decumbent - stems that lie flat on the ground and turn upwards at the ends.
Fruticose - stems that grow shrublike with woody like habit.
Herbaceous - non woody, they die at the end of the growing season.
Pseudostem - a false stem made of the rolled bases of leaves, which may be 2 or 3 m tall as in
banana
Rhizome - a horizontal underground stem that functions mainly in reproduction but also in
storage, e.g. most ferns, iris
Runner (plant part) - a type of stolon, horizontally growing on top of the ground and rooting at
the nodes, aids in reproduction. e.g. garden strawberry, Chlorophytum comosum. Scape - a stem that holds flowers that comes out of the ground and has no normal leaves. Hosta,
Lily, Iris.
Stolon - a horizontal stem that produces rooted plantlets at its nodes and ends, forming near the
surface of the ground.
Thorn - a modified stem with a sharpened point
Tree - a woody stem that is longer than 5 metres with a main trunk.
Tuber - a swollen, underground storage stem adapted for storage and reproduction, e.g. potato.
Woody - hard textured stems with secondary xylem.
Stem structure
Flax stem cross-section, showing locations of underlying tissues. Ep = epidermis; C = cortex; BF = bast
fibres; P = phloem; X = xylem; Pi = pith
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See also: Stele (biology)
Stem usually consist of three tissues, dermal tissue, ground tissue and vascular tissue. Thedermal tissue covers the outer surface of the stem and usually functions to waterproof, protect
and control gas exchange. The ground tissue usually consists mainly of parenchyma cells and
fills in around the vascular tissue. It sometimes functions in photosynthesis. Vascular tissueprovides long distance transport and structural support. Most or all ground tissue may be lost in
woody stems. The dermal tissue of aquatic plants stems may lack the waterproofing found in
aerial stems. The arrangement of the vascular tissues varies widely among plant species.
Dicot stems
Dicot stem with primary growth have pith in the center, with vascular bundles forming a distinctring visible when the stem is viewed in cross section. The outside of the stem is covered with an
epidermis, which is covered by a waterproof cuticle. The epidermis also may contain stomata for
gas exchange and multicellular stem hairs. A cortex consisting of Hypodermis (collenchyma
cells) and Endodermis (starch containing cells)is present above the pericycle and vascularbundles.
Woody dicots and many nonwoody dicots have secondary growth originating from their lateral
or secondary meristems: the vascular cambium and the cork cambium or phellogen. The vascular
cambium forms between the xylem and phloem in the vascular bundles and connects to form acontinuous cylinder. The vascular cambium cells divide to produce secondary xylem to the
inside and secondary phloem to the outside. As the stem increases in diameter due to production
of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed.
Before the cortex is destroyed, a cork cambium develops there. The cork cambium divides toproduce waterproof cork cells externally and sometimes phelloderm cells internally. Those three
tissues form the periderm, which replaces the epidermis in function. Areas of loosely packedcells in the periderm that function in gas exchange are called lenticels.
Secondary xylem is commercially important as wood. The seasonal variation in growth from the
vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basisof dendrochronology, which dates wooden objects and associated artifacts. Dendroclimatology is
the use of tree rings as a record of past climates. The aerial stem of an adult tree is called a trunk .
The dead, usually darker inner wood of a large diameter trunk is termed the heartwood and is the
result of tylosis. The outer, living wood is termed the sapwood.
Monocot stems
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Stems of two Roystonea regia palms showing characteristic bulge, leaf scars and fibrous roots, Kolkata,
India
Vascular bundles are present throughout the monocot stem, although concentrated towards theoutside. This differs from the dicot stem that has a ring of vascular bundles and often none in thecenter. The shoot apex in monocot stems is more elongated. Leaf sheathes grow up around it,
protecting it. This is true to some extent of almost all monocots. Monocots rarely produce
secondary growth and are therefore seldom woody, with Palms and Bamboo being notableexceptions. However, many monocot stems increase in diameter via anomalous secondary
growth.
Gymnosperm stems
The trunk of this redwood tree is its stem.
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Tasmanian tree fern
All gymnosperms are woody plants. Their stems are similar in structure to woody dicots exceptthat most gymnosperms produce only tracheids in their xylem, not the vessels found in dicots.Gymnosperm wood also often contains resin ducts. Woody dicots are called hardwoods, e.g. oak ,
maple and walnut. In contrast, softwoods are gymnosperms, such as pine, spruce and fir.
Fern stems
Most ferns have rhizomes with no vertical stem. The exception is tree ferns, with vertical stemsup to about 20 meters. The stem anatomy of ferns is more complicated than that of dicots
because fern stems often have one or more leaf gaps in cross section. A leaf gap is where the
vascular tissue branches off to a frond. In cross section, the vascular tissue does not form a
complete cylinder where a leaf gap occurs. Fern stems may have solenosteles or dictyosteles orvariations of them. Many fern stems have phloem tissue on both sides of the xylem in cross-
section. .
Relation to xenobiotics
Foreign chemicals such as air pollutants,[3]
herbicides and pesticides can damage stem structures.
In the case of herbicides many chemicals act by surficial effects, and other agents cause damagethrough uptake of the chemical herbicide, which is typically a complex organic chemical.
Economic importance
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White and green asparagus - crispy stems are the edible parts of this vegetable
There are thousands of species whose stems have economic uses. Stems provide a few majorstaple crops such as potato and taro. Sugarcane stems are a major source of sugar. Maple sugar is
obtained from trunks of maple trees. Vegetables from stems are asparagus, bamboo shoots, cactus pads or nopalitos, kohlrabi, and water chestnut. The spice, cinnamon is bark from a tree
trunk. Cellulose from tree trunks is a food additive in bread, grated Parmesan cheese, and other
processed foods.[citation needed ] Gum arabic is an important food additive obtained from the trunks
of Acacia senegal trees. Chicle, the main ingredient in chewing gum, is obtained from trunks of the chicle tree.
Medicines obtained from stems include quinine from the bark of cinchona trees, camphor distilled from wood of a tree in the same genus that provides cinnamon, and the muscle relaxant
curare from the bark of tropical vines.
Wood is a used in thousands of ways, e.g. buildings, furniture, boats, airplanes, wagons, car
parts, musical instruments, sports equipment, railroad ties, utility poles, fence posts, pilings,
toothpicks, matches, plywood, coffins, shingles, barrel staves, toys, tool handles, picture frames, veneer, charcoal and firewood. Wood pulp is widely used to make paper, paperboard, cellulose
sponges, cellophane and some important plastics and textiles, such as cellulose acetate and
rayon. Bamboo stems also have hundreds of uses, including paper, buildings, furniture, boats,
musical instruments, fishing poles, water pipes, plant stakes, and scaffolding. Trunks of palm
trees and tree ferns are often used for building. Reed stems are also important building materialsin some areas.
Tannins used for tanning leather are obtained from the wood of certain trees, such as quebracho.
Cork is obtained from the bark of the cork oak . Rubber is obtained from the trunks of Hevea
brasiliensis. Rattan, used for furniture and baskets, is made from the stems of tropical viningpalms. Bast fibers for textiles and rope are obtained from stems include flax, hemp, jute and
ramie. The earliest paper was obtained from the stems of papyrus by the ancient Egyptians.
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Amber is fossilized sap from tree trunks; it is used for jewelry and may contain ancient animals.
Resins from conifer wood are used to produce turpentine and rosin. Tree bark is often used as amulch and in growing media for container plants. It also can become the natural habitat of
lichens.
Some ornamental plants are grown mainly for their attractive stems, e.g.:
White bark of paper birch
Twisted branches of corkscrew willow and Harry Lauder's walking stick (Corylus avellana
'Contorta')
Red, peeling bark of paperbark maple
References
1. ^ Raven, Peter H., Ray Franklin Evert, and Helena Curtis. 1981. Biology of plants. New York, N.Y.:
Worth Publishers.ISBN 0-87901-132-7
2. ^ Goebel, K.E.v. (1905/1969). Organography of plants, especially of the Archegoniatae and Spermaphyta. Hofner publishing company.
3. ^ C.Michael Hogan. 2010. Abiotic factor . Encyclopedia of Earth. eds Emily Monosson and C.
Cleveland. National Council for Science and the Environment. Washington DC
Leaf
From Wikipedia, the free encyclopedia
For other uses, see Leaf (disambiguation).
This article may require cleanup to meet Wikipedia's quality standards. (Consider using morespecific cleanup instructions.) Please help improve this article if you can. The talk page may
contain suggestions. (October 2009)
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The leaves of a Beech tree.
In botany, a leaf is an above-ground plant organ specialized for the process of photosynthesis. Leaves are typically flat (laminar) and thin, which evolved as a means to maximise the surfacearea directly exposed to light. Likewise, the internal organisation of leaves has evolved to
maximise exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the
absorption of carbon dioxide, in a process called photosynthesis. Most leaves have stomata, which regulate carbon dioxide, oxygen, and water vapour exchange with the atmosphere. The
shape and structure of leaves vary considerably depending on climate, primarily due to theavailability of light and potential for water loss due to temperature and humidity. Leaves are also
the primary site, in most plants, where transpiration and guttation take place. Leaves can alsostore food and water, and are modified in some plants for these purposes. The concentration of
photosynthesis in leaves makes them rich in protein, minerals, and sugars. Because of their
nutritional value, leaves are prominent in the diet of many animals, including humans as leaf vegetables. Foliage is a mass noun that refers to leaves.
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A leaf shed in autumn.
Many plants retain their leaves for long periods but other plants periodically shed all of theirleaves. In areas where winters are cold, deciduous plants shed their leaves in autumn. In areas
with a severe dry season, some plants may shed their leaves until the dry season ends.
Not all plants have true leaves. Bryophytes (i.e., mosses and liverworts) are non-vascular plants,
and, although they have flattened, leaf-like structures that are rich in chlorophyll, these are not
considered true leaves by all botanists, since they lack vascular tissue. Vascularised leaves firstevolved following the Devonian period, when carbon dioxide concentration in the atmosphere
dropped significantly. This occurred independently in two separate lineages of vascular plants:
the microphylls of lycophytes and the euphylls ("true leaves") of ferns, gymnosperms, and
angiosperms. Euphylls are also referred to as macrophylls or megaphylls ("large leaves").
Contents
[hide]
1 Anatomy
o 1.1 Large scale features
o 1.2 Medium scale features
o 1.3 Small-scale features
o 1.4 Major leaf tissues
1.4.1 Epidermis 1.4.2 Mesophyll
1.4.3 Veins
2 Seasonal leaf loss
3 Morphology
o 3.1 Basic types
o 3.2 Arrangement on the stem
o 3.3 Divisions of the blade
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o 3.4 Characteristics of the petiole
o 3.5 Venation
o 3.6 Morphology changes within a single plant
4 Terminology
o 4.1 Shape
o 4.2 Edge (margin)
o 4.3 Tip
o 4.4 Base
o 4.5 Surface
o 4.6 Hairiness
5 Adaptations
6 Interactions with other organisms
7 Bibliography
8 Footnotes
9 See also
10 External links
Anatomy
Large scale features
A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf
blade), and stipules (small structures located to either side of the base of the petiole). Not everyspecies produces leaves with all of these structural components. In certain species, paired
stipules are not obvious or are absent altogether. A petiole may be absent, or the blade may not
be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to
species is presented in detail below under morphology.
The petiole mechanically links the leaf to the plant and provides the route for transfer of waterand sugars to and from the leaf. The lamina is typically the location of the majority of
photosynthesis.
Medium scale features
Leaves are normally extensively vascularised and are typically covered by a dense network of
xylem, which supply water for photosynthesis, and phloem, which remove the sugars producedby photosynthesis. Many leaves are covered in trichomes (small hairs) which have a diverse
range of structures and functions.
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Small-scale features
A leaf is a plant organ and is made up of a collection of tissues in a regular organisation. The
major tissue systems present are:
1. The epidermis that covers the upper and lower surfaces
2. The mesophyll inside the leaf that is rich in chloroplasts (also called chlorenchyma)
3. The arrangement of veins (the vascular tissue)
These three tissue systems typically form a regular organisation at the cellular scale.
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Major leaf tissues
Cross section of a leaf.
Epidermal cells
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Palisade mesophyll cells
Spongy mesophyll cells
Epidermis
SEM image of Nicotiana alata leaf's epidermis, showing trichomes (hair-like appendages) and stomata
(eye-shaped slits, visible at full resolution).
The epidermis is the outer layer of cells covering the leaf. It forms the boundary separating theplant's inner cells from the external world. The epidermis serves several functions: protection
against water loss by way of transpiration, regulation of gas exchange, secretion of metabolic
compounds, and (in some species) absorption of water. Most leaves show dorsoventral anatomy:The upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may
serve different functions.
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The epidermis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer
side with a waxy cuticle that prevents water loss. The cuticle is in some cases thinner on thelower epidermis than on the upper epidermis, and is generally thicker on leaves from dry
climates as compared with those from wet climates.
The epidermis tissue includes several differentiated cell types: epidermal cells, epidermal haircells (trichomes) cells in the stomate complex; guard cells and subsidiary cells. The epidermal
cells are the most numerous, largest, and least specialized and form the majority of theepidermis. These are typically more elongated in the leaves of monocots than in those of dicots.
The epidermis is covered with pores called stomata, part of a stoma complex consisting of a poresurrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells
that lack chloroplasts. Opening and closing of the stoma complex regulates the exchange of
gases and water vapor between the outside air and the interior of the leaf and plays an important
role in allowing photosynthesis without letting the leaf dry out. In a typical leaf, the stomata aremore numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis and more
numerous in plants from cooler climates.
Mesophyll
Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma
(ground tissue) or chlorenchyma tissue called the mesophyll (Greek for "middle leaf"). This
assimilation tissue is the primary location of photosynthesis in the plant. The products of photosynthesis are called "assimilates".
In ferns and most flowering plants, the mesophyll is divided into two layers:
An upper palisade layer of tightly packed, vertically elongated cells, one to two cells thick,directly beneath the adaxial epidermis. Its cells contain many more chloroplasts than the spongy
layer. These long cylindrical cells are regularly arranged in one to five rows. Cylindrical cells, with
the chloroplasts close to the walls of the cell, can take optimal advantage of light. The slight
separation of the cells provides maximum absorption of carbon dioxide. This separation must be
minimal to afford capillary action for water distribution. In order to adapt to their different
environment (such as sun or shade), plants had to adapt this structure to obtain optimal result.
Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the
soil are single-layered.
Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more rounded
and not so tightly packed. There are large intercellular air spaces. These cells contain fewer
chloroplasts than those of the palisade layer. The pores or stomata of the epidermis open into
substomatal chambers, which are connected to the air spaces between the spongy layer cells.
These two different layers of the mesophyll are absent in many aquatic and marsh plants. Even
an epidermis and a mesophyll may be lacking. Instead for their gaseous exchanges they use a
homogeneous aerenchyma (thin-walled cells separated by large gas-filled spaces). Theirstomata are situated at the upper surface.
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Leaves are normally green in color, which comes from chlorophyll found in plastids in the
chlorenchyma cells. Plants that lack chlorophyll cannot photosynthesize.
Veins
The veins of a bramble leaf .
The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll.They are typical examples of pattern formation through ramification. The pattern of the veins is
called venation.
The veins are made up of:
Xylem: tubes that bring water and minerals from the roots into the leaf.
Phloem: tubes that usually move sap, with dissolved sucrose, produced by photosynthesis in the
leaf, out of the leaf.
The xylem typically lies on the adaxial side of the vascular bungle and the phloem typically lieson the abaxial side. Both are embedded in a dense parenchyma tissue, called the pith or sheath,
which usually includes some structural collenchyma tissue.
Seasonal leaf loss
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Leaves shifting color in fall
Leaves in temperate, boreal, and seasonally dry zones may be seasonally deciduous (falling off
or dying for the inclement season). This mechanism to shed leaves is called abscission. After theleaf is shed, a leaf scar develops on the twig. In cold autumns, they sometimes change color, and
turn yellow, bright-orange, or red, as various accessory pigments (carotenoids and xanthophylls)
are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll
production. Red anthocyanin pigments are now thought to be produced in the leaf as it dies,possibly to mask the yellow hue left when the chlorophyll is lost - yellow leaves appear to attract
herbivores such as aphids.[1]
Morphology
The Citrus leaf is identified by the pores and pigments, as well as the margins.
External leaf characteristics (such as shape, margin, hairs, etc.) are important for identifyingplant species, and botanists have developed a rich terminology for describing leaf characteristics.
These structures are a part of what makes leaves determinant; they grow and achieve a specific
pattern and shape, then stop. Other plant parts like stems or roots are non-determinant, and will
usually continue to grow as long as they have the resources to do so.
Classification of leaves can occur through many different designative schema, and the type of
leaf is usually characteristic of a species, although some species produce more than one type of leaf. The longest type of leaf is a leaf from palm trees, measuring at nine feet long. The
terminology associated with the description of leaf morphology is presented, in illustrated form,
at Wikibooks.
Basic types
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Leaves of the White Spruce (Picea glauca) are needle-shaped and their arrangement is spiral
Ferns have fronds
Conifer leaves are typically needle-, awl-, or scale-shaped
Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina
Lycophytes have microphyll leaves.
Sheath leaves (type found in most grasses)
Other specialized leaves (such as those of Nepenthes)
Arrangement on the stem
Different terms are usually used to describe leaf placement (phyllotaxis):
The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles
to each other ("decussate") along the red stem. Note the developing buds in the axils of these leaves.
Alternate — leaf attachments are singular at nodes, and leaves alternate direction, to a greater
or lesser degree, along the stem.
Opposite — Two structures, one on each opposite side of the stem, typically leaves, branches,
or flower parts. Leaf attachments are paired at each node; decussate if, as typical, each
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successive pair is rotated 90° progressing along the stem; or distichous if not rotated, but two-
ranked (in the same geometric flat-plane).
Whorled — three or more leaves attach at each point or node on the stem. As with opposite
leaves, successive whorls may or may not be decussate, rotated by half the angle between the
leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc.).
Opposite leaves may appear whorled near the tip of the stem.
Rosulate — leaves form a rosette
As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of
light. In essence, leaves form a helix pattern centered around the stem, either clockwise orcounterclockwise, with (depending upon the species) the same angle of divergence. There is a
regularity in these angles and they follow the numbers in a Fibonacci sequence: 1/2, 2/3, 3/5, 5/8,
8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit close to 360° x 34/89 = 137.52 or
137° 30', an angle known in mathematics as the golden angle. In the series, the numerator indicates the number of complete turns or "gyres" until a leaf arrives at the initial position. The
denominator indicates the number of leaves in the arrangement. This can be demonstrated by the
following:
alternate leaves have an angle of 180° (or 1/2)
120° (or 1/3) : three leaves in one circle
144° (or 2/5) : five leaves in two gyres
135° (or 3/8) : eight leaves in three gyres.
Divisions of the blade
A leaf with laminar structure and pinnate venation
Two basic forms of leaves can be described considering the way the blade (lamina) is divided. A
simple leaf has an undivided blade. However, the leaf shape may be formed of lobes, but the
gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided
blade, each leaflet of the blade separated along a main or secondary vein. Because each leafletcan appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a
compound leaf. Compound leaves are a characteristic of some families of higher plants, such as
the Fabaceae. The middle vein of a compound leaf or a frond, when it is present, is called arachis.
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Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers
off the palm of a hand, e.g. Cannabis (hemp) and Aesculus (buckeyes).
Pinnately compound leaves have the leaflets arranged along the main or mid-vein.
o odd pinnate: with a terminal leaflet, e.g. Fraxinus (ash).
o even pinnate: lacking a terminal leaflet, e.g. Swietenia (mahogany).
Bipinnately compound leaves are twice divided: the leaflets are arranged along a secondary vein
that is one of several branching off the rachis. Each leaflet is called a "pinnule". The pinnules on
one secondary vein are called "pinna"; e.g. Albizia (silk tree).
trifoliate (or trifoliolate): a pinnate leaf with just three leaflets, e.g. Trifolium (clover), Laburnum
(laburnum).
pinnatifid : pinnately dissected to the central vein, but with the leaflets not entirely separate, e.g.
Polypodium, some Sorbus (whitebeams). In pinnately veined leaves the central vein in known as
the midrib.
Characteristics of the petiole
The overgrown petioles of Rhubarb (Rheum rhabarbarum) are edible.
Petiolated leaves have a petiole (leaf stem). Sessile leaves do not: The blade attaches directly tothe stem. In clasping or decurrent leaves, the blade partially or wholly surrounds the stem, often
giving the impression that the shoot grows through the leaf. When this is the case, the leaves are
called "perfoliate", such as in Claytonia perfoliata. In peltate leaves, the petiole attaches to theblade inside from the blade margin.
In some Acacia species, such as the Koa Tree ( Acacia koa), the petioles are expanded or
broadened and function like leaf blades; these are called phyllodes. There may or may not benormal pinnate leaves at the tip of the phyllode.
A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base
of the petiole resembling a small leaf. Stipules may be lasting and not be shed (a stipulate leaf,
such as in roses and beans), or be shed as the leaf expands, leaving a stipule scar on the twig (an
exstipulate leaf).
The situation, arrangement, and structure of the stipules is called the "stipulation".
o free
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o adnate : fused to the petiole base
o ochreate : provided with ochrea, or sheath-formed stipules, e.g. rhubarb,
o encircling the petiole base
o interpetiolar : between the petioles of two opposite leaves.
o intrapetiolar : between the petiole and the subtending stem
Venation
Branching veins on underside of taro leaf
The venation within the bract of a Lime tree.
The lower epidermis of Tilia × europaea
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Palmate-veined leaf
There are two subtypes of venation, namely, craspedodromous, where the major veins stretch up
to the margin of the leaf, and camptodromous, when major veins extend close to the margin, but
bend before they intersect with the margin.
Feather-veined, reticulate (also called pinnate-netted, penniribbed, penninerved, orpenniveined)— the veins arise pinnately from a single mid-vein and subdivide into veinlets.
These, in turn, form a complicated network. This type of venation is typical for (but by no means
limited to) dicotyledons.
Three main veins branch at the base of the lamina and run essentially parallel subsequently, as
in Ceanothus. A similar pattern (with 3-7 veins) is especially conspicuous in Melastomataceae.
Palmate-netted, palmate-veined, fan-veined; several main veins diverge from near the leaf base
where the petiole attaches, and radiate toward the edge of the leaf, e.g. most Acer (maples).
Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel— veins run parallel for the length
of the leaf, from the base to the apex. Commissural veins (small veins) connect the major
parallel veins. Typical for most monocotyledons, such as grasses.
Dichotomous— There are no dominant bundles, with the veins forking regularly by pairs; found
in Ginkgo and some pteridophytes.
Note that, although it is the more complex pattern, branching veins appear to be plesiomorphic
and in some form were present in ancient seed plants as long as 250 million years ago. A pseudo-
reticulate venation that is actually a highly modified penniparallel one is an autapomorphy of
some Melanthiaceae, which are monocots, e.g. Paris quadrifolia (True-lover's Knot).
Morphology changes within a single plant
Homoblasty - Characteristic in which a plant has small changes in leaf size, shape, and growth
habit between juvenile and adult stages.
Heteroblasty - Characteristic in which a plant has marked changes in leaf size, shape, and growth
habit between juvenile and adult stages.
Terminology
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Chart illustrating some leaf morphology terms
A portion of a coriander leaf
Shape
Main article: Leaf shape
Edge (margin)
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ciliate: fringed with hairs
crenate: wavy-toothed; dentate with rounded teeth, such as Fagus (beech)
crenulate finely or shallowly crenate
dentate: toothed, such as Castanea (chestnut)
o coarse-toothed: with large teeth
o glandular toothed: with teeth that bear glands.
denticulate: finely toothed
doubly toothed: each tooth bearing smaller teeth, such as Ulmus (elm)
entire: even; with a smooth margin; without toothing
lobate: indented, with the indentations not reaching to the center, such as many Quercus (oaks)
o palmately lobed: indented with the indentations reaching to the center, such as
Humulus (hop).
serrate: saw-toothed with asymmetrical teeth pointing forward, such as Urtica (nettle)
serrulate: finely serrate
sinuate: with deep, wave-like indentations; coarsely crenate, such as many Rumex (docks)
spiny or pungent: with stiff, sharp points, such as some Ilex (hollies) and Cirsium (thistles).
Tip
Leaves showing various morphologies. Clockwise from upper left: tripartite lobation, elliptic with
serrulate margin, peltate with palmate venation, acuminate odd-pinnate (center), pinnatisect, lobed,
elliptic with entire margin
acuminate: long-pointed, prolonged into a narrow, tapering point in a concave manner.
acute: ending in a sharp, but not prolonged point
cuspidate: with a sharp, elongated, rigid tip; tipped with a cusp.
emarginate: indented, with a shallow notch at the tip.
mucronate: abruptly tipped with a small short point, as a continuation of the midrib; tipped with
a mucro.
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mucronulate: mucronate, but with a smaller spine.
obcordate: inversely heart-shaped, deeply notched at the top.
obtuse: rounded or blunt
truncate: ending abruptly with a flat end, that looks cut off.
Base
acuminate: coming to a sharp, narrow, prolonged point.
acute: coming to a sharp, but not prolonged point.
auriculate: ear-shaped.
cordate: heart-shaped with the notch towards the stalk.
cuneate: wedge-shaped.
hastate: shaped like an halberd and with the basal lobes pointing outward.
oblique: slanting.
reniform: kidney-shaped but rounder and broader than long.
rounded: curving shape.
sagittate: shaped like an arrowhead and with the acute basal lobes pointing downward.
truncate: ending abruptly with a flat end, that looks cut off.
Surface
Scale-shaped leaves of a Norfolk Island Pine, Araucaria heterophylla.
farinose: bearing farina; mealy, covered with a waxy, whitish powder.
glabrous: smooth, not hairy.
glaucous: with a whitish bloom; covered with a very fine, bluish-white powder.
glutinous: sticky, viscid.
papillate, or papillose: bearing papillae (minute, nipple-shaped protuberances).
pubescent: covered with erect hairs (especially soft and short ones).
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punctate: marked with dots; dotted with depressions or with translucent glands or colored dots.
rugose: deeply wrinkled; with veins clearly visible.
scurfy: covered with tiny, broad scalelike particles.
tuberculate: covered with tubercles; covered with warty prominences.
verrucose: warted, with warty outgrowths.
viscid , or viscous: covered with thick, sticky secretions.
The leaf surface is also host to a large variety of microorganisms; in this context it is referred toas the phyllosphere.
The parallel veins within an iris leaf.
Hairiness
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Common Mullein (Verbascum thapsus) leaves are covered in dense, stellate trichomes.
Scanning electron microscope image of trichomes on the lower surface of a Coleus blumei (coleus) leaf.
"Hairs" on plants are properly called trichomes. Leaves can show several degrees of hairiness.
The meaning of several of the following terms can overlap.
arachnoid , or arachnose: with many fine, entangled hairs giving a cobwebby appearance.
barbellate: with finely barbed hairs (barbellae).
bearded: with long, stiff hairs.
bristly: with stiff hair-like prickles.
canescent: hoary with dense grayish-white pubescence.
ciliate: marginally fringed with short hairs (cilia). ciliolate: minutely ciliate.
floccose: with flocks of soft, woolly hairs, which tend to rub off.
glabrescent: losing hairs with age.
glabrous: no hairs of any kind present.
glandular: with a gland at the tip of the hair.
hirsute: with rather rough or stiff hairs.
hispid: with rigid, bristly hairs.
hispidulous: minutely hispid.
hoary: with a fine, close grayish-white pubescence.
lanate, or lanose: with woolly hairs.
pilose: with soft, clearly separated hairs.
puberulent , or puberulous: with fine, minute hairs.
pubescent: with soft, short and erect hairs.
scabrous, or scabrid: rough to the touch.
sericeous: silky appearance through fine, straight and appressed (lying close and flat) hairs.
silky: with adpressed, soft and straight pubescence.
stellate, or stelliform: with star-shaped hairs.
strigose: with appressed, sharp, straight and stiff hairs.
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tomentose: densely pubescent with matted, soft white woolly hairs.
o cano-tomentose: between canescent and tomentose.
o felted-tomentose: woolly and matted with curly hairs.
tomentulose: minutely or only slightly tomentose.
villous: with long and soft hairs, usually curved.
woolly:' with long, soft and tortuous or matted hairs.
Adaptations
The embedded lists in this article may contain items that are not encyclopedic. Please help out
by removing such elements and incorporating appropriate items into the main body of the
article. (February 2008)
Poinsettia bracts are leaves which have evolved red pigmentation in order to attract insects and birds to
the central flowers, an adaptive function normally served by petals (which are themselves leaves highly
modified by evolution).
In the course of evolution, leaves have adapted to different environments in the following ways:
A certain surface structure avoids moistening by rain and contamination (See Lotus effect ).
Sliced leaves reduce wind resistance.
Hairs on the leaf surface trap humidity in dry climates and create a boundary layer reducing
water loss.
Waxy leaf surfaces reduce water loss.
Large surface area provides large area for sunlight and shade for plant to minimize heating and
reduce water loss.
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In more or less opaque or buried in the soil leaves, translucent windows filter the light before
the photosynthesis takes place at the inner leaf surfaces (e.g. Fenestraria).
Succulent leaves store water and organic acids for use in CAM photosynthesis.
Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g.
eucalypts).
Inclusions of crystalline minerals deter herbivores (e.g. silica phytoliths in grasses, raphides in
Araceae).
Petals attracts pollinators.
Spines protect the plants (e.g. cacti).
Insect traps feed the plants directly (see carnivorous plants).
Bulbs store food and water (e.g. onions).
Tendrils allow the plant to climb (e.g. peas).
Bracts and pseudanthia ( false flowers) replace normal flower structures when the true flowers
are greatly reduced (e.g. Spurges).
Interactions with other organisms
Some insects mimic leaves (Kallima inachus shown)
A girl playing with leaves
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Leaf after being eaten by Caterpillar
Although not as nutritious as other organs such as fruit, leaves provide a food source for manyorganisms. Animals which eat leaves are known as folivores. The leaf is one of the most vital
parts of the plant, and plants have evolved protection against folivores such as tannins, chemicals
which hinder the digestion of proteins and have an unpleasant taste.
Some animals have cryptic adaptations to avoid their own predators. For example, somecaterpillars will create a small home in the leaf by folding it over themselves, while other
herbivores and their prey mimic the appearance of the leaf. Some insects, such as the katydid, take this even further, moving from side to side much like a leaf does in the wind.
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with somestored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which
occurs after fertilization and some growth within the mother plant. The formation of the seed
completes the process of reproduction in seed plants (started with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the
integuments of the ovule.
Seeds have been an important development in the reproduction and spread of flowering plants,
relative to more primitive plants like mosses, ferns and liverworts, which do not have seeds and
use other means to propagate themselves. This can be seen by the success of seed plants (bothgymnosperms and angiosperms) in dominating biological niches on land, from forests to
grasslands both in hot and cold climates.
The term seed also has a general meaning that predates the above — anything that can be sown,
e.g. "seed" potatoes, "seeds" of corn or sunflower "seeds". In the case of sunflower and corn
"seeds", what is sown is the seed enclosed in a shell or hull, and the potato is a tuber.
Contents
[hide]
1 Seed structure o 1.1 Kinds of seeds
2 Seed production
o 2.1 Seed development
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o 2.2 Seed size and seed set
3 Seed functions
o 3.1 Embryo nourishment
o 3.2 Seed dispersal
3.2.1 By wind (anemochory)
3.2.2 By water (hydrochory) 3.2.3 By animals (zoochory)
o 3.3 Seed dormancy
o 3.4 Seed persistence and seed banks
4 Seed germination
o 4.1 Inducing germination
5 Origin and evolution
6 Economic importance
o 6.1 Edible seeds
o 6.2 Poison and food safety
o 6.3 Other uses
7 Seed records 8 See also
9 References
10 External links
[edit] Seed structure
The parts of an avocado seed (a dicot), showing the seed coat, endosperm, and embryo.
A typical seed includes three basic parts: (1) an embryo, (2) a supply of nutrients for the embryo,
and (3) a seed coat.
The embryo is an immature plant from which a new plant will grow under proper conditions.
The embryo has one cotyledon or seed leaf in monocotyledons, two cotyledons in almost alldicotyledons and two or more in gymnosperms. The radicle is the embryonic root. The plumule
is the embryonic shoot. The embryonic stem above the point of attachment of the cotyledon(s) is
the epicotyl. The embryonic stem below the point of attachment is the hypocotyl.
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Within the seed, there usually is a store of nutrients for the seedling that will grow from the
embryo. The form of the stored nutrition varies depending on the kind of plant. In angiosperms,the stored food begins as a tissue called the endosperm, which is derived from the parent plant
via double fertilization. The usually triploid endosperm is rich in oil or starch and protein. In
gymnosperms, such as conifers, the food storage tissue is part of the female gametophyte, a
haploid tissue. In some species, the embryo is embedded in the endosperm or femalegametophyte, which the seedling will use upon germination. In others, the endosperm is
absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the
embryo become filled with this stored food. At maturity, seeds of these species have noendosperm and are termed exalbuminous seeds. Some exalbuminous seeds are bean, pea, oak ,
walnut, squash, sunflower, and radish. Seeds with an endosperm at maturity are termed
albuminous seeds. Most monocots (e.g. grasses and palms) and many dicots (e.g. brazil nut andcastor bean) have albuminous seeds. All gymnosperm seeds are albuminous.
The seed coat (or testa) develops from the tissue, the integument, originally surrounding theovule. The seed coat in the mature seed can be a paper-thin layer (e.g. peanut) or something more
substantial (e.g. thick and hard in honey locust and coconut). The seed coat helps protect theembryo from mechanical injury and from drying out.
In addition to the three basic seed parts, some seeds have an appendage on the seed coat such an
aril (as in yew and nutmeg) or an elaiosome (as in Corydalis) or hairs (as in cotton). There mayalso be a scar on the seed coat, called the hilum; it is where the seed was attached to the ovary
wall by the funiculus.
[edit] Kinds of seeds
Many structures commonly referred to as "seeds" are actually dry fruits. Sunflower seeds are
sold commercially while still enclosed within the hard wall of the fruit, which must be split opento reach the seed. Different groups of plants have other modifications, the so-called stone fruits
(such as the peach) have a hardened fruit layer ( the endocarp) fused to and surrounding the
actual seed. Nuts are the one-seeded, hard shelled fruit, of some plants, with an indehiscent seed,such as an acorn or hazelnut.
[edit] Seed production
Immature Elm seeds.
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Seeds are produced in several related groups of plants, and their manner of production
distinguishes the angiosperms ("enclosed seeds") from the gymnosperms ("naked seeds").Angiosperm seeds are produced in a hard or fleshy structure called a fruit that encloses the seeds,
hence the name. (Some fruits have layers of both hard and fleshy material). In gymnosperms, no
special structure develops to enclose the seeds, which begin their development "naked" on the
bracts of cones. However, the seeds do become covered by the cone scales as they develop insome species of conifer.
Seed production in natural plant populations vary widely from year-to-year in response to
weather variables, insects and diseases, and internal cycles within the plants themselves. Over a
20-year period, for example, forests composed of loblolly pine and shortleaf pine produced from0 to nearly 5 million sound pine seeds per hectare.[1] Over this period, there were six bumper
seeds, five poor seeds crops, and nine good seed crops, when evaluated in regard to producing
adequate seedlings for natural forest reproduction.
[edit] Seed development
The inside of a Ginkgo seed, showing a well-developed embryo, nutritive tissue(megagametophyte), and a bit of the surrounding seed coat.
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Diagram of the internal structure of a dicot seed and embryo. (a) seed coat, (b) endosperm, (c)
cotyledon, (d) hypocotyl.
The seed, which is an embryo with two points of growth (one of which forms the stems the other
the roots) is enclosed in a seed coat with some food reserves. Angiosperm seeds consist of three
genetically distinct constituents: (1) the embryo formed from the zygote, (2) the endosperm,which is normally triploid, (3) the seed coat from tissue derived from the maternal tissue of the
ovule. In angiosperms, the process of seed development begins with double fertilization andinvolves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is
the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary
endosperm. Right after fertilization, the zygote is mostly inactive but the primary endospermdivides rapidly to form the endosperm tissue. This tissue becomes the food that the young plant
will consume until the roots have developed after germination or it develops into a hard seed
coat. The seed coat forms from the two integuments or outer layers of cells of the ovule, which
derive from tissue from the mother plant, the inner integument forms the tegmen and the outerforms the testa. When the seed coat forms from only one layer it is also called the testa, though
not all such testa are homologous from one species to the next.
In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double
fertilization but one sperm nucleus unites with the egg nucleus and the other sperm is not used .[2]
Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed duringearly development.[3] The seed is composed of the embryo (the result of fertilization) and tissue
from the mother plant, which also form a cone around the seed in coniferous plants like Pine and
Spruce.
The ovules after fertilization develop into the seeds; the main parts of the ovule are the funicle;
which attaches the ovule to the placenta, the nucellus; the main region of the ovule were the
embryo sac develops, the micropyle; A small pore or opening in the ovule where the pollen tubeusually enters during the process of fertilization, and the chalaza; the base of the ovule opposite
the micropyle, where integument and nucellus are joined together.[4]
The shape of the ovules as they develop often affects the finale shape of the seeds. Plants
generally produce ovules of four shapes: the most common shape is called anatropous, with a
curved shape. Orthotropous ovules are straight with all the parts of the ovule lined up in a longrow producing an uncurved seed. Campylotropous ovules have a curved embryo sac often giving
the seed a tight ―c‖ shape. The last ovule shape is called amphitropous, where the ovule is partly
inverted and turned back 90 degrees on its stalk or funicle.
In the majority of flowering plants, the zygote's first division is transversely oriented in regards
to the long axis, and this establishes the polarity of the embryo. The upper or chalazal polebecomes the main area of growth of the embryo, while the lower or micropylar pole produces the
stalk-like suspensor that attaches to the micropyle. The suspensor absorbs and manufacturers
nutrients from the endosperm that are utilized during the embryos growth.[5]
The embryo is composed of different parts; the epicotyle will grow into the shoot, the radicle
grows into the primary root, the hypocotyl connects the epicotyle and the radicle, the cotyledons
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form the seed leaves, the testa or seed coat forms the outer covering of the seed.
Monocotyledonous plants like corn, have other structures; instead of the hypocotyle-epicotyle, ithas a coleoptile that forms the first leaf and connects to the coleorhiza that connects to the
primary root and adventitious roots form from the sides. The seeds of corn are constructed with
these structures; pericarp, scutellum (single large cotyledon) that absorbs nutrients from the
endosperm, endosperm, plumule, radicle, coleoptile and coleorhiza - these last two structures aresheath-like and enclose the plumule and radicle, acting as a protective covering. The testa or seed
coats of both monocots and dicots are often marked with patterns and textured markings, or have
wings or tufts of hair.
[edit] Seed size and seed set
Seeds are very diverse in size. The dust-like orchid seeds are the smallest with about one million
seeds per gram; they are often embryonic seeds with immature embryos and no significant
energy reserves. Orchids and a few other groups of plants are myco-heterotrophs which depend
on mycorrhizal fungi for nutrition during germination and the early growth of the seedling. Some
terrestrial Orchid seedlings, in fact, spend the first few years of their life deriving energy fromthe fungus and do not produce green leaves.
[6] At over 20 kg, the largest seed is the coco de mer.
Plants that produce smaller seeds can generate many more seeds per flower, while plants withlarger seeds invest more resources into those seeds and normally produce fewer seeds. Small
seeds are quicker to ripen and can be dispersed sooner, so fall blooming plants often have small
seeds. Many annual plants produce great quantities of smaller seeds; this helps to ensure that atleast a few will end in a favorable place for growth. Herbaceous perennials and woody plants
often have larger seeds, they can produce seeds over many years, and larger seeds have moreenergy reserves for germination and seedling growth and produce larger, more established
seedlings after germination.[7][8]
[edit] Seed functions
Seeds serve several functions for the plants that produce them. Key among these functions are
nourishment of the embryo, dispersal to a new location, and dormancy during unfavorable
conditions. Seeds fundamentally are a means of reproduction and most seeds are the product of
sexual reproduction which produces a remixing of genetic material and phenotype variability thatnatural selection acts on.
[edit] Embryo nourishment
Seeds protect and nourish the embryo or young plant. Seeds usually give a seedling a faster start
than a sporeling from a spore, because of the larger food reserves in the seed and themulticellularity of the enclosed embryo.
[edit] Seed dispersal
Main article: Seed dispersal
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Unlike animals, plants are limited in their ability to seek out favorable conditions for life and
growth. As a result, plants have evolved many ways to disperse their offspring by dispersingtheir seeds (see also vegetative reproduction). A seed must somehow "arrive" at a location and be
there at a time favorable for germination and growth. When the fruits open and release their
seeds in a regular way, it is called dehiscent, which is often distinctive for related groups of
plants, these fruits include; Capsules, follicles, legumes, silicles and siliques. When fruits do notopen and release their seeds in a regular fashion they are called indehiscent, which include the
fruits achenes, caryopsis, nuts, samaras, and utricles.[9]
Seed dispersal is seen most obviously in fruits; however many seeds aid in their own dispersal.
Some kinds of seeds are dispersed while still inside a fruit or cone, which later opens ordisintegrates to release the seeds. Other seeds are expelled or released from the fruit prior to
dispersal. For example, milkweeds produce a fruit type, known as a follicle,[10]
that splits open
along one side to release the seeds. Iris capsules split into three "valves" to release their seeds.[11]
[edit] By wind (anemochory)
Dandelion seeds (achenes) can be carried long distances by the wind.
The seed pod of milkweed (Asclepias syriaca)
Many seeds (e.g. maple, pine) have a wing that aids in wind dispersal.
The dustlike seeds of orchids are carried efficiently by the wind.
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Some seeds, (e.g. dandelion, milkweed, poplar) have hairs that aid in wind dispersal.[12]
Some winged seeds have two, and some have only one wing.
[edit] By water (hydrochory)
Some plants, such as Mucuna and Dioclea, produce buoyant seeds termed sea-beans ordrift seeds because they float in rivers to the oceans and wash up on beaches.[13]
[edit] By animals (zoochory)
Seeds (burrs) with barbs or hooks (e.g. acaena, burdock , dock ) which attach to animal furor feathers, and then drop off later.
Seeds with a fleshy covering (e.g. apple, cherry, juniper) are eaten by animals (birds,
mammals, reptiles, fish) which then disperse these seeds in their droppings.
Seeds (nuts) which are an attractive long-term storable food resource for animals (e.g.
acorns, hazelnut, walnut); the seeds are stored some distance from the parent plant, andsome escape being eaten if the animal forgets them.
Myrmecochory is the dispersal of seeds by ants. Foraging ants disperse seeds which have
appendages called elaiosomes[14]
(e.g. bloodroot, trilliums, Acacias, and many species of
Proteaceae). Elaiosomes are soft, fleshy structures that contain nutrients for animals that eatthem. The ants carry such seeds back to their nest, where the elaiosomes are eaten. The
remainder of the seed, which is hard and inedible to the ants, then germinates either within the
nest or at a removal site where the seed has been discarded by the ants.[15]
This dispersal
relationship is an example of mutualism, since the plants depend upon the ants to disperse seeds,while the ants depend upon the plants seeds for food. As a result, a drop in numbers of one
partner can reduce success of the other. In South Africa, the Argentine ant ( Linepithema humile)has invaded and displaced native species of ants. Unlike the native ant species, Argentine ants donot collect the seeds of Mimetes cucullatus or eat the elaiosomes. In areas where these ants have
invaded, the numbers of Mimetes seedlings have dropped.[16]
[edit] Seed dormancy
Main article: Seed dormancy
Seed dormancy has two main functions: the first is synchronizing germination with the optimal
conditions for survival of the resulting seedling; the second is spreading germination of a batch
of seeds over time so that a catastrophe after germination (e.g. late frosts, drought, herbivory) does not result in the death of all offspring of a plant (bet-hedging).[17] Seed dormancy is defined
as a seed failing to germinate under environmental conditions optimal for germination, normally
when the environment is at a suitable temperature with proper soil moisture. This true dormancyor innate dormancy is therefore caused by conditions within the seed that prevent germination.
Thus dormancy is a state of the seed, not of the environment.[18] Induced dormancy, enforced
dormancy or seed quiescence occurs when a seed fails to germinate because the external
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environmental conditions are inappropriate for germination, mostly in response to conditions
being too dark or light, too cold or hot, or too dry.
Seed dormancy is not the same as seed persistence in the soil or on the plant, though even in
scientific publications dormancy and persistence are often confused or used as synonyms.[19]
Often seed dormancy is divided into four major categories: exogenous; endogenous;combinational; and secondary. A more recent system distinguishes five classes of
dormancy:morphological, physiological, morphophysiological, physical and combinational
dormancy.[20]
Exogenous dormancy is caused by conditions outside the embryo including:
Physical dormancy or hard seed coats occurs when seeds are impermeable to water. At
dormancy break a specialized structure, the ‗water gap‘, is disrupted in response to
environmental cues, especially temperature, so that water can enter the seed and
germination can occur. Plant families where physical dormancy occurs includeAnacardiaceae, Cannaceae, Convulvulaceae, Fabaceae and Malvaceae.[21]
Chemical dormancy considers species that lack physiological dormancy, but where a
chemical prevents germination. This chemical can be leached out of the seed by rainwateror snow melt or be deactivated somehow.[22] Leaching of chemical inhibitors from the
seed by rain water is often cited as an important cause of dormancy release in seeds of desert plants, however little evidence exists to support this claim.
[23]
Endogenous dormancy is caused by conditions within the embryo itself, including:
Morphological dormancy where germination is prevented due to morphological
characteristics of the embryo. In some species the embryo is just a mass of cells whenseeds are dispersed, it is not differentiated. Before germination can take place bothdifferentiation and growth of the embryo have to occur. In other species the embryo is
differentiated but not fully grown (underdeveloped) at dispersal and embryo growth up to
a species specific length is required before germination can occur. Examples of plantfamilies where morphological dormancy occurs are Apiaceae, Cycadaceae, Liliaceae,
Magnoliaceae and Ranunculaceae.[24][25]
Morphophysiological dormancy seeds with underdeveloped embryos, and which in
addition have physiological components to dormancy. These seeds therefore require adormancy-breaking treatments as well as a period of time to develop fully grown
embryos. Plant families where morphophysiological dormancy occurs include Apiaceae,
Aquifoliaceae, Liliaceae, Magnoliaceae, Papaveraceae and Ranunculaceae.[24]
Some
plants with morphophysiological dormancy like Asarum or Trillium species havemultiple types of dormancy, one affects radicle (root) growth while the other affects
plumule (shoot) growth. The terms "double dormancy" and "2-year seeds" are used for
species whose seeds need two years to complete germination or at least two winters andone summer. Dormancy of the radicle (seedling root)is broken during the first winter
after dispersal while dormancy of the shoot bud is broken during the second winter .[24]
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Physiological dormancy means that the embryo can, due to physiological causes, not
generate enough power to break through the seed coat, endosperm or other coveringstructures. Dormancy is typically broken at cool wet, warm wet or warm dry conditions.
Abscisic acid is usually the growth inhibitor in seeds and its production can be affected
by light.
o Drying; some plants including a number of grasses and those from seasonally aridregions need a period of drying before they will germinate. The seeds are released
but need to have a lower moisture content before germination can begin. If the
seeds remain moist after dispersal, germination can be delayed for many monthsor even years. Many herbaceous plants from temperate climate zones have
physiological dormancy that disappears with drying of the seeds. Other species
will germinate after dispersal only under very narrow temperature ranges, but asthe seeds dry they are able to germinate over a wider temperature range.[26]
Combinational dormancy In seeds with combinational dormancy the seed or fruit coatis impermeable to water and the embryo has physiological dormancy. Depending on the
species physical dormancy can be broken before or after physiological dormancy isbroken.[25]
Secondary dormancy* is caused by conditions after the seed has been dispersed and
occurs in some seeds when non-dormant seed is exposed to conditions that are notfavorable to germination, very often high temperatures. The mechanisms of secondary
dormancy are not yet fully understood but might involve the loss of sensitivity in
receptors in the plasma membrane.[27]
The following types of seed dormancy do not involve seed dormancy strictly spoken as lack of
germination is prevented by the environment not by characteristics of the seed itself (see
Germination):
Photodormancy or light sensitivity affects germination of some seeds. These
photoblastic seeds need a period of darkness or light to germinate. In species with thinseed coats, light may be able to penetrate into the dormant embryo. The presence of light
or the absence of light may trigger the germination process, inhibiting germination in
some seeds buried too deeply or in others not buried in the soil.
Thermodormancy is seed sensitivity to heat or cold. Some seeds including cocklebur
and amaranth germinate only at high temperatures (30C or 86F) many plants that have
seed that germinate in early to mid summer have thermodormancy and germinate only
when the soil temperature is warm. Other seeds need cool soils to germinate, while otherslike celery are inhibited when soil temperatures are too warm. Often thermodormancy
requirements disappear as the seed ages or dries.
Not all seeds undergo a period of dormancy. Seeds of some mangroves are viviparous, they
begin to germinate while still attached to the parent. The large, heavy root allows the seed to
penetrate into the ground when it falls. Many garden plants have seeds that will germinatereadily as soon as they have water and are warm enough, though their wild ancestors may have
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had dormancy, these cultivated plants lack seed dormancy. After many generations of selective
pressure by plant breeders and gardeners dormancy has been selected out.
For annuals, seeds are a way for the species to survive dry or cold seasons. Ephemeral plants are
usually annuals that can go from seed to seed in as few as six weeks.[28]
[edit] Seed persistence and seed banks
Further information: Seed hibernation
[edit] Seed germination
Germinating sunflower seedlings.
Main articles: Seedling and Germination
Seed germination is a process by which a seed embryo develops into a seedling. It involves thereactivation of the metabolic pathways that lead to growth and the emergence of the radicle or
seed root and plumule or shoot. The emergence of the seedling above the soil surface is the next
phase of the plant's growth and is called seedling establishment.[29]
Three fundamental conditions must exist before germination can occur. (1) The embryo must bealive, called seed viability. (2) Any dormancy requirements that prevent germination must be
overcome. (3) The proper environmental conditions must exist for germination.
Seed viability is the ability of the embryo to germinate and is affected by a number of different
conditions. Some plants do not produce seeds that have functional complete embryos or the seed
may have no embryo at all, often called empty seeds. Predators and pathogens can damage or killthe seed while it is still in the fruit or after it is dispersed. Environmental conditions like floodingor heat can kill the seed before or during germination. The age of the seed affects its health and
germination ability: since the seed has a living embryo, over time cells die and cannot be
replaced. Some seeds can live for a long time before germination, while others can only survivefor a short period after dispersal before they die.
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Seed vigor is a measure of the quality of seed, and involves the viability of the seed, the
germination percentage, germination rate and the strength of the seedlings produced.[30]
The germination percentage is simply the proportion of seeds that germinate from all seeds
subject to the right conditions for growth. The germination rate is the length of time it takes for
the seeds to germinate. Germination percentages and rates are affected by seed viability,dormancy and environmental effects that impact on the seed and seedling. In agriculture and
horticulture quality seeds have high viability, measured by germination percentage plus the rateof germination. This is given as a percent of germination over a certain amount of time, 90%
germination in 20 days, for example. 'Dormancy' is covered above; many plants produce seeds
with varying degrees of dormancy, and different seeds from the same fruit can have differentdegrees of dormancy.[31] It's possible to have seeds with no dormancy if they are dispersed right
away and do not dry (if the seeds dry they go into physiological dormancy). There is great
variation amongst plants and a dormant seed is still a viable seed even though the germination
rate might be very low.
Environmental conditions effecting seed germination include; water, oxygen, temperature andlight.
Three distinct phases of seed germination occur: water imbibition; lag phase; and radicle
emergence.
In order for the seed coat to split, the embryo must imbibe (soak up water), which causes it toswell, splitting the seed coat. However, the nature of the seed coat determines how rapidly water
can penetrate and subsequently initiate germination. The rate of imbibition is dependent on the
permeability of the seed coat, amount of water in the environment and the area of contact the
seed has to the source of water. For some seeds, imbibing too much water too quickly can kill the
seed. For some seeds, once water is imbibed the germination process cannot be stopped, anddrying then becomes fatal. Other seeds can imbibe and lose water a few times without causing ill
effects, but drying can cause secondary dormancy.
[edit] Inducing germination
A number of different strategies are used by gardeners and horticulturists to break seeddormancy.
Scarification which allows water and gases to penetrate into the seed, include methods that
physically break the hard seed coats or soften them by chemicals. Means of scarification include
soaking in hot water or poking holes in the seed with a pin or rubbing them on sandpaper orcracking with a press or hammer. Soaking the seeds in solvents or acids is also effective formany seeds. Sometimes fruits are harvested while the seeds are still immature and the seed coat
is not fully developed and sown right away before the seed coat become impermeable. Under
natural conditions seed coats are worn down by rodents chewing on the seed, the seeds rubbingagainst rocks (seeds are moved by the wind or water currents), by undergoing freezing and
thawing of surface water, or passing through an animal's digestive tract. In the latter case, the
seed coat protects the seed from digestion, while often weakening the seed coat such that the
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embryo is ready to sprout when it gets deposited (along with a bit of fertilizer) far from the
parent plant. Microorganisms are often effective in breaking down hard seed coats and aresometimes used by people as a treatment, the seeds are stored in a moist warm sandy medium for
several months under non-sterile conditions.
Stratification also called moist-chilling is a method to break down physiological dormancy andinvolves the addition of moisture to the seeds so they imbibe water and then the seeds are subject
to a period of moist chilling to after-ripen the embryo. Sowing outside in late summer and falland allowing to overwinter outside under cool conditions is an effective way to stratify seeds,
some seeds respond more favorably to periods of oscillating temperatures which are part of the
natural environment.
Leaching or the soaking in water removes chemical inhibitors in some seeds that prevent
germination. Rain and melting snow naturally accomplish this task. For seeds planted in gardens,
running water is best - if soaked in a container, 12 to 24 hours of soaking is sufficient. Soakinglonger, especially in stagnant water that is not changed, can result in oxygen starvation and seed
death. Seeds with hard seed coats can be soaked in hot water to break open the impermeable celllayers that prevent water intake.
Other methods used to assist in the germination of seeds that have dormancy include prechilling,
predrying, daily alternation of temperature, light exposure, potassium nitrate, the use of plantgrowth regulators like gibberellins, cytokinins, ethylene, thiourea, sodium hypochlorite plus
others.[32]
Some seeds germinate best after a fire, for some seeds fire cracks hard seed coats while
in other seeds chemical dormancy is broken in reaction to the presence of smoke, liquid smoke isoften used by gardeners to assist in the germination of these species.[33]
[edit] Origin and evolution
The origin of seed plants is a problem that still remains unsolved. However, more and more data
tends to place this origin in the middle Devonian. The description in 2004 of the proto-seed Runcaria heinzelinii in the Givetian of Belgium is an indication of that ancient origin of seed-plants. As with modern ferns, most land plants before this time reproduced by sending spores
into the air, that would land and become whole new plants.
The first "true" seeds are described from the upper Devonian, which is probably the theater of
their true first evolutionary radiation. The seed plants progressively became one of the major
elements of nearly all ecosystems.
[edit] Economic importance
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A variety of bean seeds.
[edit] Edible seeds
Further information: List of edible seeds
Many seeds are edible and the majority of human calories comes from seeds[citation needed ]
,
especially from cereals, legumes and nuts. Seeds also provide most cooking oils, many beverages and spices and some important food additives. In different seeds the seed embryo or the
endosperm dominates and provides most of the nutrients. The storage proteins of the embryo andendosperm differ in their amino acid content and physical properties. For example the gluten of
wheat, important in providing the elastic property to bread dough is strictly an endosperm
protein.
Seeds are used to propagate many crops such as cereals, legumes, forest trees, turfgrasses and
pasture grasses. Particularly in developing countries, a major constraint faced is the inadequacyof the marketing channels to get the seed to poor farmers.[34] Thus the use of farmer-retained
seed remains quite common.
Seeds are also eaten by animals, and are fed to livestock . Many seeds are used as birdseed.
[edit] Poison and food safety
While some seeds are edible, others are harmful, poisonous or deadly.[35]
Plants and seeds often
contain chemical compounds to discourage herbivores and seed predators. In some cases, thesecompounds simply taste bad (such as in mustard), but other compounds are toxic or break down
into toxic compounds within the digestive system. Children, being smaller than adults, are more
susceptible to poisoning by plants and seeds.[36]
A deadly poison, ricin, comes from seeds of the castor bean. Reported lethal doses are anywherefrom two to eight seeds,
[37][38] though only a few deaths have been reported when castor beans
have been ingested by animals.[39]
In addition, seeds containing amygdalin — apple, apricot, bitter almond,[40]
peach, plum, cherry,
quince, and others — when consumed in sufficient amounts, may cause Cyanide poisoning.[40][41]
Other seeds that contain poisons include annona, cotton, custard apple, datura, uncooked durian,
golden chain, horse-chestnut, larkspur, locoweed, lychee, nectarine, rambutan, rosary pea, sour
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sop, sugar apple, wisteria, and yew.[42][43]
The seeds of the strychnine tree are also poisonous,
containing the poison strychnine.
The seeds of many legumes, including the common bean (Phaseolus vulgaris), contain proteins
called lectins which can cause gastric distress if the beans are eaten without cooking. The
common bean and many others, including the soybean, also contain trypsin inhibitors whichinterfere with the action of the digestive enzyme trypsin. Normal cooking processes degrade
lectins and trypsin inhibitors to harmless forms.[44]
Please see the category plant toxins for further relevant articles.
[edit] Other uses
Flax seed oil (in bottles) and coconut oil (in jars in the middle).
Cotton fiber grows attached to cotton plant seeds. Other seed fibers are from kapok andmilkweed.
Many important nonfood oils are extracted from seeds. Linseed oil is used in paints. Oil from jojoba and crambe are similar to whale oil.
Seeds are the source of some medicines including castor oil, tea tree oil and the discredited
cancer drug, Laetrile.
Many seeds have been used as beads in necklaces and rosaries including Job's tears, Chinaberry,
rosary pea, and castor bean. However, the latter three are also poisonous.
Other seed uses include:
Seeds once used as weights for balances.
Seeds used as toys by children, such as for the game Conkers.
Resin from Clusia rosea seeds used to caulk boats.
Nematicide from milkweed seeds.
