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Plant Structure, Growth, & Development
Campbell and ReeceChapter 35
Organization of Plants
• Plants like most multicellular organisms have organs made of tissues that are made of different cell types
3 Basic Plant Organs
1. Stems2. Roots3. Leaves
Roots
• Organ that 1. anchors a
vascular plant in the soil
2. absorbs water & minerals
3. stores carbohydrates
Taproot System
• In most eudicots & gymnosperms
• Taproot develops from embryonic root
• Forms lateral roots (branch roots)
• Penetrate deep• Eudicot: most flowering
plants that have 2 embryonic seed leaves
• Gymnosperm: vascular plant that bears naked seeds
Roots in Monocots
• Grasses: no tap root• Roots called adventitious: grows
in unusual locations– Example: roots arising from stems
or leaves
Adventitious Root Systems
• Each small root forms its own lateral roots fibrous root system
Root Hairs
• Emerge near tips of roots• Increase surface area for
absorption of water and mineral ions (do not help anchor plant)
• are thin, tubular extensions of a root epidermal cell
Root Adaptations
• Prop Roots: support tall, top-heavy trees
• Pneumatophores: air roots, portion above water line allows them to get O2
Root Adaptations
• Buttress Roots: tallest trees in rain forest have shallow roots; buttress roots give support to trunks
• “Strangling” Aerial Roots: seeds of these trees (strangler fig) germinate in branches of host tree
Stems
• Plant organs that 1. raise or separate leaves
allowing them to receive more sunlight
2. Raise reproductive structures facilitating dispersal of seeds or pollen
Parts of a Stem
• Each stem has alternating system of:1. Nodes– Pts @ which leaves are attached
2. Internodes – Stem segments between nodes
3. Axillary Bud– Upper angle (axil) formed by each leaf
& stem– Structure that can form a lateral shoot
(branch)
Parts of a Stem
4.Apical Bud– Part of shoot tip– The terminal bud (where most of
growth occurs)
5. Apical Dominance– Inhibits growth @ axillary buds– If eaten by herbivore or if light
more intense @ side of a shoot axillary buds break apical dominance & grow
Why Pruning Makes a Plant Bushier
Parts of a Stem
Adaptations of Stems
1. Rhizomes: horizontal shoots that grow just below surface ;vertical shoots emerge from axillary buds
• Ex: Irises, Hops
Stem Adaptations
2.Bulbs: are vertical, underground shoots made mostly of enlarged bases of modified leaves that store food
• Ex: onion, tulips
Adaptations of Stems
3. Stolons: horizontal shoots that grow along surface; aka “runners”
• Enable plant to reproduce asexually: plantlets form @ nodes
• Ex: strawberries, some grasses
Adaptations of Stems
4. Tubers: enlarged ends of rhizomes or stolons specialized for storing food. “Eye” of potato is cluster of axillary buds that mark the nodes
Ex: potato, dahlias
Leaves
• In most plants leaf is main photosynthetic organ
• General Structure:– Blade– Petiole • not on grasses or most monocots
– Veins • Patterns differ monocots & eudicots
Structure of a Leaf
Types of Leaves
Leaf Adaptations
• Tendrils: modified leaf used to support plant
• Ex: pea plants
• Some plants have tendrils that are modified stems (grapevines)
Leaf Adaptations
2. Spines: leaves adapted for protection
In cacti, stems are main photosynthetic organ
Leaf Adaptations
3. Storage Leaves: most succulents have leaves adapted to store water
Leaf Adaptations
4. Reproductive Leaves: leaves of some succulents produce adventitious plantlets which fall off & take root in soil
Ex: some succulents
Hens and Chicks
Leaf Adaptations
5. Bracts: modified leaves surrounding the real flower; function: attract pollinators
The yellow portion of poinsettia is the flower; the red leaves are bracts
Tissue Systems
• Are functional units connecting all of the plants organs
Dermal Tissue System
• Plant’s outer protective covering:
• Epidermis: nonwoody plants: tightly packed cells
• Cuticle: waxy covering on epidermal surface prevents water loss
• Periderm: in woody plants: replaces in older regions of stems & roots
Vascular Tissue System
Carries out long-distance transport of materials between the root & shoot systems
1. Xylem– H2O & dissolved
materials roots shoots
2. Phloem– Sugars roots &
sites of growth
Ground Tissue System
• Tissue that isn’t dermal or vascular tissue
• Pith: Internal to vascular tissue
• Cortex: external to vascular tissue
Plant Cells
• Cell differentiation involving changes in:– Cell walls– Cytoplasm–Organelles
Parenchymal Cells
• Mature cells have thin, slightly flexible cell walls (only 1)& large central vacuole
• Functions:– Perform most of metabolic
functions of plant• Chloroplasts• Plastids: store starch, found in roots• Make up most of fleshy part of fruits• Most able to divide & differentiate into
other cell types
Collenchyma Cells
• Come grouped in strands just below epidermis
• Support young parts of plant shoot w/out interfering with growth
• Elongated cells with thicker cell walls (compared to parenchymal cells) which can be irregularly thickened
• Remain living cells thru out plant life
Sclerenchymal Cells
• Supportive role but more rigid than collenchymal cells in regions of plant that have stopped growing
• 2º cell walls thick, contain lignin (>1/4 dry mass of wood)Lignin in all vascular plants, not in bryophytes
• Many dead at plant maturity: rigid cell walls support plant
Sclerenchymal Cells
• 2 types: (both for support & strength)
1. Sclereids– Boxy, irregularly shaped cells– Thick lignified 2º cell walls– Hardness in nutshells/grittiness in pear
2. Fibers – Grouped in strands– Long, slender, tapered– Hemp fibers for rope/flax fibers for linen
Water-Conducting Cells of Xylem
• 2 types: both elongated, tubular cells/dead at plant maturity: form tubular conduit for water flow; have pits thru which water can move laterally
1. Tracheids– In nearly all vascular plants
2. Vessel Elements– In some vascular/most
angiosperms/few gymnosperms
Sugar-Conducting Cells of the Phloem
• 4 types all alive @ plant maturity1. Sieve Cells: in seedless vascular plants &
gymnosperms2. Sieve Tubes: chains of cells/ enucleated,
no ribosomes, vacuole, or cytoskeleton sugars can diffuse thru cell more easily
3. Sieve Plates: pores for flow of sap fluid cell-to-cell
4. Companion Cells: nonconducting cells connected to sieve tube cells by plasmodesmata/their ribosomes & nucleus serve both cells
Growth in Plants
Stem Growth
Root Growth
Cross-Section of a Leaf
Anatomy of a Tree Trunk
• bark includes all tissues external to the vascular cambian (2º xylem, wood, and phloem)
• Sapwood = “living wood” has lighter color than heart wood (center) which is made of dead cells
Development of a Plant• Definitions:• Development: specific series of
changes by which cells form tissues, organs, & organisms
• Growth: irreversible increase in size
More Definitions
• Morphogenesis:cellular & tissue-based processes by which an organism takes shape, depends on cells responding to positional information from neighboring cells
• Differentiation : process by which a cell or group of cells become specialized in structure & function
Plant Cell Division
• Preprophase band made of microtubules develops in late interphase determines where cell plate will form
Asymmetrical Cell Division
• not all plant cells divided equally duting M Phase of cell cycle
• When occurs: usually signals a key event in development
• Example:– Epidermal cell divides • 1 large epidermal cell• 1 small guard cell
Cell Polarity
• The condition of having structural or chemical differences at opposite ends of an organism
• Typical plant has axis with a shoot end & a root end– 1st division of fertilized plant ova
asymmetrical which initiates polarization of plant body into shoot & root
• Cell division enhances possibility of plant growth but it is cell elongation that is responsible for plant growth
Cell Elongation
Cell Elongation
• Controlled by microtubule orientation controls the orientation of cellulose microfibrils in cell wall
Cell Differentiation
• Arises from differential gene activation
• Enables cells w/in plant to assume different functions
• Way any particular cell differentiates depends on its position in developing plant
Pattern Formation• Is the development of specific
structures in specific locations• 2 hypothesis to explain1. Lineage-based mechanism– Daughter cells have instructions
from early cells in plant development
2. Position-based mechanism– Cell’s position in emerging organ
determines what kind of cell it will become
Pattern Formation
• Position-based hypothesis– By destroying cells during
development have shown that cell’s fate determined late in development & depends mostly on signals from neighboring cells
– Cell fate in animals mostly lineage-dependent involving transcription factors
Pattern Formation
• Hox genes–Homeotic genes that code for
transcription factors – Critical for proper # & placement of
embryonic structures (legs, antennae in Drosophila)
Pattern Formation
• Knotted-1 homologous to Hox gene found in maize
• Important in development of leaf morphology
Control of Cell Differentiation
• Depends on control of gene expression: which genes are transcribed protein
• But fate of a particular cell is determined by its final position in the developing organ, not by cell lineage– If undifferentiated cell is misplaced
it will differentiate into cell type appropriate to its position
Activation of Genes
• Depends on signals from neighboring cells
Phase Changes
• Cues from plant itself or from its environment cause plant to switch from 1 developmental stage to another: called Phase Changes
• Most obvious changes in leaf shape & size
Genetic Control of Flowering
• Flower formation involves phase change from vegetative growth reproductive growth
• transition triggered by:1. Environmental cues– Length of daylight2. Internal signals– Plant hormones
Genetic Control of Flowering
• Production of a flower by a shoot apical meristem stops the primary growth of the shoot
• Is ass’c with the switching on of floral meristem identity genes
• Meristem: plant tissue that remains embryonic as long as plant live, allowing for indeterminate growth
Meristem Identity Genes
• Code for transcription factors that regulate genes needed for conversion of the indeterminate vegetative meristems determinate floral meristems
Organ Identity Genes
• A plant homeotic gene that uses positional information to determine which emerging leaves develop into which type of floral organs
Organ Identity Genes
• Provide model system for studying pattern formation:– The development of a multicellular
organism’s spatial organization: arrangement of organs & tissues in their characteristic places in 3-D
Organ Identity Genes
• 3 classes identified by studying mutants with abnormal flowers
• ABC Hypothesis• A model of flower formation
identifying 3 classes of organ identity genes that direct formation of the 4 types of floral organs
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