Plant Tissue

Plant tissues fit into 3 main categories:

Dermal tissues: epidermis, trichomes (“hairs”), guard cells, root hairs

Ground tissues: parenchyma (with chloroplasts = chlorenchyma), sclerenchyma, collenchyma

Vascular tissues: xylem (dead at maturity: tracheids and vessel elements), phloem (living: sieve tube cells [no nuclei], companion cells [with nuclei])

Meristematic tissues – localized regions of cell division

1. Apical Meristems◦ Primary or Transitional Meristem Primary growth

Protoderm gives rise to epidermis Ground meristem gives rise to ground tissue Procambium gives rise to 1

o vascular tissue

2. Lateral Meristems◦ Vascular cambium 2

o vascular tissue

◦ Cork cambium or phellogen periderm

3. Intercalary Meristems (found in the nodes of grasses)

lateral branch

root

shoot

axillarybud

shootapex

lateralbranch

node

internode

root tip

root apicalmeristem

root cap

root hairs

shoot tip

shoot apicalmeristem

budprimordium

lateral roots

meristem - region of actively dividing cells 1) apical (shoot & root);

2) lateral (vascular & cork cambia)

Cell differentiation:1) Cell expansion (elongation)2) Cell maturation / specialization

Derivation from the Apical Meristem

Plant Body Organization11

Meristems are clumps of small cells with dense cytoplasm and large nuclei

They act as stem cells do in animals-One cell divides producing a differentiating cell and another that remains meristematic

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Dermal Tissues

dermal tissues cover the organs of a plant. They are analogous to skin.

epidermisoutermost layer of cellsusually one cell layer thickepidermal cells have a layer of cutin (cuticle)

resistant to bacteria and other disease organisms

Plant Epidermis

1. Outer layer covering primary plant body

2. Interface through which plant interacts with external environment

3. Replaced by cork or bark in plants with secondary growth

Function of Epidermis

1. Protection1. Sunlight2. Other organisms3. Abiotic damage

2. Regulates water movement1. Uptake and loss2. Shoot reduces water loss3. Root regulates absorption

3. Secretion

Cell Wall and its Coverings

1. Epidermal Cell Wall – varies in thickness

2. Cuticle – made of cutin and waxes

3. Cutin – high molecular weight lipid polyester material C16Hn or C18Hn or a combination of both

1. Arrangement 1 – cutin + cell wall2. Arrangement 2 – cutin pectin cell wall

4. Wax layer 1. epicuticular (on surface) 2. intercuticular (pockets inside cuticle)

Epicuticular Waxes

Epidermal Cells – Ordinary Cells

Ordinary Cells1. Dicots – sinuous cell wall

2. Monocots – “straight” cell walls

3. No intercellular spaces

4. Typically no chloroplasts (exceptions??)

5. May have leucoplasts (plastids that do not have any pigments)

Epidermal Cells – Guard Cells and Stomata

Stomata1. Openings in epidermis2. Occur on all aerial parts of plant3. Most abundant on leaves; typically absent in

roots4. Pore size regulated by guard cells

Guard Cells1. Found on all green plants and flower parts2. Regulate gas exchange (CO2 and H2O vapor)3. Special arrangement of microfibrils in cell wall

allows pore to open and close between guard cells

Epidermal Cells – Trichomes

1. Highly variable appendages of the epidermis

2. Living or dead, uni- or multicellular

3. Lots of forms: glandular, nonglandular, scales, papillae, absorbing hairs of roots…

4. Functions – highly variable including protection, excretion, reflection, water absorption, water shedding…

Epidermal cells of root showing formation of root hairs from single cells.

Cuticle Wax

Wax MyrtleMyrica

ceriferawaxes

obtained from boiling the leaves is used to make bayberry candles

Epidermis is made of many types of cells

1. Guard cells form stomata2. Trichomes

Outgrowths – ‘hairs’ Some leaf trichomes use to

keep animals away Some used to eliminate salt Some thought to increase

reflection to reduce stem/leaf temperature

Root hairs are a type of trichome, increase surface area to increase uptake of nutrients

3. Glands

Root Hairs are extensions of

epidermal cells

Dermal Tissues: Epidermis

Glands secrete substances that protect the plant

secrete nectardigestive glandsSundewstrigger hairs of a

Venus Flytrap

Prickles grow out of the epidermis

A Few Definitions

Periderm = secondary tissue replaces epidermis in roots and stems consists of phellem, phellogen, and phelloderm

Phellem corky tissues non-living suberized cells produced by the cork cambium (phellogen) to outside of

stem

Phellogen cork cambium produces cork to the outside produces phelloderm to the inside

Phelloderm parenchyma-like cells produced toward inside of stem by the cork cambium

(phellogen)

Dermal tissue: Periderm

How is the periderm like the layer of dead skin cells on human skin?Replaces epidermis on roots and stems of woody plants with age.

The periderm composed mainly of thick, waterproof cork cells.

Protects stems and root

Ground TissuesGround tissue functions

metabolism: conduct photosynthesis; regulate gas exchangestorage: photosynthate (sugar, starch), water, & proteins support: via cellulose and lignin

Ground tissuesParenchyma

undifferentiated, thin, living cells; extremely common ‘filler;’ storage; photosynthesis; tissue healing

Ground tissuesCollenchyma

unevenly thickened, living cells; sub-epidermal FLEXIBLE support

Ground tissuesSclerenchyma

thick & lignified; non-living cells at maturityRIGID support; conduction

Ground tissue: Parenchyma

What can you see in these parenchyma cells?oThin-walled cellsoAlive at maturityoMany functions, including photosynthesis, starch storage, hormone production.

Ground Tissue: Collenchyma

How might support cells be different from other plant cells?

Plants don’t have a skeletal system as humans do. What holds a plant up?

What features do you see in these cells?

Flexible support tissue.Elongated cells with irregular shapes and

unevenly thickened walls.Living at maturity.

Ground tissue: Sclerenchyma

How are these thick-walled cells different from collenchyma cells?

What features do you see in these cells?Sclerenchyma cells are dead at maturity.

Why might that be?

Support tissue.Elongated cells with thick cell walls.Dead at maturity.Forms long fibers, or smaller

sclerids (such as stone cells in pears).

Ground Tissue

Parenchyma: Gen. metabol.1) Isodiametric to elongate

2) Primary cell wall 3) Living

Collenchyma: support

1) Elongate2) Primary cell wall thick, uneven, rich in pectins

3) Living

Ground Tissue

Sclerenchyma1) Secondary cell wall (+ primary)

2) Dead at maturity (usually)

Fibers

Elongate, sharply tapering

Ground Tissue

Sclerenchyma1) Secondary cell wall (+ primary)

2) Dead at maturity (usually)

Sclereids Isodiametric to irregular

VASCULAR TISSUE XYLEM and PHLOEM

1. Structurally and functionally complex tissue

2. Continuous through plant as veins

3. Typically associated with each other

4. Functions1. Water movement (Tracheids and Vessel members)2. Storage (parenchyma)3. Support (fibers and sclereids)

Vascular Tissue

XylemWater & mineral conductionTracheary elements + parenchyma + sclerenchyma

PhloemSugar conductionSieve members + parenchyma + sclerenchyma

- Both complex tissues

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Looking at the Vascular Bundle

Xylem and phloem found together in “collateral bundle”

In stem: Phloem toward epidermis Xylem toward center

In Leaf: Phloem toward bottom (abaxial) Xylem toward top (adaxial)

1 = apical meristem, 2 = procambium, 3 = protoderm, 4 = ground meristem, 5 = primary phloem, 6 = primary xylem, 7 = leaf trace, 8 = pith ray, 9 = pith, 10 = epidermis

Vascular tissue: Xylem

Xylem tissue moves water. What features would the cells need to carry out this function?

What features do you see in these cells?

Long, tube-like cells, joined end-to end, that transport water and minerals from soil to leaves.

Two types of cells: tracheids (in conifers) and vessel elements (in flowering plants).

Vascular tissue: Phloem

Phloem tissue moves sap, which is sugar dissolved in water. What features would the cells need to carry out this function?

What features do you see in these cells?

XYLEM in the Primary Plant Body

Found in Higher plants (tracheophytes)

Transport water and nutrients by bulk flow

Several types of cells and structures• Tracheary elements (conduct water)• Xylary Fibers (support)• Parenchyma (storage)• Perforation plate (perforated part of a cell wall usually

found at the end) Protoxylem (still elongating) and Metaxylem (elongation

complete)

Tracheary Elements: Tracheids

Found in both gymnosperms and angiospermsDead at maturitySo-so water movementSmall diameter with angular end wallsNO perforation plates in end wallsWater moves through pairs of pits (holes) in

adjacent cell walls

Tracheary Elements: Vessel Members

Found only in angiospermsDead at maturityGood water movementLarge diameter and nearly circular in cross

sectionVariety of perforation plate types in end wallsWater moves through pairs of pits (holes) in

adjacent cell walls

1 = annular thickening; 2 - 4 = helical thickenings; 5 = reticulate thickening; 6 = scalariform pitting; 7 = opposite pitting; 8 = alternate pitting

Pitting in Vessel Members

simple to more advanced

Phloem

Function is carbohydrate and nutrient transport Always from source to sink Living tissue with short functional life (days to

one season) Very fragile 1o cell wall Velocity of movement 10 – 100 cm/hr Component of phloem tissue

◦ Sieve elements (sieve cells and sieve tube elements◦ Parenchyma◦ Companion cells◦ Fibers◦ sclereids

1 = companion cells;

2 = sieve tube content;

3 = parenchyma cell;

4 = sieve plate (oblique view);

5 = one sieve element;

6 = sieve plate (side view);

Sieve tube and companion cells in the phloem of squash

SIEVE ELEMENTS

No functional nucleus

Few organelles, no vacuole when mature

Two types1. Sieve Cells (gymnosperms and lower vascular

plants only)

2. Sieve Tube Members (flowering plants)

SIEVE ELEMENTSSieve Cells

Lower vascular plants and gymnosperms

Long and gradually taperingEnd wall poorly definedModerate efficiency for moving sapSide walls covered with primary pit

fields

SIEVE ELEMENTSSieve Tube Members

Flowering plants onlyBroad cell end wall – covered by sieve

plateMore than one stacked on top of each

other = Sieve tubeTwo types of sieve plates

Simple

Compound

1 = inclined simple sieve plate; 2 = phloem parenchyma cells, 3 = companion cell; 4 = sieve tube members

“Helper” Cells

1. help load sieve elements with photosynthates in source area and unload in sink areas.

2. Ribosomes provide sieve elements with essential proteins and ATP.

3. Companion cells and sieve elements together constitute a functional unit in food conductance.

4. Two Types1. Albuminous cells in Gymnosperms with sieve

cells 2. Companion cells in Angiosperms with sieve

tube members

Albuminous Cells

Only in gymnosperms

Specialized type of parenchyma

Very small in comparison to sieve cell

The number associated with a sieve cell varies

Tracheary Elements

Tracheids - Imperforate Vessels - Perforate

Angiosperms (most)

Gnetales

A few Monilophytes

Sieve ElementsSieve cells - No sieve plates Sieve tube members -

Sieve plates

Apomorphy of Angiosperms

Vascular cambium - a lateral meristem