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Bi 151

Plant Morpho-anatomy

Lecture 6

Epidermis and Periderm

Jan Lorie M. Robil, M.Sc.

Epidermis

• covers the primary plant body

• derived from protoderm

apical meristem of Syringa vulgaris

Epidermis

• Regular epidermal cells

• cuticle

• stomata (guard cells)

• trichomes (and emergences)

• other special epidermal cells

Epidermis

• waterproof the

plant thereby

restricting

evaporation

– due to cuticle on

surface which

contains cutin

and cutan

epidermis of Psilotum nudum

Epidermis

• control gaseous

exchange into

and out of plant

– via stomatal

apparatus

• pore (stoma)

• pair of guard

cells

epidermis of Psilotum nudum Epidermis

• produce root

hairs in roots

– for water

and nutrient

absorption

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Epidermis

• other important functions

– mechanical support

– light perception

• affects photoperiodism and circadian

rhythms

Epidermis

• usually one cell layer thick (uniseriate)

Transverse section

of stem epidermis of

cosmos (Cosmos)

Epidermis

• but in some plants a multiple epidermis

(multiseriate) forms via periclinal division

Piperaceae : Peperomia caperata

Multiple epidermis

Moraceae : Ficus elastica Orchidaceae : Epidendrum radicans

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Multiple epidermis of Epidendrum radicans root

Velamen of Orchidaceous roots

Hypodermis

• originated from cortical (ground) meristem

Transverse section of

leaf blade of oleander

(Nerium oleander)

Transverse sections of the leaves of three species of Pleiochiton and of Clidemia blepharodes. (9)

P. ebracteatum. (10) P. micranthum. (11) P. setulosum. (12) C. blepharodes. Scale bar 100mm.

Medinilla magnifica M. teysmannii Melastomataceae: Medinilla teysmanni

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Melastomataceae: Medinilla magnifica

Development of Epidermal Cells

• In grasses (Poaceae), cell division is asymmetrical producing a short and long cell.

• The short cell is called the meristemoid.

– It gives rise to guard cells and other associated cells (cork cells, silica cells, trichomes etc).

Development of Epidermal Cells

• a meristemoid may inhibit the formation of

other meristemoids near it

• the meristemoid produces the

guard mother cell.

Development of Epidermal Cells

• In roots, the cell

that gives rise to

a root hair is

called the

trichoblast

Development of Epidermal Cells

• Epidermal cells (even stomates)

are totipotent

• Epidermis retains the potential for growth

for long periods of time in some plants.

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Acer pensylvanicum 20 year

old, 20 cm in diameter stems

may still retain the original

epidermis Composition of Epidermis

• Regular epidermal cells

– aka Pavement cells

• Generally, epidermal

cells are tabular in

shape

Tradescantia upper leaf epidermis

Composition of Epidermis

• Stomata – specialized complex (pore +

pair of guard cells)

– regulates transpirational water loss

– the pore where CO2 enters the plant

• may be accompanied by distinct

neighboring or subsidiary cells

Composition of Epidermis

• Trichomes - found in most plants; variety of

functions

• Idioblastic substances

– e.g. tannins, oils, crystals – In grasses (Poaceae), silica cells may be paired with

cork cells, the latter with suberized walls

• The epidermis in seeds and scales may be

composed of sclerenchyma fibers or sclereids

Epidermal Cell Wall

• Varies in thickness

among different

plants, different parts

of same plant, and

even different walls of

one cell.

– Guard cells have

uneven cell wall

thickness

Epidermal Cell Wall

• Conifers often

have very thick

leaf epidermal

cells; so thick

that the cell

lumen can be

lost via

lignification.

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Epidermal Cell Wall

• Grasses:

epidermal cell

walls are

impregnated

with silica

(silicified)

phytoliths

Tiny silica “daggers” line

the edge of a blade of

grass.

Epidermal Cell Wall

• Outer wall of epidermal cells has a cuticle

chiefly composed of cutin and cutan

• cutinization = impregnation with cutin

• cuticularization = formation of the cuticle

Cuticle

• found on all plant parts exposed to air

(even roots and root hairs)

• also varies in thickness

Nicotiana Arctostaphylos

Yucca Ficus

Structure of Plant Cuticle

Starting at base:

• Plasma membrane

• Cell wall

• Pectinaceous layer (cont. middle lamella)

• Cuticular layer

• Cuticle proper

• Epicuticular wax

Structure of Plant Cuticle

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Structure of Cuticle

• The cuticle can be variously sculptured

Syringa

Solanum

Taxus

Cuticular ”horns”

Epicuticular Wax

Pisum Sorghum bicolor)

Epicuticular Wax

Development

of cuticle

Epicuticular Wax Development of epicuticular

wax filaments on the abaxial

surface of a sorghum

(Sorghum bicolor) leaf sheath.

A, wax fi laments emerging

from cork cells adjacent to

silica cells (sc). Initially the fi

laments appear as circular

secre-tions. B, with further

development, the secretions

appear as short cylinders. C,

D, with continued

development, the secretions

form clusters of epicuticular

wax fi laments.

Stomata

• Terminology

– guard cells

– subsidiary cells

– aperture (pore)

– ledge

– substomatal chamber

– epistomatal chamber

– stomatal crypt

ledges

stomatal

crypt

Stomata

• The cuticle

covers the

guard cells and

even extends

into the

substomatal

chamber.

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Location of stomata on the leaf

• Hypostomatic - stomates restricted to the

abaxial side

• Amphistomatic - stomates on both the abaxial

and adaxial sides

• Epistomatic - stomates are on the adaxial side,

e.g. floating leaves such as Nymphaea.

• No stomata

– submerged leaves in aquatic plants

– scale leaves in holoparasites in Balanophoraceae

Location of stomata on the leaf

Transverse section of water lily leaf (Nymphaea) showing

stomata on the upper epidermis

Shapes of Stomata

• usually reniform (eudicots)

• bone- or dumbbell-shaped in grasses

• sunken in gymnosperms (e.g. Pinus)

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Position in Relation to Epidermis

• Stomates the same

level as epidermis

– with a substomatal

cavity (or chamber)

directly below

– form zones of large

intercellular spaces in

virtually every leaf Canna

Position in Relation to Epidermis

• Stomates sunken

– guard cells sunken

into the epidermis

– common in

xerophytes and

especially conifers.

Ficus

Position in Relation to Epidermis

• Stomates within stomatal crypts

– depression in the epidermis where stomates are aggregated

– these cut down on water loss

– found in xerophytes such as Nerium

Nerium

Position in Relation to Epidermis

• Stomates are

buried in deep

folds in the leaf

of xerophytes

– as seen in Yucca

and beach grass

Amophila arenaria

Yucca

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Amophila arenaria

Position in Relation to Epidermis

• Stomates are raised

above the surface

Mechanisms of Stomatal

Functions 1. Wall thickenings. Most along pore wall

(ventral side), least on anticlinal wall (dorsal side)

2. Microfibrils in radial arrangement (radial micellation).

3. K+ fluxes and osmotic condition

4. Environment influences stomatal opening and closing: heat, [CO2], abscisic acid. When turgid they are open, when flacid they are closed.

Formation of Guard Cells

• Protoderm cell divides but unequally

• Smaller one forms the guard cell

• Subsidiary cells (if present) may come

from the same or different mother cell as

guard cells

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Formation of Guard Cells Types of Stomate Development

1. Mesogenous (middle origin) - guard cells and subsidiary cells come from same mother cell

2. Perigenous (around origin) - guard cells and subsidiary cells come from different mother cells.

3. Mesoperigenous - guard cells and only one subsidiary cell from same mother cell, other s.c. of different origin.

Mesogenous

Graptopetalum

Perigenous

Dianthus

Pelargonium

Mesoperigenous

Vigna

Types of Stomatal Complexes

1. anomocytic - (irregular celled): no

differentiation of the epidermal cells

around the guard cells.

2. anisocytic (unequal celled): 3 subsidiary

cells around the guard cells, one of

different size.

3. paracytic (parallel celled): 1 or more

subsidiary cells are parallel to guard cells.

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Types of Stomatal Complexes

4. diacytic (cross celled): 2 subsidiary cells

with walls perpendicular to guard cells.

5. actinocytic (radiate celled): several

subsidiary cells radiate from around the

guard cells.

6. cyclocytic (cyclic celled): subsidiary cells

in 1-2 rings around guard cells.

Types of Stomatal Complexes

7. tetracytic (four celled): guard cells

surrounded by 4 subsidiary cells.

8. amphianisocytic: double ring, inner ring

of 3 subsidiary cells.

9. amphiparacytic: enclosed by 2 rings of 2

subsidiary cells aligned to guard cells.

Types of Stomatal Complexes Trichomes

• Originate from the epidermis

Trichomes

• Not to be confused with structures like:

– spines which are modified leaves or stipules

– thorns which are modified branches

– prickles which originate from the epidermis but include tissue beneath in the cortex

– warts (a bark feature)

– and other emergences

• hairs, trichomes and emergences are collectively termed as indumentum

spines of Acacia thorns of Gleditsia

prickles of Rosa warts of Celtis

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Trichomes

• various kinds of trichomes are not

homologous among plants that produce

them, they are analogous

• may function alive or dead

• may be classified as non-glandular and

glandular (to be discussed in external secretory structures)

Functions

• Living

– digestive hairs, e.g. in insectivorous plants

– often glandular and secrete compounds that

are beneficial, e.g. nectar

– mucilage, wastes, protects against water loss

and herbivory

– absorption

carnivorous plant Drosera showing digestive hairs

Functions

• Dead

– as a barrier to water loss and prevent animal

grazing

– aquatic plants for flotation, e.g. Pistia

(Araceae)

– protects against ionizing radiation

trichomes of floating leaves of Pistia stratiotes

trichomes of aquatic fern, Salvinia

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high-altitude, xerophytic plant, Espeletia killipii hairy inflorescence of Espeletia killipii

Review types of

Non-glandular

trichomes

Other special epidermal cells

• Bulliform cells

– common to grasses (Poaceae)

– cause the leaves of many grass species to

fold inward during hot weather to reduce

transpiration

Zea mays

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