Lecture 7

 Epidermis

I.  General features

A.  Covers the primary tissues of the plant.  Occurs on all parts, derived from protoderm (image of apical meristem of Syringa).

B.  Functions are many including
1. cuticle on surface contains cutin and cutan, fatty materials that waterproof the plant thereby restricting evaporation (stem XS of Psilotum)
2.  stomata control gaseous exchange into and out of plant
3.  mechanical support
4.  light perception – affects photoperiodism and circadian rhythms
5.  in roots produce root hairs – important in water and nutrient absorption.

C.  Epidermis usually one cell layer thick (Figure), but in some plants a multiple epidermis forms (also called multiseriate epidermis). Formed from periclinal divisions of the protoderm (vs. anticlinal – review terms). Examples:
1. Pepperomia plants.  Leaf XS.
2. Ficus elastica.  Leaf XS.
3. Epidendrum (an orchid) adventitious roots are covered in velamen. XS of adventitious root.

D.  Epidermal cell divisions
1. In grasses, cell divisions asymmetrical producing a short and long cell.  The short cell is called the meristemoid.  It gives rise to stomata, silica cells, cork cells, trichomes, etc. (Figure). Once formed, a meristemoid may inhibit the formation of other meristemoids near it.
2. In roots, the cell that gives rise to a root hair is called the trichoblast (Figure).
3. Epidermal cells (even stomates) are totipotent, capable of dividing and regenerating an entire plant. 
4. Epidermis retains the potential for growth for long periods of time in some plants. For example, Acer pensylvanicum, 20 year old, 20 cm in diameter stems may still retain an epidermis.  In most woody plants (such as this Liquidambar twig), the epidermis is lost upon the formation of the periderm during the first year (photo of Acer 1 year old stem X.S.). The legume Cercidium (Parkinsonia) goes by the name palo verde, which literally means "green stick" or "greensward" owing to its photosynthetic branches.

E.  Composition. 
1.  Unspecialized cells.  Pavement cells in Arabidopsis look like interlocking puzzle pieces.  
2. Generally, epidermal cells are tabular in shape (Figure) but may be elongated parallel to the organ axis, as shown in this Zea lower epidermis.
3.  Stomata – specialized complex that regulates transpirational water loss and is the pore where CO2 enters the plant.  Composed of guard cells and sometimes subsidiary cells. Epidermis of Taxodium showing stomates.
3.  Trichomes - found in most plants (see below).
4.  Idioblastic substances such as tannins, oils, crysals may be present. In grasses (Poaceae), silica cells may be paired with cork cells, the latter with suberized walls
5.  The epidermis in seeds and scales may be composed of sclerenchyma fibers or sclerids.

F. Epidermal cell wall
1. Varies in thickness among different plants, different parts of same plant, and even different walls of one cell.
2.  Conifers often have very thick leaf epidermal cells; so thick that the cell lumen can be lost via lignification.
3.  Grasses: epidermal cell walls are impregnated with silica (silicified)
4.  Outer wall of epidermal cells has a cuticle composed of cutin (impregnation with cutin is called cutinization, whereas formation of the cuticle is called cuticularization).
5.  Cuticle is on all plant parts exposed to air (even roots and root hairs). Varies in thickness, thin (Nicotiana) and thick (Arctostaphylos, Yucca, Ficus)
6.  Structure of plant cuticle (Figure).  Starting at base:
a.  Plasma membrane
b.  Cell wall
c.  Pectinaceous layer
d.  Cuticular layer
e.  Cuticle proper
f.   Epicuticular wax
7.  The cuticle can be variously sculptured, as shown in Syringa, Solanum, Taxus,

II.  Stomata

A.  Basic structure and terminology.  Guard cells, subsidiary cells, aperture (pore), ledge, substomatal chamber.  The cuticle covers the guard cells and even extends into the substomatal chamber.

B. Location of stomata on the leaf
1. Hypostomatic - stomates restricted to the abaxial side, most common (~60%). Good example are gymnosperm leaves, like Tsuga (hemlock). The stomata are in rows along the midrib on the abaxial surface, protected by cuticular wax.
2. Amphistomatic - stomates on both the abaxial and adaxial sides (~30%)
3. Epistomatic - stomates are on the adaxial side, e.g. floating leaves such as Nymphaea.
4. No stomata, for example:
a. submerged leaves in aquatic plants
b. scale leaves in holoparasites in Balanophoraceae (e.g. this Sarcophyte)
c. stems of mycoheterotrophs such as Monotropa

C.  Stomatal structural differences in different taxonomic groups (Figure)
1.  Eudicots. Guard cells have reniform shape. Grasses and sedges.  Guard cells osteoform (bone-shaped). Figure
2.  Gymnosperms.  Reniform in shape (as in this Taxodium), but sunken into epidermis, as if suspended from subsidiary cell. Sciopitys.  Abies balsamea.

D. Position of stomates in relation to epidermis
1. Stomates the same level as epidermis, with a substomatal cavity (or chamber) directly below. These form zones of large intercellular spaces in virtually every leaf
2. Stomates sunken - guard cells sunken into the epidermis, common in xerophytes and especially conifers. Example: Ficus.
3. Stomatal crypts - depression in the epidermis where stomates are aggregated, these cut down on water loss, found in xerophytes such as Nerium (oleander).
4. Stomates are buried in deep folds in the leaf of the xerophytic beach grass Amophila arenaria. The photosynthetic mesophyll cells line these folds and buliform cells on the lower epidermis can cause the whole leaf to roll up under drought stress. A similar evolutionary adaptation is seen in Yucca.
5. Raised above the surface.  Populus.
6. Floating stomates (adetostomy). Present in fern Anemia phyllitidis, the guard cell pair is completely surrounded by one epidermal cell (see Mickel and Lersten 1967, AJB 54:1181-1185).

E.  Stomatal function (Figure)
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 openning and closing: heat, [CO2], abscisic acid. When turgid they are open, when flacid they are closed.

F. Formation of guard cells. Protoderm cell divides but unequally (Figure). Smaller one forms the guard cell. Subsidiary cells (if present) by come from the same or different mother cell as g.c. Developmental differences in stomates in relation to subsidiary cells come in three types:
1. Mesogenous (middle origin) - guard cells and subsidiary cells come from same mother cell (Figure).  Example: Graptopetalum (Aizoaceae) which shows a final pattern called amphianisocytic.
2. Perigenous (around origin) - guard cells and subsidiary cells come from different mother cells.  This developmental pattern is seen in Avena (oats) (Figure). Example: Pelargonium (Geraniaceae) which shows a final pattern called actinocytic. Also Dianthus
3.  Mesoperigenous - guard cells and only one subsidiary cell from same mother cell, other s.c. of different origin. Example: Vigna (which shows a final pattern called paracytic, see below).

G.  Stomatal development in monocots as described by Tomlinson (Figure). Guard cell precursor in contact with 4 cells.  One pattern, oblique divisions produce 4 cells surrounding guard cells. Another pattern, there are no oblique divisions.  This latter one comes in three types:
1.  g.c. surrounded by neighboring cells,
2.  2 neighboring cells and 2 derivatives of lateral cells
3.  2 derivatives of lateral cell, 2 derivatives of terminal cell

H. Stomatal complexes. Two guard cells and epidermal cells directly around them. The Metcalf and Chalk (1950) classification scheme for the first six of these types is shown HERE
1. anomocytic (irregular celled): no differentiation of the epidermal cells around the guard cells. Syringa, Gynura
2. anisocytic (unequal celled): 3 subsidiary cells around the guard cells, one of different size. Sedum kamtschaticum
3. paracytic (parallel celled): 1 or more subsidiary cells are parallel to guard cells. Magnolia, Phoradendron
4. diacytic (cross celled): 2 subsidiary cells with walls perpendicular to guard cells.  Dianthus
5. actinocytic (radiate celled): several subsidiary cells radiate from around the guard cells. Olea. Quercus
6. cyclocytic (cyclic celled): subsidiary cells in 1-2 rings around guard cells. Rhizophora
7. tetracytic (four celled): guard cells surrounded by 4 subsidiary cells. See Tomlinson Figure. Iris
some more unusual types:
8. amphianisocytic: double ring, inner ring of 3 subsidiary cells. Kalanchoe. Graptopetalum (Aizoaceae).
9. amphiparacytic: enclosed by 2 rings of 2 subsidiary cells aligned to guard cells. Normanbokea (Cactaceae).   

III. Trichomes

A. Originate from the epidermis.  Figure from Esau showing a wide variety of trichome types.

B. Not to be confused with structures like:
1. spines which are modified leaves or stipules. Example Acacia cornigera.
2. thorns which are modified branches. Example Gleditsia triacanthos
3. emergences such as prickles which originate from the epidermis but include tissue beneath in the cortex. Example Rosa.
4. warts (a bark feature). Example Celtis (hackberry)

C. Trichomes may function alive or dead - various kinds of trichomes are not homologous among plants that produce them, they are analogous
1. Living
a. digestive hairs, e.g. in insectivorous plants (Drosera)
b. often glandular and secrete compounds that are beneficial, e.g. nectar (Chpt. 13)
c. mucilage, wastes, protects against water loss and herbivory
d.  absorption, e.g. cells at base of the scales in Tilandsia usneoides (Spanish moss).
2. Dead
a. as a barrier to water loss and prevent animal grazing
b. aquatic plants for flotation, e.g. Pistia (Araceae) and Salvinia (a water fern).
c. protects against ionizing radiation, e.g. high altitude plants such as Espeletia: habit, lanate inflorescence.

D. Glandular Trichomes – we will cover later under secretory structures (Esau chapter 13)

E. Non-glandular trichomes
1. Simple
a. Unicellular - e.g. root hair, an extension of the epidermal cells
b. Uniserate - one cell layer thick, it is filamentous. Gynura aurantiaca, Nicotiana, Solanum lycopersicum (with glandular trichome)
c. Multiserate - filamentous, several cell layers thick. Salvinia.
2. Branched non-glandular trichomes
a. Unicellar.  Such as the trichomes of Arabidopsis (usually has three branches).
b. Multicellular
• Uniseriate, e.g. Digitalis, Asarum,
• Stellate - star shaped. Some have stalks, such as this Solanum quitoense. Some don't have a stalk, like Tilia and Hibiscus rosa-sinensis.
• Dendroid - tree-like, such as the candelabra trichome of Verbascum (mullein); Platanus occidentalis (sycamore)
• Multicellular and flattened - peltate (umbrella shaped), e.g. Olea europaea (olive), here in sectional view,  Elaeagnus (Russian olive)
• Audron (hook-tipped) hairs in Mentzelia.
• Farinose trichomes (mealy). Chenopodium. Flowers and fruits of C. fremontii showing farinose trichomes.
• Squamiform hairs on lower epidermis of Olea europaea (olive) prevent stomatal closure and damage of underlying tissues by UV radiation.
• Lithocysts that prodrude OUT of the leaf (not in) - Cannabis. SEM of hemp trichomes.

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Last updated: 10-Oct-22 / dln