Dr. T logo BIO 410/510 Plant Anatomy
Dermal tissue: Epidermis


The epidermis is the surface tissue covering all organs of the plant during primary growth

•  Root

Ranunculus root, x.s.
Cross section of Ranunculus root
Micrograph by Biodisc

•  Stem

Helianthus stem, x.s.
Cross section of Helianthus (sunflower) stem with major tissues labeled
Micrograph by John Tiftickjian

•  Leaf

Ligustrum leaf, x.s.
Cross section of a typical dicot leaf. Note upper and lower epidermis, palisade and spongy mesophyll, veins, stomata.
Micrograph by John Tiftickjian

•  Epidermis in surface view

Epidermis surface view
Epidermal peel of Lily epidermis
Biodisc

Functions

•  Protection from desiccation

•  Protection from disease organisms

•  Protection from insect damage

•  Gas exchange

•  Secretion

Origin

•  Epidermis differentiates from the protoderm

•  Usually one cell thick unless there are periclinal division of protoderm cells. Then you have multiple epidermis.

Lithocyst
Ficus leaf, x.s. showing lithocyst. Note how the crystal (the cystolith) hangs from a peg attached to the top of the lithocyst.
Micrograph by John Tiftickjian

Cuticle and waxes

•  In shoots, the outer epidermal cells are covered with the cuticle.

Phormium leaf, x.s.
Cross section of Phormium leaf. Note large areas of supporting fibers and thin-walled parenchyma cells that function in water storage.
Micrograph by John Tiftickjian

•  The cuticle contains the lipid cutin, which is largely impermeable to water (and gases).

•  The development of the cuticle is regarded as one of the important evolutionary steps in the origin of the land plants.

•  Some plants have additional wax layers

•  Cuticle thickness and presence of wax layers is partly a function of habitat.

•  Arid climate species tend to have thicker cuticles

•  Mesic and aquatic plants have thinner cuticles

Cell types

•  ordinary epidermal cells

•  guard cells (in pairs) form stomata

•  trichomes

Stomata

•  Stucture of guard cells

Stoma seen in face view
Stoma seen in epidermal peel (DIC)
Micrograph by John Tiftickjian

•  Mechanism of opening and closing

•  Radial micellation

Radial micellation
Drawing showing radial micellation and its effect on changing shape of guard cells.
Introduction to Plant Physiology, Copyright John Wiley and Sons

•  Stomata open when guard cells take up water and expand

Stoma mechanism
Illustration of how a stoma opens as guard cells take up water.
Brooker Biology textbook, Copyright McGraw-Hill companies

•  The stomatal complex

•  The pair of guard cells is often surrounded by special cells that differ in shape from the ordinary epidermal cells

•  These surrounding cells are termed subsidiary cells

•  The shape and placement of subsidiary cells result from the pattern division of protoderm cells leading up to the formation of the guard cells.

•  Stomatal complex type has taxonomic value. Many taxa (genus, family) are characterized by a particular type.

•  Common stomatal complex types

Stomatal complexes
Stomatal complex types. Types are defined by the presence and placement of subsidiary cells.

•  Anomocytic. No subsidiary cells. Cells surrounding guard cells are not different that other epidermal cells.

•  Anisocytic. Guard cells surrounded by three subsidiary cells of unequal size.

•  Paracytic. One subsidiary cell on each side of the guard cells oriented parallel to the stoma.

•  Diacytic. One subsidiary cell on each side of the guard cells oriented perpendicular to the stoma.

•  Actinocytic. Several subsidiary cells that radiate from the center of the stoma forming a ring.

Trichomes

•  Trichomes are appendages of the epidermis.

•  Most are hair-like are are commonly called plant hairs.

•  There is a large diversity of forms - examples:

Epidermal trichomes
Types of epidermal trichomes.

•  Can be unicellular or multicellular

•  Can be simple or branched

•  May have secretory function (glandular hairs)

•  Epidermal "peel" with trichomes

Pelargonium leaf epidermis
Pelargonium (geranium) leaf epidermis, w.m. showing stomata and two kinds of trichomes.
Micrograph by John Tiftickjian

•  Complex trichomes are sometimes called scales

•  Peltate scales on leaves of Elaeagnus (whole mount)

Elaeagnus leaf scales
Elaeagnus leaf scales, w.m.
Micrograph by John Tiftickjian

•  Cross section of Elaeagnus leaf showing scale

Elaeagnus leaf scales
Elaeagnus leaf, x.s., showing scales
Micrograph by John Tiftickjian

•  Root hairs

Root hairs
Root hairs on a radish seedling
Photo by John Tiftickjian

•  Root hairs are simple, usually unicellular trichomes found in large numbers near the tips of growing roots.

•  They greatly increase the surface area of the epidermis that is in contact with the soil solution.

•  They increase the efficiency with which the root can absorb water and mineral nutrients.

•  Trichomes are valuable taxonomically and can aid in plant identification.

•  Trichomes have even been used in forensic science.

•  For example, marijuana (Cannabis sativa) has distinctive glandular and nonglandular hairs that are diagnostic. This has provided evidence in drug-related court cases.

Periderm

•  In plants having secondary growth, the epidermis is temporary. It is replace by the periderm, the secondary dermal tissue.

•  We will cover the periderm later in connection with secondary growth

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