Dr. T logo BIO 410/510 Plant Anatomy

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

•  Root (Ranunculus)

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

•  Stem (Helianthus)

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

•  Leaf (Ligustrum)

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

Leaf epidermis with stomata
Epidermal peel showing epidermal cells and stomata [Micrograph by John Tiftickjian]


•  Protection from desiccation

•  Protection from disease organisms

•  Protection from insect damage

•  Gas exchange

•  Secretion

•  Absorption of water and mineral nutrients (in roots)


•  Epidermis differentiates from the protoderm

•  Because protoderm cells usually do not divide periclinally, the epidermis is only one cell thick.

•  In a few plants, the protoderm cells do divide periclinally, increasing the number of cell layers.

•  This produces an epidermis several cells thick called multiple epidermis

•  Cells of multiple epidermis often function in water storage.

•  Multiple epidermis in the leaf of Ficus.

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

•  Multiple epidermis (velamen) in aerial roots of some orchids

Orchid velamen
Cross section of orchid root showing multiple epidermis (velamen) [Micrograph by John Tiftickjian]

Cuticle and waxes

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

•  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 (Phormium)

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

•  Mesic and aquatic plants have thinner cuticles

Cell types in the epidermis

•  Ordinary epidermal cells

•  These make up the bulk of the epidermis

•  Usually tabular in basic shape

•  Tabular means that the thickness of the cell is less than the width and length.

•  A book has a tabular shape

•  The shape of the cells in surface view varies among species.

•  Rectangular shape is common

•  Also common are undulating, resembling pieces of a jigsaw puzzle

Leaf epidermis, surface view
Epidermal peel showing undulate epidermal cells as seen with differential interference microscope. [Micrograph by John Tiftickjian]

•  No intercellular spaces between cells (except for stomata)

•  Usually lack chloroplasts (present in some species)

•  Guard cells (in pairs) form stomata

•  Subsidiary cells can be associated with stomata in some species

•  Trichomes - any appendages of the epidermis

•  Other specialized cells my be present in some species.


•  A stoma is a pore (intercellular space) surrounded by two guard cells

•  Terminology of stomata

•  Stoma is singular, stomata is plural

•  Technically, the stoma is just the pore

•  The pore + guard cells = stomatal complex

•  Many people use the term stoma to refer to both pore and guard cells

•  The stomatal complex may include additional subsidiary cells

•  Alternate terminology (you see this sometimes)

•  Stomate (same as stoma)

•  Stomates (same as stomata)

•  If you want to be picky, stomate is actually the condition of having stomata.

•  Guard cells

•  In nearly all plants, guard cells are kidney shaped.

•  The concave side of each cell faces the other guard cell.

•  The cells become more curved when the stoma opens.

•  In grasses and sedges, guard cells are dumbbell shaped.

•  Structure of guard cells seen in surface view

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

•  Guard cells have chloroplasts, unlike ordinary epidermal cells

Chloroplasts in guard cells
Surface view of stoma showing chloroplasts in guard cells, differential interference microscope [Micrograph by John Tiftickjian]

•  Position of stomata with respect to epidermal plane

•  Stomata may be at the same level as rest of epidermis

•  Stomata may be below the epidermal plane - sunken stomata

•  Sunken stoma is a water-saving feature, common in xerophytes

•  Sunken stomata in the grass Poa

Poa leaf, x.s.
Cross section of Poa leaf. Bundle sheath and bundle sheath extension are composed of fibers. A stoma is visible in the lower epidermis. [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

•  Microfibrils are oriented radially from the center of the pore.

•  Microfibrils are resistant to stretching, but can slide apart.

•  This causes the shape of the guard cells to change when cell turgor changes.

•  Stomata open when guard cells take up water and expand

Open and closed stomata
Surface view of leaf epidermis showing closed (left) and open (right) stomata [Micrograph by John Tiftickjian]

•  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.

•  Dicot with anomocytic stomatal complexes (SEM)
Dicot leaf epidermis (SEM)
Dicot leaf surface with anomocytic stomatal complexes (SEM) [Micrograph by John Tiftickjian]

•  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.

•  Monocot with parasitic stomatal complexes (SEM)
Monocot leaf epidermis (SEM)
Monocot leaf epidermis (SEM) [Micrograph by John Tiftickjian]
•  Higher magnification of one stomatal complex. Note also wax deposits on epidermal cells.
Monocot stoma (SEM)
Monocot stoma with paracytic arrangement of subsidiary cells (SEM) [Micrograph by John Tiftickjian]

•  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 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)

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

•  Scanning electron microscope (SEM) is a great tool for studying trichomes. This image shows unicellular and multicellular hairs and short glandular trichomes.

Dicot leaf epidermis (SEM)
Dicot leaf surface with trichomes on vein (SEM) [Micrograph by John Tiftickjian]

•  Root hairs

•  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.

•  Root hairs on radish seedling

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

•  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 non-glandular hairs that are diagnostic. This has provided evidence in drug-related court cases.

Other specialized cells in the grass family

•  Bulliform cells

•  Large cells in small groups on one side (adaxial) of the leaf

•  These cells shrink when water is in short supply. As they lose turgor, the leaf rolls or folds so that less surface area is exposed and transpiration is reduced.

•  Bulliform cells in Zea (corn) leaf

Zea leaf, x.s.
Zea leaf xs. Note prominent bundle sheaths (Kranz anatomy), stomata, and bulliform epidermal cells [Micrograph by John Tiftickjian]

•  Silica cells and cork cells

•  Often occur in pairs dispersed among ordinary epidermal cells

•  Their functions are unclear

•  Silica is also found in the epidermal cells of Equisetum (horsetails). This gives their stems their characteristic rough texture.


•  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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