Epidermis in Plants :

In this article we will discuss about the structure of epidermis in plants. This will also help you to draw the structure and diagram of epidermis in plants.

The outermost layer or layers of cell covering all plant organs are the epidermis. It is in direct contact with the environment and so it modifies itself to cope up with the natural surroundings.

It thus protects the inner tissues from any adverse natural calamities like high temperature, desiccation, mechanical injury, external infection etc. In some plants the epidermis may persist throughout the life, while in others it is replaced by periderm when the epidermis is sloughed off along with underlying tissues.

Origin:

The epidermis of all organs originates from the outermost layer of apical meristem. Haberlandt, Hanstein and Schmidt called this surface layer of meristem as protoderm, dermatogen and tunica respectively. In cryptogams epidermis originates from a single initial cell that also forms cortex and stele.

The epidermis of gymnospermous root originates, in association with root cap, from periblem. In dicotyledonous root, epidermis develops from initials of dermatogen, which are not distinct from those of root cap. The epidermis of monocotyledonous root owes its origin from the periblem along with the cortex.

Structure and Contents:

Usually the epidermis consists of one layer of cells. Several-layered epidermis, termed multiple epidermis, is found in the leaves of Ficus, Nerium and in the aerial roots of orchid. The initials of epidermis divide periclinally to form multiple epidermis. The multiple epidermis of orchid root has the special name —velamen.

The epidermis of aerial parts of a plant consists of living parenchyma cells whose shape, size and arrangement may differ. The epidermal cells are more or less tabular (=horizontally flattened) in cross sectional view. In leaves, the epidermal cell walls appear as sinuous in dicots and in monocots they appear as straight or sinuous in surface view. Usually the cells of epidermis are compactly set with none or few intercellular spaces (e.g. flower petals).

The epidermal cells are devoid of chloroplasts. The guard cells of stomata that are specialized epidermal cells contain chloroplastids. Other pigment like anthocyanin may occur in epidermal cells. In some plants silicon may be deposited in the epidermal cells cither in the lumen or wall. The wall of trichome may be silicified.

Silicon containing cell can be differentiated from the adjacent epidermal cells by its shape and size. This cell is solitary and may be either scattered over the leaf surface or situated over the veins in longitudinal rows. Silicon is deposited in the bracts of rice, in the marginal trichomes of oat, in the leaves of Cyperus, Avena etc.

In the internode of Avena sativa, the epidermal cells at the intercostal position form cork-silica cell pairs, i.e. cork and silica containing cells are in close contact with each other. In the leaf of Ficus, some of the epidermal cells contain crystals of calcium carbonate, known as cystolith. These cells are easily distinguishable from the other epidermal cells by their large size and these specialized epidermal cells containing cystolith are called lithocyst.

Usually the walls of epidermal cells are thin. Thick walled lignified epidermal cells occur in some gymnosperms. Cutin, a fatty substance, is very often deposited on the outer surface of the epidermal cell wall to form cuticle over which wax may also be deposited. The cuticle is resistant to decay and is well preserved in fossils.

The cuticle often preserves the characteristic features of the epidermal surfaces such as the types and distribution of hairs and stomata. Thus the fossil plants may be identified by cuticular studies. Palmer (1976) used the fossil grass cuticle instead of grass pollen, as a new palaeoecological tool to reconstruct the nature of past vegetation of East African lake sediments.

Now a days cuticular pattern is used in recognizing small fragments of plants, which are necessary in forensic medicine, animal nutrition, pharmacognosy etc. Cuticular pattern is also taxonomically useful to characterize genus and species. The cuticle is impervious to water but in grapes water diffuses out when it is transformed to sultana. It has protective function. Cutin is resistance to microorganisms and prevents the entry of the pathogen.

In many plants wax is deposited on the surface of the cuticle. This forms a powdery coating on various fruits, e.g. plum, grapes etc. and on leaves. This gives a glossy appearance to the surface of leaves and fruits (e.g. grapes). Wax is deposited either in the form of granules, rods or tubes, which form various specific patterns on the surface.

The deposition may also be in the form of projections and folds. Wax is also deposited on the inside of the pitcher of Nepenthes in the form of overlapping scales. The scales adhere to the feet of insects, which fall inside the pitcher. So the insects cannot climb out of the pitcher. The morphological form of the deposition of wax is typical for the species.

With the aid of scanning electron microscope the wax and cuticular pattern can be observed directly. The study of wax pattern on the epidermal surface is extremely useful in agricultural practices. Waxy epidermis is not wetted. So the effectiveness of fungicide and herbicide can be obtained by studying the extent of wax deposition. Wax obtained from the wax palm Copernicia cerifera is commercially used in making polishes and phonographs records.

Hairs:

Hairs are present all over the surfaces of plant organs namely — roots, stems, leaves, floral parts, seeds (e.g. Gossypium) and stamens (ex. Tradescantia).

Root hair:

They are present at a short distance behind the root tip of most monocots and dicots. Root hairs in some species are formed from distinct epidermal cells termed trichoblast.

Trichoblasts may be morphologically similar to other epidermal cells or they are distinguishable by their smaller size with dense cytoplasm. Trichoblast prolongs to form unicellular root hairs. The cell wall is generally thin and is covered by a thin layer of cuticle; mucilage may also occur on the surface (Fig. 12.1).

Apart from anchorage the other main function of root hairs is absorption. It has been found that the rate of absorption in the epidermal surface with and without root hairs is same. Moreover the short root hairs are more efficient in absorption than longer ones. So it appears that the longer hairs absorb the distantly situated water. The root hairs synthesize cellulose at their tips.

Hairs other than root hairs:

Hairs or trichomes are the outgrowths of epidermal cells. They are either unicellular or multicellular. Multicellular hairs may be composed of a single linear row of cells or several rows.

Trichomes, either unicellular or multicellular, are classified into glandular and non-glandular hairs. The former is secretory in function and the latter is the covering hair and does not secrete. It is believed that non-glandular trichomes are protective in function and may prevent undue water loss (Fig. 12.2).

The covering trichomes may have a star like appearance (stellate hair) or a miniature tree (dendroid hair, e.g. Verbascum). In Hamamelis the covering hairs occur in tufts. Hair like projections are present in may flower petals termed papillae. The glandular hairs consist of a stalk and a head that may be unicellular or multicellular. A cuticle like structure covers the head.

The secretory substances accumulate in the sac formed between the cuticle and the cell of head. Example: Oils, resins, camphor, peppermint (e.g. Mentha) etc. The leaves of Olea have scales, which are composed of a short stalk and head, consisting of discoid plate of cells.

Some of the epidermal cells of Mesembryanthemnm may enlarge where water accumulates. These specialized epidermal cells are termed as vesicles or bladders. Most trichomes have thin and cellulosic cell wall; lignified cell walls also occur (ex. seed coat of Strychnos nuxvoniica).

Trichomes with their different types may be of taxonomic significance. The types of trichome can identify the several species of Oleaceae and Rhododendron to some extent.

Bulliform cell:

It is a group of outer epidermal cells that can be easily distinguished from the typical epidermal cells by their fan like appearance in cross section and larger sizes. The median cell of bulliform cell is the tallest and the size of the other cells, present on the two sides, diminishes gradually. The cells are thin walled, hyaline and have large vacuole.

The cells contain much water and are devoid of chloroplastids. The cell walls are composed of cellulose and pectic substances; cutin occurs on the outermost wall that is covered by cuticle. They usually form isolated strips that are situated on parallel between the veins. They are present on the outer epidermis of the leaves of Poaceae and other mono­cotyledons except Helobiae (Fig. 12.3).

Opinion varies regarding the functions of bulliform cell, which are:

(i) The rolled leaf in bud unrolls with the help of bulliform cell.

(ii) The turgidity and flaccidity of bulliform cell due to water uptake and loss respectively cause the closing and opening of mature leaves.

(iii) These cells act as water reservoir.

(iv) These cells are often filled with silica and their outer walls become thick and cuticularized thus providing mechanical rigidity to leaves.

Stomata:

Stoma (pl. stomata, physiologists usually call stomate) that occurs predominantly on leaves and young stems can be defined as a pore enclosed by two specialized cells —the guard cells that move to open and close the pore, and thus control gaseous exchange during transpiration, respiration and photosynthesis.

In Greek the word stoma means mouth. ‘Link and de Candole in 1827 jointly claimed to be the first to have called the pores by that name’. Stoma is reported as early as lower Devonian period (about 390 million years ago) from the extinct genera Rlnjnia, Asteroxylon, Zosterophyllum and Drepattophycus.