Epidermal Tissue System

 The internal tissues of the plants are protected by a well organised tissue system called the epidermal or integumentary tissue system. It forma an outermost covering of various organs of plants e.g., young stems, young roots, foliar structures, flower, fruit and seeds.

Epidermal tissue system is in direct contact with the external environment and has, therefore, to perform many functions, which are responsible for bringing about a number of structural variations in this tissue system. The chief functions performed by this tissue system include – protection, absorption, secretion, excretion, gaseous exchange, restriction of transpiration.
Epidermis (Greek, epi – upon, derma – skin) is the outermost layer of cells in the various plant organs. The epidermis of stem,l eaves and floral parts originates from the surface layer of the shoot apical meristem, whereas, in roots it originates from an independent set of initials or has a common origin with root cap and cortex. The epidermis of root differs from that of shoot in origin, function and structure and is designated as epiblema, rhizodermis or piliferous layer.
Epidermis persists as such in plant that do not undergo secondary growth, e.g., majority of monocots and herbaceous plants. In plants which undergo secondary growth, the epidermis is replaced by periderm.
Uniseriate and Multiseriate Epidermis:
In most of the angiosperms, the epidermis is composed of a single layer of cells- uniseriate epidermis. But in some (e.g., Members of Bignoniaceae, Moraceae and Piperaceae) it is made up of 2 or more layer of cells – multiseriate or multiple epidermis. In the leaves of Indian rubber plant  (Ficus elastic) epidermis is two layered. The multiple epidermis of Peperomia pereskiaefolia has as many as 13 to 14 layers.
Multiple epidermis is usually found in the leaves of xerophytic plants. In Begonia the leaf epidermis is more than two layers thick and the innermost layer functions as water storage tissue. Specialised cells called mucilage cells have been reported in the outermost layer of multiple epidermis of some leaves.
The multiple epidermis saves the underlying tissue from injurious effects of heat in plants growing in deserts and hot regions. It may also reduce the transpiration rate by preventing the underlying mesophyll cells from heating. The Velamen of the aerial roots is also regarded as a multiple epidermis meant especially for the function of absorption of moisture from the surrounding air.
Epidermis is a continuous layer except for certain small pores called stomata. Further, epidermal cells bears various types of outgrowths. The epidermis has several distinct types of components and can be studied under the following –
1. The epidermal cells
2. Stomata
3. Epidermal appendages or outgrowths.

Epidermal Cells:
The epidermal cells make up most of the epidermis and are compactly arranged. They are of varying shapes and sizes and form a continuous layer, which may be interrupted by stomata. In some case, e.g., petals of some flowers, the epidermal cells are separated by intercellular spaces, which are always covered by cuticle, i.e., there is no pore.
The epidermal cells are mostly tubular in shape and appear flattened and rectangular in a cross-section. In some cases they are isodiametric. In the stems of Calotropis, Salvadora epidermial cells are elongated. The outer walls of the epidermal cells may be flat or convex or may show localised protuberances. The inner walls may be flat or convex.
The epidermal cells are parenchymatous and living with a large central vacuole and thin peripheral cytoplasm with distinct nucleus. The cells may contain leucoplast, anthocyanins or chromoplasts, but not chloroplasts except in guard cells of stomata.
In the epidermal cells of certain aquatic plants (e.g., Hydrilla) and shady plants the chloroplasts are also found. Sometimes, the substances like mucilage, tannins and calcium carbonate crystals (cystoliths) are also found in these cells.
The walls of epidermal cells are unevenly thickened. The outer tangential wall of the epidermal cells are usually thick due to the presence of cuticle, wax, resin etc., and the inner tangential wall is thin. The innermost layer of the outer walls of epidermal cells contains cellulose and layers external to this contain varying amounts of cutin.

Cuticle:
In addition to the true wall layers of epidermal cells, there is another layer lying external to the outermost walls of epidermal cells known as cuticle. This is a separate layer and is made up of a fatty substance called cutin.
It is a covering layer of all parts of shoot including stems, leaves, flowers, etc., the active growing regions of the root do not possess cuticle.
The thickness of the cuticle varies in different plants and even in different organs of the same plant. The cuticle has also been observed to extend upon varying depths in the stomatal opening and sometimes it lines the stomatal chamber, e.g., Musa and Ficus elastica.
The cuticle may be smooth or in some cases rough with ridges and nodules. In some plants, the cuticle may be coated with wax which may be deposited in the form of granules (grapes), rods (sugarcane), hooked projections. The leaves of Copernica cerifera (wax palm) are covered with wax called the cornauba wax, which is used in the manufacture of phonograph records and polishes. Oils and resins may also get deposited on the cuticle.
The cuticle is impervious to water and restricts transpiration. It also provides protection. It is also resistant to micro-organisms as well as decay.
Epidermal cells with special structure or contents:
I. In some members of the families Gramineae and Cyperaceae, the epidermis is composed of two kinds of  cells, the long cells and the short cells. The short cells are again of two types – cork cells – which have suberized cell walls and silica cells – cells filled with silica. The cork cells and silica cells generally occur in pairs.

II. Bulliform or Motor cells:
Many grasses have group of large epidermal cells called bulliform cells (bulliform – bubble like). These cells are larger in size than the adjacent epidermal cells and bulge slightly above the surface of epidermis. They are characterised by –
i. They contain sufficient water and are turgid
ii. They lack solid contents
iii. The cell walls are made up of cellulose and pectic substances.
Functions:-
1. These cells play an important role in unfolding the developing leaves which is brought about by their sudden expansion.
2. These cells bring about rolling of the leaves during dry season. They loose water in dry weather and contract, thus, bringing about rolling of the leaves as in Poa. By doing so, these cells help in reducing the rate of transpiration.

III. In some plants Silica, Calcium Oxalate or Calcium Carbonate are deposited in the cell wall or lumen of epidermal cells. Silicification is common in the leaves of Palms, whereas, Calcium Oxolate occurs in the leaves of many gymnosperms and angiosperms in the form of granules and crystals.
Crystals of Calcium Carbonate, known as Cystoliths accumulate in specialised cells, the lithocysts. They usually occur in multiseriate epidermis. Cystholiths occur frequently in Apocyanaceae, Acanthaceae, Moraceae and Utricaceae.
Lithocysts are usually larger than the adjacent epidermial cells. At maturity each lithocyst consists of an invaginated growth of outer wall in the form of a stalk. The inner end of this stalk is dilated and bears enormous deposition of Calcium Carbonate crytstals to form the cystolith

Stomata
Stomata (stoma – singular) are small openings in the epidermis of most aerial parts of plants and most abundant on leaves. They are absent in roots.
A stoma consists of an opening or pore bounded by two specialised, usually kidney shaped epidermal cells known as guard cells. The guard cells have unevenly thickened walls. The inner wall facing the pore is highly thickened, while the one away from the pore is thin and extensible.
In grasses, the guard cells are characteristically dumb-bell shaped i.e., narrow in middle and enlarged or bulbous at both ends. The central narrow part has a thick wall, the bulbous ends have thinner walls.
Due to the unevenly thick walls, the guard cells are helpful in the opening (turgid condition) and closing (flaccid condition) of the stomata.
The guard cells are covered with cutcle which extends to the inner wall forming the boundary of the pore. Mitochondria, dictyosomes, ribosomes and abundant endoplasmic reticulum are present in the guard cells. They also contain starch grains.
Unlike other epidermal cells, guard cells contain chloroplast which are supposed to have fewer and less well organized lamellae than those of mesophyll cells. Due to the presence of chloroplasts, active carbon assimilation takes place in guard cells. Consequently, increased turgor pressure after photosynthesis results in the opening of stomata, while  a decrease in turgor pressure results in their closing.
The guard cells are surrounded by a variable number of epidermal cells which are called subsidiary or accessory cells. These cells may be morphologically similar to the other epidermal cells or very different from them.
The two guard cells of a stoma, its aperture or opening and adjoining subsidiary cells together constitute the stomatal apparatus. A cavity is generally present beneath the stoma, known as sub-stomatal cavity which communicates with the intercellular spaces of the mesophyll.
Distribution of stomata:
On the basis of stomatal distribution, the following 3 categories of leaves are recongnised –
Amphistomatic – stomata present on both surfaces of leaf. Eg.,- most of the herbs.
In amphistomatic leaves, the lower surface usually has more stomata (multi stomatic) and the upper surface has comparatively less stomata (paucistomatic)
Hypostomatic – stomata confined to the lower surface of leaf. Eg., - most of the trees.
Epistomatic – stomata confined to the upper surface of leaf. Eg., - Nymphaea.
The stomata may occupy 3 different positions in relation to the epidermal cells –
1. They may be at the same level as the adjoining epidermal cells as in most of the mesophytic plants;
2. In xerophytic plants, as in Hakea, the stomata are sunken as they are located in a cup shaped drepression
3. Sometimes, as in Cucurbita, Solanum etc., the stomata re slightly raised above the surface of epidermis.
There is a great variation in stomatal frequency of leaves. In dicot leaves, their number varies from 1,000 – 10,000 per cm2.
The average size of stomata ranges between 6.7 and 17.7 microns.

Types of  Stomata:-
The first classification of stomata was given by Vesque (1889). He assumed that developmental pattern of stomatal apparatus shows absolute constancy in the majority of the families and thus are can easily define the autogeny by studying the structure of mature type.
The four stomatal types distinguished by Vesque were named after the families in which they were well observed for the first time – 1. Ranunculaceous types, 2. Cruciferous type,  3. Rubiaceous type and 4.Labiateous or Caryopohyllaceous type.
The terminology of Vesque was faulty since the types occur in many other families.
Later Metcalfe and Chalk (1950) proposed the following types of stomata:
1. Anomocytic (irregular – celled ) type: - in this type, the stomata are surrounded by a limited number of epidermal cells which are undistinguishable from other epidermal cells. Eg.,Ranunculaceae, Papaveraceae, Capparidaceae and Nyctaginaceae.
2. Animocytic (unequal – celled ) type :- this type is characterised by the presence of 3 subsidiary cells of which one is distinctly smaller than the other two. Eg., Cruciferae, Umbelliferae, Solanceae
3. Paracytic (parallel – celled ) type:- in this type, the stoma is surrounded on either side by two subsidiary cells which lie paralled to the long axis of the pore and guard cells. Eg.,Magnoliaceae, Rubiaceae.
4. Diacytic (cross – celled ) type : in this type, the stomata is surrounded by a pair of subsidiary cells whose common wall is at right angles to the long axis of the guard cells. Eg.,Caryophyllaceae, Labiatae, Acanthaceae.
In addition to these four types, two other are recognised by Stace (1965) :-
5. Actinocytic type:- with four or more subsidiary cells elongated radially to the stoma
6. Cyclocytic type:- with four or more subsidiary cells arranged in a close ring around the stoma.
According to Stebbins and Khush, stomata with many subsidiary cells are primitive.
Pant (1965) proposed a classification of stomata based on autogeny. He divided the stomata in the following 3 types –
1. Mesogenous:-  Guard cells and the subsidiary cells are derived by consecutive divisions of the same mother cell.
2. Perigynous :- Guard cells and subsidiary cells have an independent origin. Subsidiary cells develop from the cells lying around the mother cell of the guard cells.
3. Perimesogenous-  one of the subsidiary cells has a common origin with the guard cells whereas other subsidiary cells have an independent origin. Eg., members of Ranunculaceae, Caryophyllaceae.
The development of stomata in Gymnosperms as given by Florin is of 2 types:-
1. Haplocheilic type:- the subsidiary cells and guard cells arise from different origin i.e., they arise from different mother cells. Eg., Cycads, Conifers, Ginkgo
2. Syndetocheilic type:- the subsidiary cells and guard cells have a common origin i.e., they arise from same mother cell. Eg.,Gnetum, Welwitschia.

Functions of Stomata:
Regulate transpiration and gaseous exchange and therefore, they play an important role in the vital phenomenon of respiration and photosynthesis.

Epidermal Appendages or Outgrowths:
The outgrowth present on the epidermis are called epidermal appendages. They are of two types:-
1. Trichomes and 2. Root hairs.

Trichomes:-
All the unicellular and multicellular appendages of the epidermis are designated by the term trichomes.
They are highly variable in structure, form and function and occur on both vegetative and reproductive parts of the plant.
They may be living or dead and may persist throughout the life of a plant or short lived. They usually have thin cell walls of cellulose but sometimes have depositions of lignin, cutin and suberin.
Types of Trichomes:-
Uphof classified trichomes on the basis of morphological characters into two types – 1. Non-glandular trichomes and 2.Glandular trichomes.
I. Non- Glandular Trichomes:-

They are of two types:-
1. Unicellular Trichomes:- these are the elongation of the epidermal cells. They may be straight, curved, bifurcated (forming Y) or they become extended laterally to form T-shaped structures.
Unicerlluar trichomes of Cannabis are highly silicified and have warty prtouberances.

2. Multicellular Trichomes:-
They are made up of more than one cell. They may be uniseriate( with a single row of cells) or multiseriate ( with several rows of cells) and branched or unbranched.
A multiseriatetrichome is usually divided into an embedded foot and a projecting body.
If in uniseriatetrichomes, the terminal cell become rounded bladder like, then it is called as vesiculatetrichome, eg.,Chenopodium.
If terminal cell is capitate or sub-capitate, then it is called as capitatetrichome.
Multiacelluar trichomes may be complex branching structures – ramulose trichomes or Tufted where several arms of the body diverge in various directions at right angles to the surface of the organ.
In Gossypium, Sida, Solanum, several arms of the body spread out in a plane parallel to the surface of the organ giving a star-shaped appearance, it is called as stellate trichome.

In Olea and Humulus, the trichome has a multicellular disocoid head which is borne on a stalk or attached directly to the foot, it is called as peltate trichome.

II. Glandular Trichomes:
The unicellular or multicellular trichomes which secrete various substances are called as glandular trichomes. They are of various shapes. The simplest type of glandular trichomes are stigmatic papillae which are the extenstions of the protodermal cells.
The simple multicellular glandular trichomes consists of one or more celled stalk and a unicellular or multicellular head. A peltate glandular trichome has a discoid shield of variable number of glandular cells attached to a stalk.
Glandular trichomes are involved in the secretion of various substances and they are often called glands.
Salt-secreting trichomes:- they occur in subsidiaryhytes and help in removing the excess salt. They consists of a bladder-like secretory cell or cells at the end of a narrow stalk.
At maturity, the secretory cell dries out, the salt content remains on the leaf as white powdery layer. The salt glands are often in the form of many celled trichomes.
Ex- Avicennia has multicellular salt glands with a 2 celled foot, one celled stalk and a head of 3 or 4 salt-secretingcells. The nuclei are relatively large.
The head of the salt gland is covered with a thick cuticle which has pores at top of the gland. An amorphous substance of pectic nature appears between the secretory cells and the cuticle which is supposed to increase the apoplastic transport of salt solution.

Nectar-secreting trichomes:- in many plants nectar is secreted through unicellular or multicellular hairs (Malvaceae) which are especially rich in endoplasmic reticulum.

Mucilage or slime secreting glands:- mucilage or slime secreting trichomes occur in Rumex and Rheum. The secreted mucilage is mainly a polysaccharide and extrudes through the pores in the cutilcle or by its rupture.

Stinging Hairs:- the stinging haris of Urtica are highly specialised glandular trichomes. They consists of a single, long cell which has a broad, bladder-like base and narrow, needle-like upper part.
The broad base is surrounded by epidermal cells which are raised above the level of the other epidermal cells. The wall of the distal needle-like part of the secreting cell is impregnated with silica at the tip and with calcium somewhat lower.
The very tip is spherical and breaks off along a predetermined line, when the hair is touched. The broken tip resembles the tip of a syringe and thus easily penetrates the skin, into which the poisonous, irritating cell contents (histamine and acetylcholine) are injected.

Shaggy glands:- such as those of Viola have a multicellular central core of cells which is surrounded by a layer of palisade-like elongated secretory cells.

Colleters:-trichomes secreting sticky substances. These are glandular trichomes consisting usually of a multicellular head and a stalk. All external and often also the neighbouring epidermal cells have the ability to secrete.
The secreted sticky substance, frequently a mixture of terpenes and mucilage, reaches the gland surface usually by rapid rupture of the cuticle. Colleters appear mostly on bud scales of Rosa, Coffea, Syringa.

Functions:-
1. Foliar trichomes are helpful in reducing the transpiration and the intensity of illumination.
2. Hair present on stigma are helpful in pollination.
3. When present on seeds these are helpful in dispersal.
4. Secretory trichomes help in chemical defense.
5. Stinging hairs protect plants from animals.
Importance of Trichomes:
1. Taxonomic and phylogenetic importance:-trichomes are of characteristic form in many taxa and have been successfully used in taxonomy in the classification of genera and species in some families.
2. Commercial importance : the unicellular hairs of cotton seeds are the source of cotton. The unicellular hairs of seed coat of Bombaxceiba are also of commercial importance.

Root  Hair:
The root hairs arise just behind the zone of active cell division. They are always unicellular and unbranched and are direct lateral extensions of the epidermal cells of the root. Their walls are generally thin and made up of cellulose with an outer covering of calcium pectate.
The cytoplasm forms a thin lining with the nucleus embedded in it near the tip. There is single large central vacuole that is filled with absorbed water and mineral substances. The root hair is short-lived and dies after a few days or a few weeks.
The new root hair develops near the tip as the root grows and the old ones collapse. The new root hair do no arise among the older ones.
In some plants, the rhizodermal (epidermal) cells giving rise to root hairs are smaller and with dense cytoplasm. They are called trichoblasts or piliferous cells.
Functions:
1. Root hairs increase the absorbing surface of the root.
2. Root hair by means of their lateral extension come into contact with otherwise untapped sources of water.

Functions of Epidermal Tissue System:
1. Protects the underlying tissues.
2. Reduces the rate of transpiration due to the cuticularisation of the outer walls of the cells of this layer.
3. Helps in gaseous exchange due to the presence of stomata.
4. Storage of water and metabolic products. In some cases the epidermal cells contain chloroplasts and help in photosysthesis.
5. Trichomes of aerial parts are useful in the dispersal of seeds, pollination, defence etc. various glandular trichomes secrete various products.
6. The layer has also meristematic potentialities and resumes active cell division during the formation of phellogen and in the healing of wounds.
7. The root hairs in root epidermis serve the function of absorption of water and also increase the surface area of absorption of the root.