The tissue system that forms the skin or exterior of the plant body is called the epidermal tissue system. Different parts of cutaneous tissue system are-
1. Epidermis: The outer layer of leaves and stems of plants is called epidermis. The exterior of the root is called epibeoma. Skin is formed from the protoderm or dermatogen of the dividing tissue at the root and stem apex. Cutin and suberin accumulate in the epidermis to form a thick layer of cuticle. The leaves of plants like sugarcane, rice, bamboo, grass, reeds etc. have silica cells and cork cells, so the edges are sharp. Leaves of plants like banyan, saffron, fig, rubber etc. contain large cells called lithocysts and contain calcium carbonate called cystoliths. Myrosine cells are present in the leaves of mustard family plants (mustard, rye, radish). It secretes myrosin enzyme. Plants like wheat, wheat, sugarcane etc. have large cells called bulliforms in their leaves. Bulliform stores water. Flower petals and fruit skin contain pigments called anthocyanins. Outside the cell wall there is a thin coating of cutin, suberin, lignin, wax etc. The root cuticle is called the pilosebaceous layer because the root hairs arise from the root cuticle. The skin of bots, ostriches, rubbers, crabs, corbi and orchids is multi-layered.
Function of Epidermis
(i) Epidermis protects the inner plant cells from external injury and adversity.
(ii) The skin stops water loss. Cutin, silica and wax reduce the rate of skin absorption.
(iii) Poison glands protect against animal attacks.
(iv) Stomata present in leaves allow exchange of gaseous substances.
(v) Rhizomes absorb water and mineral salts.
(vi) Skin containing chloroplasts produces food.
(vii) Skin cells divide and heal wounds.
(viii) Bulliform cells store water.
(ix) It stores water and waste matter.
(x) The waxy layer of the skin resists fungal attack.
2. Trichom: The unicellular or multicellular appendages in the roots and stems of plants are called epidermal appendages or trichomes. Different types of trichomes in plants are-
(i) Hair: Hair follicles are unicellular and devoid of cuticle and chondromes are multicellular and have cuticles. It may be branched or unbranched or stellate or stellate (Althaea -leaves). Carpus The hairs formed from the cotton seed coat are fibrous in appearance and form the cotton fibers. Monocotyledonous stems do not have chondromes. Rhizomes absorb water and mineral salts from the soil. Leaves and stem hairs reduce sensation.
(ii) Colleters: Colleters are special types of multicellular trichomes. It is filled with sticky sticky substance. Digestive glands of insectivorous plants contain colleters. Punarnabha, Lemon, Rosa and Carya plants contain colaters.
(iii) Stigma: Unicellular, long, pointed and poisonous juice-filled trichomes are called Stigma. When the stinger comes in contact with the animal, the needle tip breaks and the poisonous juice is released. Skin burns due to toxic sap. Such as Tragia involucrata, Mucuna pruriens, Laportea aestuans, Agnichutra etc.
(iv) Scale: Special hair with thin coating is called scale. It is composed of sheets of flat, multicellular and disc-like cells. It reduces the rate of respiration.
Functions of Trichomes
(i) Glandular hairs present on stems and leaves reduce plant sensation and light intensity.
(ii) Glandular hairs protect the plant from insect attack.
(iii) The hairs of the pistil of the flower help in the pollination of the plant by receiving the pollen.
(iv) Hair present on fruit and seed skin helps in spreading.
(v) Glandular rhombs of insectivorous plants secrete enzymes which aid in digestion of insect bodies.
(vi) Plants absorb water and mineral salts from soil by rhizomes.
(vii) Stem hairs protect the plant from external injury by secreting glue, mucilage and toxins.
3. Stomata: A pore or pore surrounded by two crescent-shaped guard cells located in the leaves and young green stems of plants is called stomata. The lower skin of the opposite-sided leaves and the two skins of the equilateral leaves have stomata. The aerial parts of plants such as leaves, shoots, petals, fruits etc. have stomata. Below each stomata is a large stomata or stomata. Guard cells contain chlorophyll and participate in photosynthesis. Different types of stomata are-
(i) Diacytic : It is surrounded by two subsidiary cells and the cells are at right angles to the two guard cells. Plants like Chirata, Kalsi etc. have diacytic stomata.
(ii) Paracytic : It is surrounded by two subsidiary cells and the cells lie parallel to the two guard cells. Dye plants have paracytic stomata.
(iii) Anisocytic : It is surrounded by three subsidiary cells and one of them is small. Plants like mustard, tobacco, brinjal etc. have anisocytic stomata.
(iv) Tratacytic: It is surrounded by four subsidiary cells.
(v) Actinocytic: It is surrounded by many elongated cells. Plants in the family Ebenaceae have actinocytic stomata.
(vi) Anomocytic : The cells surrounding the stomata are associated with normal cells. Plants such as Nayantara and Cucurbita have anomocytic stomata.
(vii) Gramineous : The stomata are covered by two guard cells and two accessory cells. Plants like paddy, wheat, sugarcane, bhutra, grass etc. have gramineous stomata.
Anatomy of stoma
Leaves may have 1,000-60,000 pores per square centimeter. When open, stomata are 10–40 µm in length and 3–12 µm in width. Different parts of stomata are-
1. Stomata pore: There is a special type of pore in the center of the stomata. This is called stomatal pore. Different types of gaseous substances are exchanged through it.
2. Guard cells: Stomata are covered by two crescent-shaped cells. It is called Rakshikosh. Each guard cell contains cytoplasm, chloroplasts, mitochondria, cytoplasm and a large nucleus. The guard cell has two walls. Outer wall and inner wall. The inner wall is thick and inelastic. But the outer wall is thin, semipermeable and elastic. Guard cells are covered by 2-4 epidermal cells. It controls the opening and closing of stomata.
3. Supporting cells: There are some special types of parenchyma cells surrounding the stoma. These are called accessory cells. Each stomata with accessory cells is called a stomatal complex. Some plants do not have accessory cells. Like- Pumpkin, Cucumber, Orchid etc.
4. Air chamber : There is a large air-filled cavity below the stomata. It is called substomatal cavity. This chamber is also known as air cavity or respiratory cavity.
The leaf pores are hidden deep in the skin. This is called hidden or submerged stomata. The stomata are open during the day and closed at night. However, the stomata of Patharkuchi plants are closed during the day and open during the night. The plant’s stomata are fully open from 10-11 am and 1-3 pm.
Function of stoma
(i) Photosynthesis: The biochemical process by which green plants produce sugary food by chemically combining water and carbon dioxide with the help of chlorophyll in the presence of sunlight and releasing oxygen as a by-product is called photosynthesis. Carbon dioxide needed for photosynthesis enters the cell through the stomata. Carbon dioxide reacts with water to form sugars and release oxygen as a byproduct. Oxygen is released out of the cell through the stomata. Therefore, stomata play an important role in carrying out the process of photosynthesis.
(ii) Respiration : The biochemical process in which respired matter is completely oxidized in the presence of free oxygen to produce water, CO2 and energy is called respiration. Oxygen required for plant growth enters the cells through the stomata. Oxygen reacts with oxygen to produce water, CO2 and energy. The carbon dioxide produced is released out of the cell through the stomata. Therefore, stomata play an important role in carrying out the reproductive process.
(iii) Transpiration: The process by which the excess water absorbed by the roots of the plant evaporates through the stomata is called transpiration. Green plants use only 1-2% of the water absorbed by their roots. The remaining 98-99% of water is useless. Excess water is released by evaporation through the stomata. Therefore, stomata play an important role in carrying out the process of respiration.
(iv) Temperature regulation: A lot of heat is generated as a result of various biological processes in the plant body. During respiration, heat is released through the stomata. If the heat wasn’t released outside, the plant would burn to death.
(v) Gaseous exchange: It involves the exchange of gaseous substances or O2 and CO2.
(vi) Opening and closing of stomata : Guard cells control the opening and closing of stomata.
(vii) Synthesis and storage of organic food: Glucose is synthesized in the guard cells and temporarily stored as glucose.
Role of stomata in food production
(i) In the process of photosynthesis, the raw material for making food, carbon dioxide, enters the cell through the stomata. Sugars are produced from carbon dioxide. Plants with more leaves have more stomata. Due to this, the supply of carbon dioxide increases and food production increases.
(ii) Water is an important raw material in the process of photosynthesis. In the transpiration process, excess water is released through the stomata in the evapotranspiration. As a result water flow is created and water enters different cells. This water is used in the process of photosynthesis. (iii) After making food in the process of photosynthesis, oxygen released as a by-product escapes through the stomata. Oxygen is an essential element in photosynthesis. It reduces the rate of photosynthesis. Food production is accelerated when oxygen escapes through the stomata.
(iv) Each stomata contains two guard cells. The guard cell contains a large number of chloroplasts. This chloroplast produces food in the process of photosynthesis.
(v) Green accessory cells surround the stomata. Chloroplasts of accessory cells take part in the process of photosynthesis to make food.
4. Hydathodes: The pores located along the edges of plant leaves through which water is released in liquid form are called hydathodes. The release of water in liquid form is known as gation or water excretion or water drinking. Hydathodes are more active at night than during the day. Hydathodes are located along the edges of plant leaves. That’s why in the morning water is seen on the edges of the leaves. Hydathodes are present in plants like grasses, squash, tomato, saffron, strawberry, primula, rose, balsam, Ranunculus fluitans, Campanula rotundifolia. Hydathods named Esau in 1965.
There are two types of water pore. Passive and active hydathodes. Passive hydathodes emit water passively and active hydathodes emit water actively.
The cells that make up the hydathode are called epithems. Aqueducts are made up of a bunch of living cells. Cells are watery and have intercellular spaces. The cells do not have chloroplasts. Each hydathode has one or more pores. Its pore is structurally similar to a leaf pore. But somewhat large in size and unable to move. It consists of one or more chambers. The chambers open into windpipe or trachea.
Importance of hydathode
(i) Water passes through the hydathode in liquid form.
(ii) There is a water chamber below it. Water accumulates there.
(iii) Hydathodes are more active at night than during the day.
(iv) Hydathodes cause water droplets to appear on leaf edges in the morning.
(v) Salts are released during water evaporation.
(vi) Water excretion is high when perspiration is low.
5. Water sac: Water accumulates in the skin cells of ice plants and takes the form of a sac. In winter this water turns into ice. For example, Mesembrayanthemum crystallinum.