A group of tissues carrying out a common function irrespective of their position is called a tissue system. There are 3 types of tissue systems in plants.
- Dermal /Epidermal tissue: They are protective tissues.
- Vascular tissue: Conduction of water, minerals and organic food materials.
- Ground tissue: All tissue except dermal and vascular tissues.
Dermal Tissue
- Dermal tissue (or epidermal tissue) forms the outermost protective covering of a plant body.
- It is derived from specialized cells called dermatogens.
- It is composed of epidermal cells, stomata and different epidermal outgrowths such as trichomes and hairs.
Epidermis:
- The epidermis is usually uniserate (single layered).
- It is the outermost covering of a plant body.
- The cells are parenchymatous with small amount of cytoplasm and a large nucleus.
- The epidermis is covered on the outer surface by a thick waxy coating called cuticle. It protects the plant from water loss, mechanical injury and pathogen attack.
- Cuticle is absent in the roots.
Stomata
- Stomata are special structures found on the epidermis of leaves.
- Gaseous exchange and transpiration takes place through the stomata.
- Each stoma is composed of two bean shaped cells called guard cells which enclose a stomatal pore.
- Certain epidermal cells around the guard cells modify their shape according to the guard cells and are called subsidiary cells.
- In grasses, the guard cells are dumbbell shaped.
- The outer walls of guard cells are thin while the inner walls are highly thickened. The guard cells contain chloroplasts.
- The opening and closing of stomatal pore is regulated by the guard cells.
- The stomatal pore, guard cells and the surrounding subsidiary cells are together known as the stomatal apparatus.
Epidermal outgrowths
- In some plants, unicellular or multicellular hairs are found on the epidermis.
- Some hairs are secretory in function (e.g. Nepenthes).
- Trichomes: Epidermal stem hairs are called trichomes. They are usually multicellular and prevent water loss by transpiration.
- Epidermis of root is called piliferous layer or epiblema. Unicellular outgrowths from this piliform layer are called root hairs. They help in absorbing water from soil.
