A tissue may be defined as an aggregation of similar cells, governed by the same laws of growth and development and adapted for the performance of the same function.
These may be either plant tissues or animal tissues.
Plant tissues are put into two main groups:
I. Meristematic Tissues:
Meristematic tissues are found at the growing points and the cells forming them possess the power of dividing and forming new cells. According to their origin and development, Meristematic tissues may be of the following types:
It is the very foundation stage and the region of formation of new organs and tissues.
Primary meristems are composed of cells which are the direct descendants of embryonic cells. Primary meristems build up the primary body of plants, e.g., tissues located at the tips of stem, root and appendages.
Secondary meristems are derived from primary meristems which have the power of cell division, e.g., cork cambium.
According to the position in the plant body, meristems are divided into three groups:
(i) Apical Meristem:
Apical meristems occur at the apices of the systems’ roots and lateral of vascular plants. Growth in the length of the axis is entirely due to their activities.
(ii) Intercalary Meristem:
These tissues are formed between masses of permanent tissues. They are short-lived and soon become permanent. They are found in stems and leaf-sheaths of many monocotyledons, particularly in grasses.
(iii) Lateral Meristem:
These meristems occur laterally in the axis. They are composed of initials which divide periclinally. The growth in thickness is due to their activities, e.g., cork cambium.
II. Permanent Tissues:
They perform a variety of functions and are divided into the following groups:
(i) Thin Walled Parenchyma:
The cells are oval, spherical or polygonal in shape with a thin cell wall and made up of cellulose. The cells contain chloroplasts and are concerned with the manufacture of food and its storage. They occur in leaves and herbaceous stems.
Cells are oval or spherical with a cell wall which is thickened at the corners. It serves to give strength to the plant and to manufacture starch. It occurs in leaves and herbaceous stems.
(iii) Wood Parenchyma:
The cells are lignified because lignin is deposited in their cell walls. They are found in woody parts of a plant and give mechanical support.
These tissues consist of loosely arranged cells with large intercellular spaces where air is restored. The air space gives buoyancy to plants in addition to normal aeration.
The cells are spindle shaped with or without protoplasm. It is mechanical in function and also serves to transport water from the roots to other parts.
It is a mechanical tissue and consists of long cells known as fibres. The wall of the cell is thick and lignified. The cells are dead due to the absence of protoplasm. The cells are of two types:
(a) Sclerotic Cells:
These are oval, rounded or polygonal in shape. The walls are thick, lignified and have small pits. They occur in the soft parenchymatous tissue in the fleshy parts of certain fruits like pears. They are also called stone cells and are found in the shells of coconut and walnut.
(b) Sclerenchymatous Fibres:
Their walls are thickened and have small pits. They occur in bundles. The fibres are long and make excellent textile fibres as in hemp, jute and coconut.
3. Vascular Tissues
They are long spindle- shaped cells. Their walls are lignified. They serve to conduct water and give support to the plant.
(ii) Trachea or Wood Vessels or Xylem:
These are long tube-like bodies ideally suited for the conduction of water and solutes. Translocation of solutes becomes easier in trachea than in tracheids.
(iii) Sieve Tube Tissue or Phloem:
The sieve tubes are elongated, thin walled cells without the nuclei. Their main function is associated with the conduction of manufactured food material like proteins and carbohydrates from the leaves to the places of storage in the plants.
Associated with the sieve tubes are thin, elongated cells with prominent nuclei called the companion cell? Their functional association is evident from the fact that companion cells continue so long as the sieve tubes function and die when the tubes are disorganised.
(iv) Laticiferous Tissue:
This consists of elongated (i) latex cells or coenocytes and (ii) latex vessels which contain milky fluid called latex. The prosenchymatous cells with numerous nuclei are thus called coenocytes. The latex vessels are formed by the fusion of irregular cells. These are found in sunflower plant and poppy. The latex cells are found in Euphorbia (Chandibuti) and Calotropis.
4. Glandular Tissues:
The cells in this tissue secrete mucilage, resin, gum, tannin, etherial oils and nectar. They may be of the following types:
(i) Digestive Glands:
The insectivorous plants possess specific digestive glands which secrete enzymes, and thus, part of their nitrogen requirement is obtained from the bodies of the insects they catch. In Drosera (sundew), the gland is located at the tip of the tentacle; in Nepenthes (pitcher-plant), glands occur along the walls of the pitcher. In Dionoea, the glands are normally inactive, but contact with the insect body stimulates them.
These are special glands usually located on the floral parts and secrete sugary substances such as nectar or honey which attract the pollinating insects.
(iii) Resin Ducts and Oil Ducts:
Substances like resin, oil and gums are secreted through special glands known as ducts, e.g., resin duct in Pinus, oil ducts in Umbellifers and oil glands in citrus fruits like lemon.
These are specialized structures through which exudation of water takes place. So, they are also called water-stomata. They are commonly found in tomato and Garden Nasturtium.