Tissue Types & Tissue System
The tissues, i.e., complex of cells are of two types:
It is a tissue composed of immature, not well differentiated cells which has the capacity of division.
The term Meristem is derived from a Greek word Meritos meaning Divisible. It emphasizes the cell division activity characteristic of the tissue which bears this name. During early stages of development, all cells of an embryo divides but as the growth proceeds the cell division becomes restricted to special parts of the plants. In these parts the tissue remains embryonic in character and cell retain the ability to divide. These tissues, when present in the mature plant body, are called Meristems. The cell division can also take place in tissues other then meristems, e.g., in cortex of stem and vascular tissue of the young, but the number of divisions is limited while cells of the meristems add new cells continuously to the plant body.
The meristems are described as Formative derivatives – Tissues, i.e., they add new cells to the plant body and at the same time perpetuates themselves (increase their own number) as meristems. In an active meristem, some products of cell division remain meristematic (able to divide) and are called Initiating Cells whereas other develop into various tissue elements and are known as the derivatives of the initiating cells. The tissues which undergo differentiation lose the embryonic characteristics and are known as Mature or Permanent Tissues. The permanent cells with nuclei, have the ability to grow and divide and re-differentiate, e.g., the cells of cortex, phloem, etc. Forming the cork cambium (phellogen).
Meristematic cells are usually thin-walled, isodiametric, spherical, oval or polygonal in shape and are rich in protoplasm. Generally, the protoplast is at protoplastid stage. Usually, the vacuoles are small and are scattered throughout the protoplast. However, some cells of the apical meristems of pteridophytes and spermatophytes have conspicuous vacuoles. Also, the cells of vascular cambium are highly vacuolated.
Classification of Meristem:
The meristems are classified on the basis of their origin, plane of division, functions and their position in the plant body, into various categories. These are:
According to the position of meristem in the plant body, the meristem can be classified into following three types:
Apical Meristem: The meristems which are found in the apices of the main and lateral shoots and roots.
Intercalary Meristem: The meristems which are found between the mature tissues, e.g., in the basis of the internodes of the grasses.
Lateral Meristem: The meristems situated parallel to the circumference of the organ in which they are found. The examples are the vascular cambium and the phellogen (cork cambium).
On the basis of the origin and development of the meristems, the meristems are classified as follow:
Those meristems which develop directly from the embryonic cell and therefore constitute a direct continuation of the embryo. The primary meristems are responsible for the development of fundamental body parts like epidermis, cortex and mesophyll and the primary vascular tissue.
Those meristems which develop from the cells which first differentiated and function as mature tissues and then resume meristematic activity, for example, phellogen and callus tissue. From the secondary meristems, the secondary tissue and the protective tissues develop.
In some monocots, such as Palms, Banana, etc., the stem is thickened at apices. The meristem which is responsible for this thickening is termed as Primary Thickening Meristems.
In primary meristems, different regions in various stages of differentiation can be recognized. These are:
In apical meristem, a zone consisting of apical initials and the cells derived from them, which are still close to the initials, is recognized and is termed as Pro-meristem. The pro meristem is present at root and shoot apices.
Below the pro-meristem, a meristematic zone is recognized on the basis of function, whose cells have undergone a certain degree of differentiation. The zone consists of three meristems:
Protoderm: It is the outermost layer of meristematic zone from which the epidermis system of plant develops.
Procambium: It is responsible for the development of the primary vascular system of the plant. It consists of narrow elongated cells. It occupies the center of meristematic zone.
Ground Meristem: It is composed of large, isodiametric, thin walled cells from which the ground tissue of the plant develops, i.e., cortex, pith, etc.
On the basis of growth patterns and planes of division, the meristematic tissues can be grouped as follow:
Mass Meristem: In this type of meristem the growth takes place by division in all planes which results in the formation of isodiametric, spherical or irregular bodies, e.g., growth resulting in the formation of endosperm and that which takes place in young embryos of some plants.
Rib Meristem: In this type of meristem. the cell division is at right angles to the longitudinal axis of the cell row and also to the longitudinal axis of the plant organ. This type of growth pattern result in the formation of parallel longitudinal files (ribs) of cells. This type of growth occurs in cylindrical parts of the plants, e.g., during development of cortex root and pith and the cortex of the stem.
Plate Meristem: In this type of meristem the cell divides by anticlinal divisions (divisions at right angles to the surface of the apex) with result that number of layers do not increase and a plate like structure is formed, e.g., growth shown by Hal blades of angiosperm leaves.
Apical Organization of Meristems:
In 1759 Wolff discovered that the new leaves and tissues of the stem arise in the apex of the stem. He termed this region the “Punctum Vegetationis”. Now-a-days the term Shoot Apex is generally used. The shoot apex is considered as terminal part of the shoot immediately above the uppermost leaf primordium. The size and shape of the shoot apex varies among the spermatophytes. It may be convex or in the form of a narrow cone with a rounded tip or slightly concave.
Before the initiation of each leaf the apical meristem widens and after the appearance of the leaf primordium it again becomes narrow. The period between the successive initiation of two leaves or two pairs of leaves is called a Plastochorn. The variation shown by the apical meristem are termed as plastochronic variations.
In case of angiosperms, the apices are usually small while in case of gymnosperms, the apices of the conifers are cone-shaped and narrow like those of angiosperms and apices in Ginkgo and Cycas are 3-8 times as wide as high.