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Tissue Types and Tissue Systems.pptx.ALLEROI0TP93T043T

Plant systematic

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Tissue Types and Tissue Systems A group of closely associated cells that perform related functions and are similar in structure. Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissue systems fall into one of two general types: meristematic tissue and permanent (or non-meristematic) tissue.
Meristematic Tissues Meristematic Tissues (meristos=divisible) It is a tissue composed of immature, not well differentiated cells which has the capacity of division. It emphasized the cell division activity. 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 the cells retain the ability to divide. These tissues when present in the mature plant body are called Meristems. The meristems are described as Formative Tissues i.e. they add new cells to the plant body and the same time perpetuates themselves as meristems.
In an active meristem, some products of cell division remain meristematic and are called initiating cells whereas others develop into various tissue elements and are known as the Derivatives of the initiating cells. The derivatives undergo morpho-physiological changes and this process is called Differentiation. The tissue which undergo differentiation lose the embryonic characteristics and are know as Mature or Permanent Tissues. Structure of Meristematic Tissues Meristematic cells are usually thin walled, isodiametric, spherical, oval or polygonal in shape and are rich in protoplasm. Generally, the protoplast of meristematic cells is without reserve materials and crystals and the plastids are at proplastid stage. The vacuoles are small and are scattered throughout the protoplast. The size of the meristematic cells vary. Thick walled meristematic cells are found in apical meristems.
Apical Cell Theory According to this theory the apical meristem consists of a single apical cell (also called apical initial) and this cell is interpreted as the ‘structural and functional unit of apical meristem’. The apical cell is tetrahedral in shape and has three or four cutting faces among which single face is directed upward and the rest points downward. The concept that single apical cell is responsible for the formation different parts of the plant body is know n as Apical Cell Theory.
Histogen Theory The apical cell theory was superceded by Histogen Theory suggested and developed by Hanstein after studying angiosperm shoot apices and embryos. According to this theory, the main plant body develops from a mass of meristem present deeply and consists of three parts or histogens. The outermost is Dermatogen (skin). This histone is responsible for the formation of epidermis. Inner to the dermatogen is Periblem (clothing). This histone gives rise to the cortex of root and shoot apex. The inner one is called Plerome (fills) and forms the entire inner mass of the axis, i.e. stele.
Tunica Carpus Theory Schmidt in 1924 postulated tunica- corpus theory on the basis of studies of shoot apices of angiosperm. This theory is concerned with planes of cell division in the apex. In contrast to apical cell theory and histogen theory tunica-corpus theory is applicable only to shoot apex and not to root. Schmidt distinguishes two tissue zones in the shoot apex and termed them as tunica and corpus. Majority of angiosperm shoot apex exhibits tunica consisting of two layers of cells and corpus. Researchers designate the layers as L1, L2 and L3 to denote respectively outer layer of tunica, inner layer of tunica and corpus. The two regions are usually distinguished by the planes of cell divisons in them. The plane of cell division in tunica is principally anticlinal which result in surface growth. In corpus, the planes of cell division are in all directions and as a result whole mass grows in volume.
Functions of Meristematic Tissues 1. Meristems are actively dividing tissues of the plant. 2. They are responsible for primary (elongation) and secondary (thickness) growth of the plant. 3. All new organs and their growth occur by the division of meristematic tissue. 4. Secondary tissues such as, wood, cork are also formed due to activity of meristematic tissue.