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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents The Tissues The Tissue System Anatomy of Dicotyledonous and Monocotylendous Plants Secondary Growth The study of internal structures of organisms is called anatomy. It includes the study of type of cells present in the body; whether eukaryotic or prokaryotic, approximate number of cells in the body, their organisation into tissues and in turn, the tissues are organised into organs, etc. The study of internal structure of plants helps us to classify them because one can easily find several similarities as well as differences in internal organisations. THE TISSUES A tissue can be defined as a group of structurally similar or dissimilar cells that perform a common function and have a common origin. Each tissue has a specific function to perform and different types of tissues of a plant body coordinate with one another to maintain the life of a plant. A plant is made up of different kinds of tissues. The plant tissues are broadly classified into two main groups, namely, meristematic and permanent tissues, based on whether the cells being formed are capable of dividing or not. The meristematic tissues are further of different kinds but the basic characteristic of all of them is that the cells constituting them persist the power of division. On the other hand, the cells constituting the permanent tissues become structurally and functionally specialised and temporarily or permanently lose the ability to divide. Hence, the basic criterion for classifying the plant tissues is the persistence or loss of ‘ability of division’. Meristematic Tissues 1. These tissues consist of cells that retain the power of division. 2. The protoplasm within the cell is dense, the vacuoles are smaller or absent. 3. These cells are isodiametric without intercellular spaces. 4. The nucleus is bigger in size 5. These cells have thin cellulosic cell wall. Chapter 6 Anatomy of Flowering Plants
194 Anatomy of Flowering Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 6. Metabolically active cells with high surface area per unit volume and nucleocytoplasmic ratio. 7. Ergastic substances are absent. 8. Colourless proplastids are present in cells. Classification of Meristems The meristems can be divided on the basis of 1. Origin or order of appearance in the life of a plant. 2. Their position in the body of a plant. 1. On the basis of origin in life of a plant : On this basis, the meristems can be distinguished into (a) Primary meristems : The meristematic cells that appear early in the life of a plant and contribute to the formation of primary plant body are called primary meristems. These cells are always in active state of division. They form primary permanent tissues. (b) Secondary meristems : The meristems that appear later than primary meristems in life of a plant and are responsible for producing the secondary tissues, particularly the woody areas, are called secondary meristems. 2. On the basis of position in the plant body : The meristems occupy different areas in the body of plants and depending upon this, they can be divided into : (a) Apical meristem : These tissues are found at the apices or tips of stem, root or branch, i.e., root apical meristem and shoot apical meristem. Central cylinder Cortex Protoderm Initial of cap root Root cap Root apical meristem Leaf primordium Shoot apical meristem Meristematic zone Axillary bud Differentiating vascular tissue A B Fig. : Apical meristem : A. Root, B. Shoot Function of apical meristem : The apical meristems produce the primary tissues of plant body and are responsible for the primary growth of the plants. They principally contribute to increase in the length, i.e., elongation of the plants along their axis. Formation of axillary bud : Plant bud is an under developed shoot. While the stem is elongating and leaves are forming on it, some cells ‘left behind’ from shoot apical meristem in the axil of leaves and constitute the axillary bud. Axil is the angle between the upper surface of a leaf stalk and the stem that bears it. These buds are capable of forming a branch or a flower. (b) Intercalary meristem : These meristems are intercalated in between the mature tissues, i.e., the permanent tissues. Hence, these meristems are separated from the apex of the organ by mature tissue. The activity of intercalary meristem also adds to the length of plant or its organs.
NEET Anatomy of Flowering Plants 195 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Fig. : A. Locations of meristematic tissues in L.S. of a shoot, B. T.S. of A at a-a Intercalary meristems are found in grasses where they help to regenerate the parts removed by the grazing herbivores. They help in elongation of organs and also allow fallen stems of cereals to become erect. Note : Both apical meristems and intercalary meristems are the primary meristems because they appear early in the life of a plant and contribute to the formation of primary plant body. (c) Lateral meristem : These meristems are present along the lateral sides of roots and shoots of many plants. They are found in the mature regions of roots and shoots. These do not occur in all plants. They occur only in those plants that produce the woody axis or show secondary growth. These are generally not present from the very beginning of life of a plant and appear later than the primary meristems, that is why they are called secondary meristems. The lateral meristems are the cylindrical meristems which divide in the radial direction resulting in the increase in girth of the stems and roots. Lateral meristems are responsible for producing the secondary tissues like secondary xylem, secondary phloem, secondary medullary rays, cork, secondary cortex, etc. The various examples of lateral meristems are: (i) Fascicular vascular cambium : It is the meristematic tissue which develops within the vascular bundles. It is also called intrafascicular vascular cambium. It is primary in origin. (ii) Interfascicular cambium : The meristematic tissue which develops between the vascular bundles is called interfascicular cambium. (iii) Cork cambium : This lateral meristem forms the cork, a tough protective material during the secondary growth. Hence, lateral meristems are secondary meristems on the basis of origin (except intrafascicular cambium) and lateral on the basis of location.
196 Anatomy of Flowering Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Permanent Tissues The cells of primary and secondary meristems undergo division and differentiation to produce primary and secondary permanent tissues of the plant body respectively, which become structurally and functionally specialised and lose the ability to divide, and become mature. These mature or permanent tissues perform specific functions in the body of plants. Classification of permanent tissues : Permanent tissues are basically of two types : 1. Simple permanent tissues : It is made up of only one type of cells. Here ‘one’ type of cells means that they are structurally and functionally ‘similar’ to one another. Various simple tissues are found in plants which make up the body of plants. These are : (i) Parenchyma (ii) Collenchyma (iii) Sclerenchyma (i) Parenchyma : (a) This tissue forms the major component within various organs of plants and is present in all of them, i.e., roots, stems, leaves, flowers, fruits and seeds. It forms the main bulk of the plant body. (b) The parenchymatous cells do not lose their protoplasm (the living contents of a cell) during their maturation hence, they are living cells. Fig. : Parenchyma (c) The cells of the parenchyma are generally isodiametric. ‘Iso’ means equal and ‘diametric’ refers to diameter. These cells may be spherical, oval, round, polygonal or elongated in shape (d) The parenchymatous cells are thin-walled. Their cell walls are made up of cellulose which is a major structural component of cell walls of plants. (e) Cells in the parenchyma tissue may either be closely packed with no intercellular spaces or have small intercellular spaces. Fig. : A. No intercellular spaces, B. Intercellular spaces present