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The Glocal University 14/03/2020 PLANT ANATOMY GBBT/GBMB-202

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The Glocal University 14/03/2020 PLANT ANATOMY GBBT/GBMB-202 The Glocal University 14/03/2020 PLANT ANATOMY GBBT/GBMB-202 Plant Tissue Lower plants are composed of either single cells or colonies of cells which are more or less similar in size, shape and origin. Higher plants are built up of diverse types of cells, varied in shape, size, origin and function. A group of cells more less alike in size and shape, having same origin, same methods of development and same function are called tissue. Study of tissue is called histology. But, sometime distinctly dissimilar cells are often found to be associated in the formation of a tissue, as for example the complex tissue, xylem and phloem. Tissue may be classified on the basis of different characters, like, nature of cells, origin and methods of development, position in the plant body, etc. However, the tissues are put into two main groups on the basis of development: (i) Meristematic tissue (ii) Permanent tissue Meristematic tissue In the early stages of the development of the embryo, all the cells undergo" division, but with further growth and development cell division and multiplication become restricted to special parts of the plant in which the tissues remain embryonic in character and the cells retain the ability to divide. These embryonic tissues in the mature plant body, are called meristems. Cell division can also occur in tissues other than meristems, for instance, in the cortex of the stem and in young, developing vascular tissues. However, in these tissues the number of divisions is limited. On the other hand, the cells of the meristems continue to divide indefinitely and as a result new cells are continually added to the plant body. Meristems may also be found in a temporary resting phase, for instance, in perennial plants that are dormant in certain seasons and in axillary buds that may be dormant even during the active phase of the plant. The process of the growth and morpho-physiological specialization of the cells produced by the meristems is called differentiation. Theoretically, it was believed that the tissues that undergo differentiation gradually loses the embryonic characteristics of the meristem and acquire the mature state. Such tissues are called mature or permanent. Recently it has been shown that the term permanent tissues can only be used in relation to certain cells which have undergone irreversible differentiation, as, for instance, sieve elements which have no nucleus and dead cells, such as tracheids, vessel elements and cork cells. All cells which contain nuclei possess, to 'a certain degree, the ability to grow and divide and redifferentiate if the appropriate stimulus is present. The meristematic cells have generally some distinctive feature. The cells are usually isodiametric in shape; they are compactly set without evident intercellular spaces; have dense cytoplasm with small vacuoles and large prominent nuclei; ergastic (like starch and tannins) matters are lacking; the plastids are in pro-plastid stages; cell wall made of cellulose is thin and homogenous. Various system of classification of meristem have been proposed, based on characters like origin and nature of initiating cells, stage of development, topography and function. No system is exclusive and rigid. The common important types of meristem according to their origin and development are following: (i) Pro-meristem: pro-meristem is the very foundation stage, the region of formation of new organs and tissues. It may be called the earliest embryonic condition consisting of young initials and their derivatives. All the cells possess nuclei, proplastids, absence of ergastic matters and thin walls. (ii) Primary meristem: primary meristem is composed of cells which are direct descendants of embryonic cells and which have all through retained meristematic nature. So primary meristem may be called a later development stage. Chief primary meristem is those located at the tips of the stem, root and appendages. Primary meristems build up the primary body of the plants. (iii) Secondary meristem: some living permanent cells may retain the power of cell division. They constitute the secondary meristem, as they originate from the permanent cells. The cork cambium or phellogen, arising from the epidermis, cortical and other cells during secondary increase in the thickness, is an example of secondary meristem. According to their position in the plant body meristems are put into following three groups: (i) Apical meristem (ii) Intercalary meristem (iii) lateral meristem Apical Meristem The apical meristem is the growth region in plants found within the root tips and the tips of the new shoots and leaves. Apical meristem is one of three types of meristem, or tissue which can differentiate into different cell types. Meristem is the tissue in which growth occurs in plants. Apical is a description of growth occurring at the tips of the plant, both top and bottom. Intercalary meristem is found between branches, while lateral meristem grow in girth, such as in woody plants. Apical meristem is crucial in extending both access to nutrients and water via the roots and access to light energy via the leaves. Plants must expand in both of these directions in order to be successful. Some plants show apical dominance, in which only one main shoot apical meristem is the most prominent. In plants like this, there is a single main trunk which reaches to great heights. If the apical meristem is cut off, the branches below will start to assume the role of primary apical meristem, which will lead to a bushier plant. Horticulturalists use this phenomenon to increase the bushiness and yield of certain agricultural crops and ornamental plants. Apical Meristem Function The apical meristem, found just below the surface of the branches and roots furthest from the centre of the plant, is continually dividing. Some cells divide into more meristematic cells, while other cells divide and differentiate into structural or vascular cells. There are two apical meristem locations in most plants. The shoot apical meristem is found in the tips of plants. This apical meristem is responsible for creating cells and growth to drive the plant into the light and air, where it can photosynthesize and exchange built up gases. The root apical meristem is found at the tips of roots. Sensing the conditions of the soil around the root, signals are created within the apical meristem which direct the plant towards water and desired nutrients. It is for this reason that roots often invade pipes for water and drainage, which carry many of the nutrients they need. The apical meristem, protected by the root cap continues to produce cells even as the root cap is scraped away as it pushes through the dirt. The apical meristem must produce enough cells to not only extend into the soil, but also to replace the cells lost to abrasion. The apical meristem is located just below the root cap in the roots, as seen in the image below. The actual apical meristem is a cluster of densely packed and undifferentiated cells. From these cells will come all of the various cell structure the plant uses. An undifferentiated apical meristem cell will divide again and again, slowly becoming a specialized cell. In the root apical meristem, the cells are produced in two directions. In the shoot apical meristem, cells are only created in one direction. The shoot apical meristem may exist at the tips of plants, as in many dicots, or may start slightly below the soil and generate leaves which grow upward, like most monocots. However, in both groups the shoot apical meristem is the growth centre of all above ground growth. Interestingly, the shoot apical meristem in most plants is capable of producing an entire plant, whereas the root apical meristem cannot. Scientists have used the ability of the shoot apical meristem to clone many species of plant. By simply cutting off the apical meristem and transferring it to an appropriate growth medium, the apical meristem will develop roots and differentiate into a whole new plant. As an added benefit, more apical meristems form on the plant, and can be harvested for more clones. In this way, a desirable plant can be replicated almost indefinitely. Permanent tissue It has been stated earlier that meristematic cells gradually differentiate and become mature or permanent. Permanent tissue is thus composed of cells in which growth has stopped. Of course, all the living cells retain the potentialities of further growth. Classification of permanent tissue has been a perplexing problem. Mainly on the basis of constituent cells, permanent tissues are put into two groups 1. Simple 2. Complex Simple Tissues Simple tissues are composed of similar cells, and so they are homogeneous in nature. The complex tissues are heterogeneous, being made up of different types of cells elements. Mainly simple permanent tissue of plants are parenchyma, collenchyma and sclerenchyma. 1. Parenchyma tissues: It is the most common simple tissue of the plants with relatively little specialization. It is composed of cells which are usually isodiametric in shape with intercellular spaces. The cells have active protoplast. This tissue is universally distributed in the all the plants, the soft portion like epidermis, cortex, pith, pericycle, whole or part of stems and roots, mesophyll of leaves, pulp of fleshy fruits, embryo and endosperm of seeds being composed of parenchyma cells. It is called the fundamental tissue of the plant, because it really constitutes the ground substance where other tissues remain embedded. Bodies of lower plants are made of parenchyma cells. The meristematic cells are also parenchymatous in nature. Thus, parenchyma is the precursor of all other tissues. In mesophyll of the leaves they are slightly elongated. Irregular shapes as a result of folding, lobation etc. are also very common. Parenchyma cells have usually thin cell wall made of cellulose.
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