Tiêu đề PLANT GROWTH & DEVELOPMENT.pdf ✅

 Digital www.allendigital.in [ 173 ] 1. Introduction : Growth : • Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. • Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. • Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. • Therefore, for example, expansion of a leaf is growth. (A) Characteristics of Plant Growth : (i) Plant growth generally is indeterminate : • Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. • This ability of the plants is due to the presence of meristems at certain locations in their body. The cells of such meristems have the capacity to divide and self-perpetuate. • The product, however, soon loses the capacity to divide and such cells make up the plant body. (ii) This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth (iii) Plant growth is of two types : (a) Primary growth : Root apical meristem and shoot apical meristem are responsible for the primary growth of the plants and principally contribute to the elongation of the plants along their axis. (b) Secondary growth : In dicotyledonous plants and gymnosperms the lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth and contribute to the increase in the girth of the organs (root, stem). Plant Growth and Development 05 Shoot Root Shoot Apical Meristem Vascular Cambium Vascular Cambium Root Apical Meristem
NEET : Biology [ 174 ] www.allendigital.in  Digital Growth is Measurable : • At cellular level growth can be measured by measuring the increase in the amount of protoplasm but it is very difficult to measure directly, so growth is measured by a variety of parameters, they are: (a) Increase in fresh weight (b) Increase in dry weight (c) Increase in surface area/volume (d) Increase in number or size of cells. • One single maize root apical mersitem can give rise to more than 17,500 new cells per hour, whereas cells in a watermelon may increase in size by upto 3,50,000 times. In the former, growth is expressed as increase in cell number; the latter expresses growth as increase in size of the cell. While the growth of a pollen tube is measured in terms of its length, an increase in surface area denotes the growth in a dorsiventral leaf. Growth is measured by Auxanometer. (B) Phase of Growth : The period of growth is generally divided into three phases, namely : (i) Meristematic phase (ii) Elongation phase (iii) Maturation phase (i) Meristematic phase : The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in this region are characterised by : (a) Cells are small in size with abundant plasmodesmal connections. (b) Intercellular spaces are absent, if present then very small. (c) Cell walls are primary in nature, thin and cellulosic. (d) Cells are rich in protoplasm, possess large conspicuous nuclei. (ii) Elongation phase : The cells proximal to the meristematic zone represent the phase of elongation. Cells in this region are characterised by : (a) Increased vacuolation (b) Cell enlargement (c) New cell wall deposition (iii) Maturation phase : The cells more proximal to the phase of elongation represent the phase of maturation. Cells of this zone, attain their maximal size in terms of wall thickening and protoplasm modifications. (C) Growth Rate : • Increased growth per unit time is termed as growth rate. • The growth rate shows an increase that may be arithmetic or geometric. (i) Arithmetic growth : In arithmetic growth only one daughter cell among the two further divides while other differentiates and become mature (stop dividing). A B C D E F G Parallel line technique to detect zone of elongation in root
Plant Growth and Development  Digital www.allendigital.in [ 175 ] • Ex. Root & Shoot elongation at constant Rate. • It is mathematically expresesed as Lt = L0 + rt Where Lt : length at time 't' L0 : length at time 'zero' r : growth rate / elongation per unit time. (a) Arithmetic (b) Geometric (c) Zygote divided Geometric phase : all cell divide Arithmetic phase : These cells divide These cells do not divide Cells capable of division Cells that lose capacity to divide Diagrammatic representation of : (a) Arithmetic (b) Geometric growth and (c) Stages during embryo development showing geometric and arithmetic phases
NEET : Biology [ 176 ] www.allendigital.in  Digital • It's curve is linear. (ii) Geometric Growth : Here both the progeny cells following mitotic divisions retain the ability to divide and continue to do so. Ex : Early embryonic development/division in zygote, division in unicellular organism. It is mathematically represented as W1 = W0ert Where W1 - final size (Weight, height, number etc.) W0 - initial size at the begining of period. r - growth rate e - base of natural logarithms. • In most systems the initial growth is slow (lag phase) and it increases rapidly thereafter at an exponential rate (log or exponential phase), it is also called as "grand phase of growth". However, with limited nutrient supply, the growth slows down leading to a stationary phase or steady phase. If we plot the parameter of growth against time, we get typical sigmoid or S-curve. A sigmoid curve is a characteristic of living organisms growing in a natural enviroment. It is typical for all cells, tissues and organs of a plant. • Here r = relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Quantitative comparisions between the growth of living system can also be made in two ways : (a) Absolute growth rate : Measurement and comparision of total growth per unit time is called the absolute growth rate. (b) Relative growth rate : The growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate. Height of the plant Time Steady phase or Stationary phase Lag phase (slow) Size/Height of the organ Time Grand phase of growth