Title Med-RM_Bot_SP-2_Ch-12-Plant Growth and Development.pdf ✅

Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Growth Differentiation, Dedifferentiation and Redifferentiation Development Plant Growth Regulators Photoperiodism Vernalisation Development is the sum of two processes: growth and differentiation. To begin with, it is essential and sufficient to know that a mature plant develops from a single-celled zygote which divides and differentiates to form various types of cell. These different types of cells organise to form tissues and organs of the plant. The development of a mature plant from a zygote follows a precise and highly ordered succession of events. During this process, a complex body i.e., plant is formed which constitutes the roots, leaves, branches, flowers, fruits and seeds. GROWTH Growth is a fundamental characteristic of all living organisms. It is defined as an irreversible permanent increase in size, volume and weight of an organ or its parts or even of an individual cell. Plant Growth is Generally Indeterminate Fig.: Diagrammatic representation of locations of root apical meristem, shoot apical meristem and vascular cambium. Arrows exhibit the direction of growth of cells and organ. Shoot apical meristem Vascular cambium Shoot Root Vascular cambium Root apical meristem Plants have the capacity for indeterminate or unlimited growth which continues throughout their life. Due to which, the growth of a plant is known as unique. The growth of the plants is due to the presence of different types of meristems at specific locations in their body. Meristems are the cells which are capable of self- perpetuation i.e., have the power to continue indefinitely through division. The continued growth due to which new cells are always being added to the plant body by the activity of the meristem is called the open form of growth. Chapter 12 Plant Growth and Development
160 Plant Growth and Development NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Growth is Measurable Growth can be measured by various parameters, as shown below: Increase in cell size Increase in surface area Increase in length Increase in cell number Increase in volume Increase in fresh and dry weight Estimation of growth Auxanometer is an instrument used to measure the growth in length of the plant. Phases of Growth Plant growth takes place in three phases : (1) Meristematic phase or phase of cell division : It is the first phase of growth in plants which occurs in the areas where meristematic cells are present. For example, at shoot and root tip of the plants where meristems are constantly dividing. As a result this phase is also known as cell division. The cells at this phase of growth show following features : (i) Have dense protoplasm. (ii) Contain large nucleus. (iii) Have high respiration rate. (iv) Cell wall is primary in nature. It means that the cell wall is made up of cellulose, thin walls with abundant plasmodesmatal connections so that cells can communicate with each other. (2) Elongation phase or phase of enlargement : The cells present just next to the cells of meristematic zone represent the elongation phase. The cells found in this zone show the following features : (i) Increased vacuolation : The number and size of vacuoles increase as growth occurs. (ii) New cell wall deposition : The cell wall of the cells starts depositing new materials resulting in cell enlargement. (iii) Cell size : The size of the cells of this phase increases mainly due to vacuolation. (3) Maturation phase or phase of differentiation : The cells present just next to the cells of elongation phase represent the phase of differentiation. The enlarged cells develop into special or particular type of cells by undergoing structural and physiological differentiation. (i) Structural differentiation : A cell attains a particular shape, size and internal constitution. (ii) Physiological differentiation : A cell attains a particular function. For example, absorption of water and minerals by root hair, photosynthesis by mesophyll cells etc. So, cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.
NEET Plant Growth and Development 161 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 After differentiation, the mature cells do not grow further and remain unchanged till death. G F E D C B A Fig. : Detection of zones of elongation by the parallel line technique. Zones A, B, C, D immediately behind the apex have elongated most. Growth Rates An organism or a part of the organism can produce more cells in a variety of ways. The increase in growth per unit time is defined as growth rate. Thus, it can be expressed mathematically, being either arithmetical or geometrical. Fig. : Diagrammatic representation of : (a) Arithmetic (b) Geometric growth and (c) Stages during embryo development showing geometric and arithmetic phases. (a) Arithmetic (b) Geometric Zygote divides Geometric phase: all cells divide (c) = Cells capable of division = Cells that lose capacity to divide Arithmetic phase: These cells divide These cells do not divide
162 Plant Growth and Development NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (i) Arithmetic growth : It is the growth rate in which growth occurs at a constant rate from the very beginning and progresses arithmetically. Here, following mitotic cell division only one daughter cell divides continuously whereas other cells undergo differentiation and become mature. For example, we can study the arithmetic growth pattern in root elongation where elongation occurs at a constant rate. If a graph is plotted by taking length of the organ of plant at Y-axis against the time at X-axis, a linear curve is obtained. Mathematically, it can be expressed by following equation : L L rt t 0   where Lt = Length of the organ at time ‘t’ L0 = Length of the organ at time ‘zero’ r = Growth rate or elongation per unit time. Fig. : Constant linear growth, a plot of length L against time t a Height of the plant b Time (ii) Geometrical growth : It is a growth rate where every cell divides with all the daughters growing and dividing again. We can take the example of microorganisms to study the pattern of geometrical growth. In microorganisms, growth occurs when they are provided with enough food and space. During their growth, three phases can be observed . (1) Lag phase : This is the initial phase of growth when the rate of growth is very slow. It represents the beginning of growth of microorganisms where their cell number is small. (2) Log phase or exponential phase : In this phase, growth progresses rapidly or exponentially i.e., reaches to its maximum. Here, both the progeny or daughter cells obtained after mitotic cell division have the ability to divide continuously. However, such a growth cannot be sustained for long and growth reaches to next phase. (3) Stationary phase : Due to the shortage of space, food and accumulation of toxins, growth slows down leading to a phase known as stationary phase. If we plot all these phases graphically then we will obtain S-shaped or sigmoid curve. Similarly, we can study the growth of cells, tissues and organs of a plant. For example, if we plot the parameters of growth such as size, weight of the organ at Y-axis against the time at X-axis, we get a typical sigmoid or S-shaped curve which explains the geometrical growth.