Plant Growth and Development

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Plant Growth and Development
Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate. (a) Growth It is an irreversible and permanent process, accomplished by an increase in the size of an organ or organ parts or even of an individual cell. (b) Differentiation It is a process in which the cells derived from the apical meristem (root and shoot apex) and the cambium undergo structural changes in the cell wall and the protoplasm, becoming mature to perform specific functions. (c) Development It refers to the various changes occurring in an organism during its life cycle – from the germination of seeds to senescence. (d) De-differentiation It is the process in which permanent plant cells regain the power to divide under certain conditions. (e) Re-differentiation It is the process in which de-differentiated cells become mature again and lose their capacity to divide. (f) Determinate growth It refers to limited growth. For example, animals and plant leaves stop growing after having reached maturity. (g) Meristem In plants, growth is restricted to specialised regions where active cell divisions take place. Such a region is called meristem. There are three types of meristems – apical meristem, lateral meristem, and intercalary meristem. -------------------------------------------------

(h) Growth rate It can be defined as the increased growth in plants per unit time. -------------------------------------------------

Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant? -------------------------------------------------
In plants, growth is said to have taken place when the amount of protoplasm increases. Measuring the growth of protoplasm involves many parameters such as the weight of the fresh tissue sample, the weight of the dry tissue sample, the differences in length, area, volume, and cell number measured during the growth period. Measuring the growth of plants using only one parameter does not provide enough information and hence, is insufficient for demonstrating growth. -------------------------------------------------

Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates
(a) Arithmetic growth
In arithmetic growth, one of the daughter cells continues to divide, while the other differentiates into maturity. The elongation of roots at a constant rate is an example of arithmetic growth. (b) Geometric growth

Geometric growth is characterised by a slow growth in the initial stages and a rapid growth during the later stages. The daughter cells derived from mitosis retain the ability to divide, but slow down because of a limited nutrient supply. (c) Sigmoid growth curve

The growth of living organisms in their natural environment is characterised by an S-shaped curve called sigmoid growth curve. This curve is divided into three phases – lag phase, log phase or exponential phase of rapid growth, and stationary phase. Exponential growth can be expressed as:

W1 = Final size
W0 = Initial size
r = Growth rate
t= Time of growth
e = Base of natural logarithms
(d) Absolute and relative growth rates
Absolute growth rate refers to the measurement and comparison of total growth per unit time. -------------------------------------------------
Relative growth rate refers to the growth of a particular system per unit time, expressed on a common basis. -------------------------------------------------

List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them. Plant growth regulators are the chemical molecules secreted by plants affecting the physiological attributes of a plant. There are five main plant growth regulators. These are: (i) Auxins

(ii) Gibberellic acid
(iii) Cytokinins...
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