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Biology, , (Chapter — 15)(Plant Growth and Development), (Class - x1), , Question 1:, , Define growth, differentiation, development, dedifferentiation, redifferentiation,, determinate growth, meristem and growth rate., , Answer 1:, , (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 fram 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., , (4) 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, , (9) 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., , Question 2:, Why is not any one parameter good enough to demonstrate growth throughout the life, of a flowering plant?, , Answer 2:, , In plants, growth is said to have taken place when the amount of protoplasm increases., Measuring the growth of protaplasm involves many parameters such as the weight of
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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, Question 3:, Describe briefly:, (a) Arithmetic growth, (b) Geometric growth, (¢) Sigmaid growth curve, (4) Absolute and relative growth rates, Answer 3:, (a) Arithmetic growth, In arithmetic growth, one of the daughter cells continues to divide, while the other, differentiates into maturity. The elongaticn cf 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 fromm mitosis retain the ability, to divide, but slow down because of a limited nutrient supply., (¢) Sigmoid growth curve, The growth of living organisms in their natural environment is characterised by an S, shaped curve called sigmaid growth curve. This curve is divided into three phases - lag, phase, log phase or exponential phase of rapid growth, and stationary phase., , Semone shee, , } J, , Exponential growth can be expressed as:, W,=W,e", Where,, W: = Final size, We = Initial size, r = Growth rate, t= Time of growth, e = Base of natural logarithms
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(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., , Question 4:, , List five main groups cf natural plant growth regulators. Write a note on discovery,, physiological functions and agricultural/horticultural applications of any one of them., Answer 4:, , 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, , (iv) Ethylene, , (wv) Abscisic acid, , (i) Auxins, , Discovery:, , The first observations regarding the effects of auxins were made by Charles Darwin and, Francis Darwin wherein they saw the colecptiles of canary gross bending toward a, unilateral source of light., , It was concluded after a series of experiments that some substance produced at the tip, of coleaptiles was responsible for the bending. Finally, this substance was extracted as, auxins from the tips of coleoptiles in oat seedlings., , Physiological functions:, , > They control plant cell-growth., , > They cause the phenomenon of apical dominance., , > They control division in the vascular cambium and xylem differentiation., , > They induce parthenocarpy and prevent abscission of leaves and fruits., , Horticultural applications:, , ¥ They are used as the rooting hormones in stem cuttings, , v 2-4 D is used weedicide to kill broadleaf, dicotyledonous weeds, , v¥ They induce parthenocarpy in tomatoes., ¥ They promote flowering in pineapple and litchi,
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(ii) Gibberellic acid, , Discovery:, , Bakane or the "foolish rice seedling” disease was first observed by Japanese farmers. In, , this disease, rice seedlings appear to grow taller than natural plants, and become slender, , and pale green. Later, after several experiments, it was found that this condition was, , caused by the infection fram a certain fungus Gibberella fujikurai. The active substance, , was isolated and identified as gibberellic acid, , Physiological functions:, , > It causes elongation of internodes., , > It promotes bolting in rosette plants., , > It helps in inducing seed germination by breaking seed dormancy and initiating the, synthesis of hydrolases enzymes for digesting reserve food, , Horticultural applications:, , ¥ It helps in increasing the sugar content in sugarcane by increasing the length of the, internodes, , ¥ It increases the length of grape stalks., , ¥ It improves the shape of apple., , ¥ It delays senescence., , ¥ It hastens maturity and induces seed-productian in juvenile conifers., , (iii) Cytekinins, , Discovery:, , Through their experimental observations, F, Skoog and his co-workers found that the, , tobacco callus differentiated when extracts of vascular tissues, yeast extract, coconut, , milk, or DNA were added to the culture medium. This led to the discovery of cytokinins,, , Physiological functions:, , > They promote the growth of lateral branches by inhibiting apical dominance., , > They help in the production of new leaves, chloroplasts, and adventitious shoots., , > They help in delaying senescence by promoting nutrient mobilisation., , Horticultural! applications:, , ¥ They are used for preventing apical dominance, , ¥ They are used for delaying senescence in leaves., , (iv) Ethylen, , Discovery:, , It was observed that unripe bananas ripened faster when stored with ripe bananas., , Later, the substance promoting the ripening was found to be ethylene
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Physiological functions:, , > It helps in breaking seed and bud dormancy., , > It promotes rapid internode-elongation in deep-water rice plants., , > It promotes root-grawth and formaticn of root hairs., , > It promotes senescence and abscission of leaves and flowers., , > It hastens the respiration rate in fruits and enhances fruit ripening., , Horticultural applications:, , ¥ It is used to initiate flowering and synchronising the fruit set in pineapples., , ¥ It induces flowering in mange., , ¥ Ethephon is used to ripen the fruits in tomatoes and apples, and accelerate the, abscission of flowers and leaves in cotton, cherry, and walnut, , ¥ It promotes the number of female flowers in cucumbers., , (¥) Abscisic acid, , Discovery:, , During the mid-1960s, inhibitor-B, abscissian II, and dormin were discovered by three, , independent researchers. These were later on found to be chemically similar and were, , thereafter called ABA (Abscisic acid)., , Physiological functions:, , > It acts as an inhibitor to plant metabolism., , > It stimulates stomatal closure during water stress., , > It induces seed dormancy., , > It induces abscission of leaves, fruits, and flowers., , Horticultural application:, , ¥ It induces seed dormancy in stored seeds., , Question 5:, , What do you understand by photoperiodism and vernalisation? Describe their, significance., , Answer 5:, , Photoperiodism refers to the response of plants with respect to the duration of light (i.e.,, period of day and night). On the basis of its response to the duration of light, a plant is, classified as a short-day plant, a long-day plant, or a day-neutral plant. Shart-day plants, flower when they are exposed to light for a period less than the critical day-length (for, example: Chrysanthemum). Long-day plants flower when they are exposed to light for, @ period more than the critical day-length (for example: radish). When no marked, correlation is observed between the duration of exposure to light and the flowering, response, plants are termed as day-neutral plants (for example: tomato).
