Title PLANT GROWTH AND DEVELOPMENT (211-266)_08.02.2020.pdf ✅

NEET BOTANY VOL-1 CH- 15 : PLANT GROWTH AND DEVELOPMENT 211 NARAYANA GROUP 211 PLANT GROWTH AND DEVELOPMENT 15 C O N T E N T S  Introduction  Growth  Characteristics of Plant Growth  Differentiation, Dedifferentiation & Redifferentiation  Development  Plant Growth Regulators  Auxins  Gibberellins  Cytokinins  Ethylene  Abscisic Acid  Photoperiodism  Vernalization  Seed dormancy and germination  Vivipary  Plant Movements  Important definitions  Review Points  Plant growth and development:  Seed germination;  Phases of Plant growth and plant growth rate; Conditions of growth; Differentiation, dedifferentiation and redifferentiation; Sequence of developmental process in a plant cell;  Growth regulators-auxin, gibberellin, cytokinin, ethylene, ABA;  Seed dormancy,  Photoperiodism. Vernalization  Tropic movements NEET SYLLABUS
212 NARAYANA GROUP CH- 15 : PLANT GROWTH AND DEVELOPMENT NEET BOTANY VOL-1 INTRODUCTION S eed germinates to produce seedling. Seedling grows first into a juvenile plant which develops only vegetative organs. Later on it begins to produce flowers, fruits and seeds. Unlike animals, plants do not stop growing after reaching maturity. They continue to grow and bear new roots, leaves, branches, flowers etc. While roots, stems and their branches have indefinite growth, other organs like leaves, flowers and fruits show limited or definite growth. They appear and fall off periodically and sometimes repeatedly. There is a highly ordered succession of events. A plant consists of billions of cells arranged in specific tissues and organs. All of them are descendents of a single celled zygote. However, by their specific differentiation and location in the plant they come to have different functions. As a result complex organization is formed that produces roots, leaves, branches, flowers, fruits and seeds. After a period of growth, differentiation and development, each plant dies. There are a number of intrinsic (internal) and extrinsic (external) factors which control various development processes. GROWTH * Plants typically continues to grow throughout their lives. Growth in a plant is the result of cell division, enlargement of the new cells and their differentiation into different types of tissues. * Growth can be defined as the permanent and irreversible change in any dimension (like size and volume) of an organism accompanying an increase in the dry weight.  In plants growth is confined only to meristem. The extreme apices of root and shoot, for instance, are occupied by primary meristem while in their older parts; secondary meristems (i.e., Cambia) give rise to additional vascular tissues and to protective cork layer cells. * The activity of each meristem facilitates the activity of the other meristems especially those near to it, giving rise to growth correlations. The relationship of the growth of one part of an organism to another part is called allometry. * Generally growth is accompanied by metabolic processes (both anabolic and catabolic) that occur at the expense of energy. During growth the rate of anabolism is more than catabolism. * The swelling of a piece of wood when placed in water is not described as growth as it comes to its original size after sometime. It represents that it is a reversible phenomenon where as growth is an irreversible one. CHARACTERISTICS OF PLANT GROWTH PLANT GROWTH IS INDETERMIINATE * Growth in plants is unlimited and occurs continuously throughout their life due to which, the growth of a plant is unique by the activity of meristematic cells. The new cells are continuosly added and produced by cell division in a meristem. Such kind of growth where in new cells are continuously added to the plant body by the activity of the meristem is called the open form of growth. * In plants, there is meristem at the apex of every root and shoot known as root apex and shoot apex respectively.  Some plants have intercalary meristems at the bases of internodes and leaf bases of grasses. * The vascular cambium is another kind of meristem which develops secondary tissues. * The root apex and shoot apex add new cells to cause linear growth parallel to the long axis of plant body. It is called primary growth of the plants.
NEET BOTANY VOL-1 CH- 15 : PLANT GROWTH AND DEVELOPMENT 213 NARAYANA GROUP 213 * The lateral meristems like vascular cambium and cork cambium add new cells to cause increase in girth. It is called secondary growth which occurs mainly in gymnosperms and dicotyledons. Root apical meristem Shoot apical meristem Shoot Root Cork Cambium Cork Cambium Vascular Cambium Vascular Cambium Fig. : Diagrammatic representation of locations of root apical meristem, shoot aplical meristem and vascular cambium. Arrows exhibit the direction of growth of cells and organ * The daughter cells, derived from cell division in the meristems, enlarge and bring about visible increase in size of the plant. The expanding cells differentiate to form the different tissues, and become part of the mature organ.  If meristems cease to divide then they take a permanent shape and size to form various structures of the plant body which are specific to a particular function. GROWTH IS MEASURABLE  Auxanometer is an instrument for measuring the growth while crescograph is used for measuring plant growth in millionth part of mm.Auxamometer was discovered by V.J. Koningsberger  Crescograph was developed by Sir J.C. Bose * Growth is a quantitative phenomenon as it involves increase in cell number and cell size followed by differentiation. It is therefore measurable. * Growth at cellular level is due to the increase in the amount of protoplasm. Since increase in protoplasm is difficult to measure directly, a variety of parameters like (i) Increase in fresh weight, dry weight (ii) Increase in volume and size (cells in watermelon increase in size upto 3,50,000 times) (iii) Increase in cell number (one single maize root apical meristem gives rise to more than 17,500 new cells per hour) (iv) Increase in surface area denotes the growth in a dorsiventral leaf (v) Increase in length denotes the growth of a pollen tube . Phases of Growth * The period of growth is generally divided into three phases namely, meristematic, elongation and maturation. * We can understand this by observing root tips. The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells at this phase of growth shows following features : (i) rich in protoplasm
214 NARAYANA GROUP CH- 15 : PLANT GROWTH AND DEVELOPMENT NEET BOTANY VOL-1 (ii) possess large conspicuous nuclei (iii) their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections (iv) have high respiratory rate A B C D E F G Fig. : Detection of zones of elongation by the parallel line technique. Zones A, B, C, D immediately behind the apex have elongated most. * The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. The cells found in this zone shows the following features: (i) increased vacuolation (ii) cell enlargement and new cell wall deposition are the characteristics of the cells in this phase. * Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis that is undergoes the phase of maturation. * The cells of this zone, attain their maximal size in terms of thickening and protoplasmic modifications. GROWTH RATES * Increase in growth per unit time is called growth rate. * Growth rate may result in arithmetic or geometric growth.  Arithmetic Growth. It is a type of growth in which the rate of growth is constant and increase in growth occurs in arithmetic progression—2, 4, 6, 8, 10, 12. Arithmetic growth is found in root or shoot elongating at constant rate. Meristematic cells at the growing point divide in such a fashion that one daughter remains meristematic while the other grows and differentiates. The process continues. Mathematically arithmetic growth is expressed as : Lt = L0 + rt Lt = length after time t. L0 = length at the beginning. r = growth rate. Time (A) (B) (C) Fig. : A. Arithmetic growth. B. formation of new cells, C. growth curve (constant linear growth, a plot of length L against time t) . * If a graph is plotted by taking length of the organ of a plant at Y-axis against the time at X-axis, a linear curve is obtained * Geometric Growth. It is quite common in unicellular organisms when grown in nutrient rich