PLANT GROWTH AND DEVELOPMENT

1 PLANT GROWTH AND DEVELOPMENT239 You have already studied the organisation of a flowering PLANT in Chapter5. Have you ever thought about where and how the structures like roots,stems, leaves, flowers, fruits and seeds arise and that too in an orderlysequence? You are, by now, aware of the terms seed, seedling, plantlet,mature PLANT . You have also seen that trees continue to increase in heightor girth over a period of time. However, the leaves, flowers and fruits of thesame tree not only have limited dimensions but also appear and fallperiodically and some time repeatedly. Why does vegetative phase precedeflowering in a PLANT ? All PLANT organs are made up of a variety of tissues; isthere any relationship between the structure of a cell, a tissue, an organand the function they perform? Can the structure and the function of thesebe altered? All cells of a PLANT are descendents of the zygote. The questionis, then, why and how do they have different structural and functionalattributes?

2 DEVELOPMENT is the sum of two processes: GROWTH anddifferentiation. To begin with, it is essential and sufficient to know that thedevelopment of a mature PLANT from a zygote (fertilised egg) follow a preciseand highly ordered succession of events. During this process a complexbody organisation is formed that produces roots, leaves, branches, flowers,fruits, and seeds, and eventually they die (Figure ). The first step in theprocess of PLANT GROWTH is seed germination. The seed germinates whenfavourable conditions for GROWTH exist in the environment. In absence ofsuch favourable conditions the seeds do not germinate and goes into aperiod of suspended GROWTH or rest. Once favourable conditions return,the seeds resume metabolic activities and GROWTH takes this chapter, you shall also study some of the factors whichgovern and control these developmental processes. These factors are bothintrinsic (internal) and extrinsic (external) to the GROWTH AND DEVELOPMENTCHAPTER , is regarded as one of the most fundamental and conspicuouscharacteristics of a living being.

3 What is GROWTH ? GROWTH can be definedas an irreversible permanent increase in size of an organ or its parts oreven of an individual cell. Generally, GROWTH is accompanied by metabolicprocesses (both anabolic and catabolic), that occur at the expense ofenergy. Therefore, for example, expansion of a leaf is GROWTH . How wouldyou describe the swelling of piece of wood when placed in water? PLANT GROWTH Generally is IndeterminatePlant GROWTH is unique because plants retain the capacity for unlimitedgrowth throughout their life. This ability of the plants is due to the presenceof meristems at certain locations in their body. The cells of such meristemshave the capacity to divide and self-perpetuate. The product, however,soon loses the capacity to divide and such cells make up the PLANT form of GROWTH wherein new cells are always being added to theplant body by the activity of the meristem is called the open form of would happen if the meristem ceases to divide? Does this everhappen?

4 In Chapter 6, you have studied about the root apical meristem andthe shoot apical meristem. You know that they are responsible for theSeed coatEpicotylhookCotyledonsCotyledonSoil lineEpicotylHypocotylHypocotylFigure Germination and seedling DEVELOPMENT in bean2021-22 PLANT GROWTH AND DEVELOPMENT241primary GROWTH of the plants and principallycontribute to the elongation of the plants alongtheir axis. You also know that in dicotyledonousplants and gymnosperms, the lateral meristems,vascular cambium and cork-cambium appearlater in life. These are the meristems that causethe increase in the girth of the organs in whichthey are active. This is known as secondarygrowth of the PLANT (see Figure ). is MeasurableGrowth, at a cellular level, is principally aconsequence of increase in the amount ofprotoplasm. Since increase in protoplasm isdifficult to measure directly, one generallymeasures some quantity which is more or lessproportional to it.

5 GROWTH is, therefore,measured by a variety of parameters some ofwhich are: increase in fresh weight, dry weight,length, area, volume and cell number. You mayfind it amazing to know that one single maizeroot apical mersitem can give rise to more than17,500 new cells per hour, whereas cells in awatermelon may increase in size by upto3,50,000 times. In the former, GROWTH isexpressed as increase in cell number; the latterexpresses GROWTH as increase in size of the the GROWTH of a pollen tube is measuredin terms of its length, an increase in surface areadenotes the GROWTH in a dorsiventral of GrowthThe period of GROWTH is generally divided intothree phases, namely, meristematic, elongationand maturation (Figure ). Let usunderstand this by looking at the root tips. Theconstantly dividing cells, both at the root apexand the shoot apex, represent the meristematicphase of GROWTH . The cells in this region are richin protoplasm, possess large conspicuousnuclei.

6 Their cell walls are primary in nature,thin and cellulosic with abundantplasmodesmatal connections. The cellsproximal (just next, away from the tip) to theShoot apicalmeristemVascularcambiumVascularcam biumRoot apicalmeristemShootRootFigure representation oflocations of root apical meristem,shoot aplical meristem andvascular cambium. Arrows exhibitthe direction of GROWTH of cells andorganGFEDCBAF igure of zones of elongation bythe parallel line technique. ZonesA, B, C, D immediately behind theapex have elongated zone represent the phase of elongation. Increasedvacuolation, cell enlargement and new cell wall deposition are thecharacteristics of the cells in this phase. Further away from the apex, ,more proximal to the phase of elongation, lies the portion of axis which isundergoing the phase of maturation. The cells of this zone, attain theirmaximal size in terms of wall thickening and protoplasmic of the tissues and cell types you have studied in Chapter 6 representthis GROWTH RatesThe increased GROWTH per unit time is termed as GROWTH rate.

7 Thus, rateof GROWTH can be expressed mathematically. An organism, or a part of theorganism can produce more cells in a variety of representation of : (a) Arithmetic (b) Geometric GROWTH and(c) Stages during embryo DEVELOPMENT showing geometric and arithematicphases2021-22 PLANT GROWTH AND DEVELOPMENT243 The GROWTH rate shows an increase that may bearithmetic or geometrical (Figure ).In arithmetic GROWTH , following mitotic celldivision, only one daughter cell continues to dividewhile the other differentiates and matures. Thesimplest expression of arithmetic GROWTH isexemplified by a root elongating at a constant at Figure On plotting the length of theorgan against time, a linear curve is , it is expressed asLt = L0 + rtLt = length at time t L0 = length at time zero r = GROWTH rate / elongation per unit us now see what happens in geometricalgrowth. In most systems, the initial GROWTH is slow(lag phase), and it increases rapidly thereafter atan exponential rate (log or exponential phase).

8 Here,both the progeny cells following mitotic cell divisionretain the ability to divide and continue to do , with limited nutrient supply, the growthslows down leading to a stationary phase. If we plotthe parameter of GROWTH against time, we get a typicalsigmoid or S-curve (Figure ). A sigmoid curveis a characteristic of living organism growing in anatural environment. It is typical for all cells, tissuesand organs of a PLANT . Can you think of more similarexamples? What kind of a curve can you expect ina tree showing seasonal activities?The exponential GROWTH can be expressed asW1 = W0 ertW1 = final size (weight, height, number etc.)W0 = initial size at the beginning of the periodr = GROWTH ratet = time of growthe = base of natural logarithmsHere, r is the relative GROWTH rate and is also themeasure of the ability of the PLANT to produce newplant material, referred to as efficiency index. Hence,the final size of W1 depends on the initial size, linear GROWTH , a plotof length L against time tFigure idealised sigmoid growthcurve typical of cells in culture,and many higher plants andplant organsSize/weight of the organExponential phaseLag phaseTimeStationary phase2021-22244 BIOLOGYQ uantitative comparisons between the GROWTH of living system canalso be made in two ways : (i) measurement and the comparison of totalgrowth per unit time is called the absolute GROWTH rate.

9 (ii) The GROWTH ofthe given system per unit time expressed on a common basis, , perunit initial parameter is called the relative GROWTH rate. In Figure leaves, A and B, are drawn that are of different sizes but showsabsolute increase in area in the given time to give leaves, A1 and B1. However,one of them shows much higher relative GROWTH rate. Which one and why? for GrowthWhy do you not try to write down what you think are necessary conditionsfor GROWTH ? This list may have water, oxygen and nutrients as very essentialelements for GROWTH . The PLANT cells grow in size by cell enlargement whichin turn requires water. Turgidity of cells helps in extension GROWTH . Thus, PLANT GROWTH and further DEVELOPMENT is intimately linked to the waterstatus of the PLANT . Water also provides the medium for enzymatic activitiesneeded for GROWTH . Oxygen helps in releasing metabolic energy essentialfor GROWTH activities. Nutrients (macro and micro essential elements) arerequired by plants for the synthesis of protoplasm and act as source addition, every PLANT organism has an optimum temperature rangebest suited for its GROWTH .

10 Any deviation from this range could bedetrimental to its survival. Environmental signals such as light and gravityalso affect certain phases/stages of comparison of absolute and relative GROWTH rates. Bothleaves A and B have increased their area by 5 cm2 in a given time toproduce A1, B1 GROWTH AND , DEDIFFERENTIATION ANDREDIFFERENTIATIONThe cells derived from root apical and shoot-apical meristems andcambium differentiate and mature to perform specific functions. This actleading to maturation is termed as differentiation. During differentiation,cells undergo few to major structural changes both in their cell walls andprotoplasm. For example, to form a tracheary element, the cells wouldlose their protoplasm. They also develop a very strong, elastic,lignocellulosic secondary cell walls, to carry water to long distances evenunder extreme tension. Try to correlate the various anatomical featuresyou encounter in plants to the functions they show another interesting phenomenon.