Plant Growth and Development

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Written by SiddSir For NEETPLAY.COM  & NEETBUDDY.COM

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All cells of a plant are descendents of the zygote. Development of a mature plant from a zygote follows a precise and highly ordered succession of events.   Development is the sum of two processes: GROWTH and DIFFERENTIATION. 

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Growth

 During this process a complex body organisation is formed that produce roots, leaves, branches, flowers, fruits, seeds and eventually they die. 

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Growth

 The first step in the process of plant growth is seed germination.  The seed germinates when favourable conditions for growth exist in the environment.

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Growth

in absence of favourable conditions they go into a period of suspended growth or rest, and resume metabolic activities on return of favourable conditions and growth takes place.

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Growth

Growth is irreversible permanent increase in size of an organ or its parts or even an individual cell.   It is one of the most fundamental and conspicuous characteristic of a living being. 

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Growth is accompanied by metabolic processes and occur at the expense of energy. 

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Plants retain the capacity of unlimited growth throughout their life due to presence of meristem at certain locations in their body. 

Root apical meristem and shoot apical meristem are responsible for primary growth, i.e., elongation along their axis. 

In dicotyledonous plants and gymnosperms, the lateral meristems like vascular cambium and cork-cambium, which appear later in life and cause increase in girth of the organs called secondary growth.

Growth is measurable: At cellular level, it is principally a consequence of increase in amount of protoplasm. It is measured by a variety of parameters like-increase in fresh weight, dry weight, length, area, volume and cell number

Detection of zones of elongation by the parallel line technique. Zones A, B, C, D immediately behind the apex have elongated most.

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The period of growth is generally divided into three phases MERISTEMATIC, ELONGATION & MATURATION.  (i) The constantly dividing cells at root apex and shoot apex represent meristematic phase of growth.

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(ii) Cells proximal to the tip, represent phase of elongation.  (iii) More proximal to the phase of elongation is phase of maturation

Increased growth per unit time is termed as growth rate.  it can be arithmetic or geometrical.  (a) Arithmetic growth: Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. So, a linear curve is obtained  e.g., root elongating at a constant rate.  

Mathematically, expressed as  Lt = L + rt  Lt = length at time 't'  t  L = length at time 'zero'  r = growth rate/elongation per unit time

Geometrical growth: In most systems, initial growth is slow (lag phase), it increases rapidly thereafter at an exponential rate (lag or exponential phase), as both progeny cells of mitotic cell division retain ability to divide and continue to do so. However with limited nutrient supply, growth slows down leading to stationary phase, giving a typical sigmoid or S-curve. 

A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.  

  Here, r = relative growth rate and measure of ability of plant to produce new material called efficiency index.

(i) Measurement and comparison of total growth per unit time called ABSOLUTE GROWTH RATE. (ii) The growth of given system per unit time expressed on a common basis, e.g., per unit initial parameter is called RELATIVE GROWTH RATE.

RELATIVE GROWTH RATE

ABSOLUTE GROWTH RATE

Diagrammatic comparison of absolute and relative growth rates. Both leaves A and B have increased their area by 5 cm2 in a given time to produce A1 , B1  leaves.

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Water: For cell enlargement, i.e., extension growth by turgidity. Water also provides medium for enzymatic activities.  Oxygen: For aerobic respiration to get metabolic energy.

CONDITIONS FOR GROWTH

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Macro and Micro nutrients: for synthesis of protoplasm.  Temperature: Optimum range for best growth.   Light and Gravity: also affect certain phases/stages of growth.

CONDITIONS FOR GROWTH

DIFFERENTIATION The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions, this act leading to maturation in termed differentiation. eg. tracheary element

DE-DIFFERENTIATION Living differentiated cells, that have lost the capacity to divide can regain capacity of division under certain conditions; this phenomenon is dedifferentiation, e.g., formation of interfascicular and cork-cambium from parenchyma cells

RE-DIFFERENTIATION De-differentiated meristems are able to divide and produce cells that once again lose capacity to divide but mature to perform specific functions, i.e., get redifferentiated.  e.g., secondary xylem, secondary cortex, cork, etc

Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called Plasticity.

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In cotton, coriander and larkspur the leaves of juvenile plant differ from that of mature plant (developmental heterophylly).  The leaves of buttercup in water and air are different due to environment  ( environmental heterophylly).

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END OF PART-1