Plant Growth And Development
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Growth
· Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an
individual cell.
· Growth is accompained by metabolic processes (both anabolic and catabolic), that occur at the expense of
energy. e.g., expansion of a leaf is growth.
Characterstics of plant growth
(1) Plant growth is generally indeterminate because plants retain the capacity for unlimited growth throughout
their life.
This ability of plants is due to the presence of meristems at certain locations in their body. The cells of such
meristems have the capacity to divide and self-perpetuate. The product, however, soon loses the capacity to
divide and such cells make up the plant body.
(2) Plant growth is localised.
Reason : Plant growth is restricted to certain locations (apical meristems, intercalary meristems, lateral meristems)
within plant body.
(3) Plant growth is open. In this form of growth new cells are always being added to the plant body by the activity
of the meristem.
(4) Plant growth is of two types :
(i) Primary growth : Root apical meristem and shoot apical meristem are responsible for the primary growth
of the plants and principally contribute to the elongation of the plants along their axis.
(ii) Secondary growth : In dicotyledonous plants and gymnosperms the lateral meristems (vascular cambium
and cork cambium) are responsible for secondary growth and contribute to the increase in the girth of the
organs (root, stem).
Growth is measurable
Growth at cellular level, is principally a consequence of increase in the amount of protoplasm. Since increase
in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less
proportional to it.
Growth is measured by a variety of parameters, they are :
(i) Increase in fresh and dry weight.
(ii) Increase in length e.g., length of pollen tube.
(iii) Increase in surface area e.g., growth in surface area of a leaf.
(iv) Increase in cell number e.g., maize root apical meristem can give rise to more than 17500 new cells per hour.
(v) Increase in cell size or volume. e.g., cells in a watermelon may increase in size upto 3,50,000 times.
(vi) Increase in girth e.g., increase in diameters of dicot root and dicot stem
Methods of growth measurement
(a) By direct observation (b) By auxanometer
(c) By crescograph : This apparatus was developed by J.C. Bose.
Phases of growth
The period of growth is generally divided into three phases, namely :
(i) Meristematic phase
(ii) Elongation phase
(iii) Maturation phase
(i) Meristematic phase : The constantly dividing cells,
both at the root apex and the shoot apex, represent the
meristematic phase of growth. The cells in this region
are characterised by :
(a) Cells are small in size with abundant plasmodesmal
connections.
(b) Intercellular spaces are absent, if present then very small.
(c) Cell walls are primary in nature, thin and cellulosic.
(d) Cells are rich in protoplasm, possess large conspicuous nuclei.
(ii) Elongation phase : The cells proximal to the meristematic zone represent the phase of elongation. Cells in this region are characterised by :
(a) Increased vacuolation (b) Cell enlargement (c) New cell wall deposition
(iii) Maturation phase : The cells more proximal to the phase of elongation represent the phase of maturation.
Cells of this zone, attain their maximal size in terms of wall thickening and protoplasm modifications.
Growth rates
The increased growth per unit time is termed as growth rate. Thus, rate of growth can be expressed
mathematically. An organism, or a part of the organism can produce more cells in two ways :
(1) Arithmetic (2) Geometric
(1) Arithmetic growth : In arithmetic growth cell undergoes mitotic cell division and produce two daughter cells.
Only one daughter cell continue to divide while the other differentiates and matures.
· The simplest expression of arithmetic growth is examplified by a root elongating at a constant rate.
· Mathematically, it is expressed as :
Lt = L0 + rt
Lt = Length at time 't'
L0 = Length at time 'zero'
r = Growth rate/elongation per unit time
· On plotting the length of the root against time,
a linear curve is obtained
(2) Geometric growth :
In geometric growth cell undergoes mitotic cell division and produce two daughter progeny. Both the progeny
cells following mitotic cell division retain the ability to divide and continue to do so.
Geometric growth has two phases - Lag and Log phase. When nutrients are limited the growth will be logistic
which show sigmoid growth curve.
Sigmoid growth curve is divided into following three stages :
(a) Lag phase : It is initial stage, where growth is slow.
(b) Log phase : At this stage growth is rapid at exponential rate.
(c) Stationary phase : At this stage, the growth slows down leading to a stationary phase.The exponential growth can be expressed as :
w1 = w0ert
w1 = final size (weight, height, number etc.)
w0 = Initial size at the begining of the period.
r = growth rate
t = time of growth
e=base of natural logrithm
ALLEN
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ALLEN Pre-Medical : Biology
(1) Arithmetic growth : In arithmetic growth cell undergoes mitotic cell division and produce two daughter cells.
Only one daughter cell continue to divide while the other differentiates and matures.
· The simplest expression of arithmetic growth is examplified by a root elongating at a constant rate.
· Mathematically, it is expressed as :
Lt = L0 + rt
Time
Height of the plant
Lt = Length at time 't'
L0 = Length at time 'zero'
r = Growth rate/elongation per unit time
· On plotting the length of the root against time,
a linear curve is obtained.
(2) Geometric growth :
In geometric growth cell undergoes mitotic cell division and produce two daughter progeny. Both the progeny
cells following mitotic cell division retain the ability to divide and continue to do so.
Geometric growth has two phases - Lag and Log phase. When nutrients are limited the growth will be logistic
which show sigmoid growth curve.
Sigmoid growth curve is divided into following three stages :
(a) Lag phase : It is initial stage, where growth is slow.
(b) Log phase : At this stage growth is rapid at exponential rate.
(c) Stationary phase : At this stage, the growth slows down leading to a stationary phase.
· The exponential growth can be expressed as :
w1 = w0ert
w1 = final size (weight, height, number etc.)
w0 = Initial size at the begining of the period.
r = growth rate
t = time of growth
e = base of natural logarithms
Here, 'r' is the relative growth rate and is also the measure of the ability of the plant to produce new plant
material, referred to as efficiency index. Hence, the final size of w1 depends upon the initial size, w0.
· If we plot the parameter of growth against time, we get a typical sigmoid or s-curve.
· A sigmoid curve is a characteristic of living organism growing in a natural environment (limited resources).
· It is typical for all cells, tissues and organs of plant. It is also idealised for cells in culture.
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