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You have already studied the organisation of a flowering plant in Chapter
5. Have you ever thought about where and how the structures like roots,
stems, leaves, flowers, fruits and seeds arise and that too in an orderly
sequence? You are, by now, aware of the terms seed, seedling, plantlet,
mature plant. You have also seen that trees continue to increase in height
or girth over a period of time. However, the leaves, flowers and fruits of the
same tree not only have limited dimensions but also appear and fall
periodically and some time repeatedly. Why does vegetative phase precede
flowering in a plant? All plant organs are made up of a variety of tissues; is
there any relationship between the structure of a cell, a tissue, an organ
and the function they perform? Can the structure and the function of these
be altered? All cells of a plant are descendents of the zygote. The question
is, then, why and how do they have different structural and functional
attributes? Development is the sum of two processes: growth and
differentiation. To begin with, it is essential and sufficient to know that the
development of a mature plant from a zygote (fertilised egg) follow a precise
and highly ordered succession of events. During this process a complex
body organisation is formed that produces roots, leaves, branches, flowers,
fruits, and seeds, and eventually they die (Figure 15.1). The first step in the
process of plant growth is seed germination. The seed germinates when
favourable conditions for growth exist in the environment. In absence of
such favourable conditions the seeds do not germinate and goes into a
period of suspended growth or rest. Once favourable conditions return,
the seeds resume metabolic activities and growth takes place.
In this chapter, you shall also study some of the factors which
govern and control these developmental processes. These factors are both
intrinsic (internal) and extrinsic (external) to the plant.
P
LANT
G
ROWTH AND
D
EVELOPMENT
C
HAPTER
15
15.1 Growth
15.2 Differentiation,
Dedifferentiation
and
Redifferentiation
15.3 Development
15.4 Plant Growth
Regulators
15.5 Photoperiodism
15.6 Vernalisation
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240 BIOLOGY
15.1 GROWTH
Growth is regarded as one of the most fundamental and conspicuous
characteristics of a living being. What is growth? Growth can be defined
as an irreversible permanent increase in size of an organ or its parts or
even of an individual cell. Generally, growth is accompanied by metabolic
processes (both anabolic and catabolic), that occur at the expense of
energy. Therefore, for example, expansion of a leaf is growth. How would
you describe the swelling of piece of wood when placed in water?
15.1.1 Plant Growth Generally is Indeterminate
Plant growth is unique because plants retain the capacity for unlimited
growth throughout their life. This ability of the 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.
This form of growth wherein new cells are always being added to the
plant body by the activity of the meristem is called the open form of growth.
What would happen if the meristem ceases to divide? Does this ever
happen?
In Chapter 6, you have studied about the root apical meristem and
the shoot apical meristem. You know that they are responsible for the
Seed coat
Epicotyl
hook
Cotyledons
Cotyledon
Soil line
Epicotyl
Hypocotyl
Hypocotyl
Figure 15.1 Germination and seedling development in bean
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PLANT GROWTH AND DEVELOPMENT
241
primary growth of the plants and principally
contribute to the elongation of the plants along
their axis. You also know that in dicotyledonous
plants and gymnosperms, the lateral meristems,
vascular cambium and cork-cambium appear
later in life. These are the meristems that cause
the increase in the girth of the organs in which
they are active. This is known as secondary
growth of the plant (see Figure 15.2).
15.1.2 Growth is Measurable
Growth, at a 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, therefore,
measured by a variety of parameters some of
which are: increase in fresh weight, dry weight,
length, area, volume and cell number. You may
find it amazing to know that one single maize
root apical mersitem can give rise to more than
17,500 new cells per hour, whereas cells in a
watermelon may increase in size by upto
3,50,000 times. In the former, growth is
expressed as increase in cell number; the latter
expresses growth as increase in size of the cell.
While the growth of a pollen tube is measured
in terms of its length, an increase in surface area
denotes the growth in a dorsiventral leaf.
15.1.3 Phases of Growth
The period of growth is generally divided into
three phases, namely, meristematic, elongation
and maturation (Figure 15.3). Let us
understand this by looking at the root tips. 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 rich
in protoplasm, possess large conspicuous
nuclei. Their cell walls are primary in nature,
thin and cellulosic with abundant
plasmodesmatal connections. The cells
proximal (just next, away from the tip) to the
Shoot apical
meristem
Vascular
cambium
Vascular
cambium
Root apical
meristem
Shoot
Root
Figure 15.2 Diagrammatic representation of
locations of root apical meristem,
shoot aplical meristem and
vascular cambium. Arrows exhibit
the direction of growth of cells and
organ
G
F
E
D
C
B
A
Figure 15.3 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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