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A Handbook of Biology
All plant cells are descendants of the zygote (fertilized egg). The zygote
develops into a mature plant through growth and differentiation forming
roots, leaves, branches, flowers, fruits and seeds. Then they eventually
die.
GRØWTH
Growth is regareded as one of the most fundamental and conspicuous
characteristics of a living being.
Growth is an irreversible permanent increase in size of an organ or its
parts or an individual cell.
It involves metabolic processes that consume energy.
PLÅÑT GRØWTH GËÑËRÅLLÝ ÏS ÏÑDËTËRMÏÑÅTË
Plant growth continues throughout the life due to the presence of
meristems.
Meristematic cells have capacity to divide & self perpetuate.
The growth where new cells are always added to the plant body by the
meristem is called open form of growth.
Primary growth: It occurs due to root apical meristem & shoot apical
meristem. It causes the elongation of the plants along the axis.
Secondary growth (In gymnosperms & dicots): It occurs due to lateral
meristems, vascular cambium & cork-cambium. It causes increase in
the girth of the organs.
GRØWTH ÏS MËÅSÜRÅBLË
At cellular level, growth occurs due to increase in the amount of protoplasm.
Increase in protoplasm is difficult to measure directly. So growth is
measured by parameters like increase in fresh weight, dry weight,
length, area, volume & cell number. E.g. A maize root apical meristem
can produce more than 17,500 new cells per hour. In the maize, growth
is expressed as increase in cell number.
Cell size: E.g. Cells in a watermelon can increase in size by up to 3,50,000
times.
Length: E.g. Growth of a pollen tube.
Surface area: E.g. Growth in a dorsi-ventral leaf.
PHÅSËS ØF GRØWTH
3 phases: meristematic, elongation & maturation.
Meristematic phase: It occurs in the meristems at the root apex & the
shoot apex. Here, cells have rich protoplasm and large nuclei. Cell walls
are primary, thin & cellulosic with abundant plasmodesmata.