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A Handbook of Biology
DËVËLØPMËÑT
It is a process that includes all changes
in the life cycle of an organism from seed
germination to senescence.
It is the sum of growth and differentiation.
Plants follow different pathways in response
to environment or phases of life to form
different kinds of structures. This ability is
called plasticity. E.g. heterophylly in cotton,
coriander and larkspur. In such plants, the
leaves of the juvenile plants and mature
plants are different in shape.
On the other hand, difference in shapes
of leaves produced in air and those
produced in water in buttercup also
represent the heterophyllous development
due to environment. This phenomenon of
heterophylly is an example of plasticity.
FÅÇTØRS ÇØÑTRØLLÏÑG THË DËVËLØPMËÑT
INTRINSIC FACTORS: Include intracellular (genetic) or intercellular factors
(such as plant growth regulators).
EXTRINSIC FACTORS: Include light, temperature, water, oxygen, nutrition,
etc.
PLÅÑT GRØWTH RËGÜLÅTØRS (PLÅÑT HØRMØÑËS ØR PHÝTØHØRMØÑËS)
Plant growth regulators (PGRs) are small, simple molecules that regulate
plant growth. Based on the functions, PGRs are 2 groups:
Plant growth promoters: For growth promoting activities like cell division
& enlargement, tropic growth, pattern formation, flowering, fruiting &
seed formation. E.g. auxins, gibberellins and cytokinins.
Plant growth inhibiters: For growth inhibiting activities like dormancy &
abscission. Respond to wounds & stresses of biotic and abiotic origin.
E.g. abscisic acid & ethylene (Ethylene fits either of the groups, but it is
largely a growth inhibitor).
1 Auxins
Charles Darwin & his son Francis Darwin observed that the coleoptiles of
canary grass responded to unilateral illumination by growing towards
the light source (phototropism). It was concluded that the tip of coleop-
tile caused the bending of the entire coleoptile.
(NEET 2015)
Juvenile
Terrestrial
habitat
Water
habitat
Adult
Heterophylly in Larkspur
Heterophylly in Buttercup