i. Plant growth:
A vital process which brings about a slow, permanent and irreversible increase in size or volume or mass accompanied by an increase in dry weight.
A process in which there is sequence of qualitative changes, towards a higher or more complex state. Plants exhibit a regular sequence of seed germination, growth, differentiation, maturation, seed formation and senescence.
iii. The growth of a multicellular organism can be divided into 3 phases i.e., cell division, cell expansion and cell differentiation.
i. Internal inhibition of germination of an otherwise viable seed even when it is placed in most favourable external conditions is called as dormancy.
ii. The suspension of growth of a viable seed may be due to exogenous control such as a change in environmental conditions and is called as quiescence.
iii. Dormancy of seed may be due to:
Tough seed coats (e.g., Capsella, Lepidium), rudimentary embryos (Erantliis liiemalis), impermeable seed coat (Chenopodium, Brassica, Xanthium), mechanically resistant seed coats, excess salts (Atriplex), after ripening (Oat, Barley, Wheat/and even by the presence of germination inhibitors e.g.,, abscisic acid, phenolic acids, short-chain fatty acids and coumarin).
iv. Natural breaking of seed dormancy:
Leaching of inhibitors and salts ; development and after-ripening of embryo ; development of growth hormones ; destruction of inhibitors by heat, cold, light and oxidation ; weakening of impermeable and tough seed coats by mechanical abrasions, microbial activity and digestive eyzymes of the alimentary canal of animals.
v. Artificial breaking of seed dormancy:
By mechanical scarification (machine threshing, chipping or filing), chemical scarification (hot water, fat solvents, mineral acid etc.), vigrous shaking, pressure (about 2000 kg for 5-20 min), chilling treatment, light (red light breaks domancy and induces seed germination in Lettuce and Tobacco), counter acting inhibitors (soaking seeds in KNO , thiourea, ethylene chlorohydrin, gibberellin etc.).
vi. Seed germination:
This is the first step towards plant growth. After seed dormancy and necessary conditions for growth are present, the dormant embryo resumes metabolic activities and growth. This process is called seed germination.
vii. Water, oxygen and temperature are the essential condition for seed germination. Red region of visible spectrum is most effective for the onset of seed germination.
viii. Internal factors for seed germination are completion of after-ripening, completion of seed-dormancy, maturity of embryo, presence of sufficient food and viability.
Phenomenon of producing young ones in advanced state of development. Seed germinate inside the fruit, while still attached to the parent plant. It is found in mangrove plants e.g., Rhizophora, Sonneratia and Heritiera, such plants grow in marshy land. The seedling of such plant is heavy and falls down into salt rich marshy water with plumule remaining outside.