Process of seed germination
Most seeds geminate, when they are provided with water, oxygen and their dormancy is over. The main step of seed germination are described below.
The first step in germination is imbibition or uptake of water by the dehydrated seed. The seeds, when placed in moist soil, absorbs water through micropyle. Imbibition causes the seed to swell as the cellular constituents are dehydrated. Imbibition takes place iwth great force. It ruptures the seed coat and enables the radicle to emerge. It causes swelling of seeds and development of the great force called imbibition pressure. Dry seeds packed in a bottle containing water can crack it as they imbibe water and swell.
Imbition makes the embryo cells active and causes resumption of metabolic activities. Their respiration isinitially anaerobic. The cells possess some simple polysaccharides for functioning as respiratory substrate. When the anaerobic respiration reaches a peak,mitochondria differentiate in the embryo cells. The respiration now becomes aerobic as oxygen starts entering the seed coats.
The activated embryo cells induce the production of hormones and digestion of reserve food. Depending on the nature of seed, the resouce may be stored chiefly in the endosperm(e.g.castor, cereal grains and other monocots) or in the cotyledons(eg. many dicotyledonssuch as pea, gram, bean etc.) The cells which are rich in proteins produce and secretes hydrolyzing enzymes. These enzymes bring about the digestion of the reserve foods. The latter are changed into sugars, amino acids and other soluble substances. They are translocated to the embryo.
On the receipt of soluble food, the cells of embryo axis undergo division and expansion. the radicle end of the embryo axis is the first to enlarge. It grows out of the seed coats and passes downwardly into the soil toestablish itself as the primary root. The plumule also comes out of the seed and soil to establish itself as shoot.