germination

germination

Fig. 171 Germination . A typical seedling.

germination

1. the removal of a germination inhibitor which can be leached out by water.
2. a period of cold temperature (STRATIFICATION).
3. exposure to the correct wavelength of light to stimulate PHYTOCHROMES in the seed, for example, lettuce seeds require red light for germination and are inhibited by far-red light.
4. rupture of a thick testa by (i) microbial breakdown, (ii) abrasive action of soil (as in desert plants), (iii) heat from bush fires, (iv) the effect of digestive juices (as when eaten by birds), (v) the softening action of water. Once dormancy is broken a regular sequence of events takes place: water is imbibed which hydrates the tissues increasing enzymic action in the ALEURONE layer (when present), a process aided by the release of GIBBERELLIN from the embryo; food stores are mobilized by production of enzymes in the ENDOSPERM or within the COTYLEDONS; AUXINS and CYTOKININS are formed which promote cell division and enlargement, causing the embryo to grow and burst through the testa. See Fig. 171 .

There are two main types of germination, depending on whether the seed cotyledons are carried above the soil (EPIGEAL) or remain below ground (HYPOGEAL).