DORMANCY: VIVIPAROUS SEEDS 431 



prevented from germinating partly by the low temperature, 

 and partly because of the necessity of after-ripening. In 

 October and November germination takes place in the 

 light ; the dark night periods have no inhibiting effect 

 because, although the day temperatures are high, and thus 

 favourable, the night temperatures are low. There is an 

 average difference of 20° C. 



Summary. — We have considered this question of dor- 

 mancy and delayed germination in some detail because it 

 is an admirable example of the variety of ways in which 

 external factors may influence the development of the young 

 plant and of the complexity of the interaction of these 

 factors with the special constitution of the embryo and its 

 protective coverings. In a great variety of ways the ger- 

 mination of the seed is postponed and its vitality retained, 

 often for long periods. Adverse conditions are guarded 

 against. Germination is delayed till a favourable season. 

 The sprouting of a single crop is spread out over a number 

 of years, and the chance of obtaining completely or excep- 

 tionally favourable conditions by some of the offspring is 

 greatly increased. 



§ 3. Viviparous Seeds 



We may here refer to the opposite conditions found in 

 the true viviparous seeds which germinate before separation 

 from the parent. The most striking example is offered by 

 the mangroves, e.g. Rhizophora and Bruguiera, in which 

 the seedling has extended its hypocotyl to a length of as 

 much as 18 in. before it falls from the tree. In the man- 

 groves, as in some other cases, e.g. Crinum and Melocanna, 

 the integuments are reduced or absent. Kidd (1914) 

 suggests that the immediate germination, or rather the 

 continuous development, is due to the absence of auto- 

 narcosis by the respiratory carbon dioxide, which this 

 condition of the integuments permits. Biologically the 

 vivipary of the mangroves is important in securing the 

 fixation of the seedling in mud subject to tidal submersion. 



