RESULTS OF FERTILISATION 399 



experimental work can tell us, in most cases, how much 

 the flower really depends on the insect. 



Cross-pollination is, almost certainly, the rule. This 

 supports the view that out-breeding is advantageous, and 

 emphasises the importance of this aspect of sexual repro- 

 duction. But we do not know much about the relative fre- 

 quency of geitonogamy, nor of its significance. We know 

 that autogamy occurs occasionally in many flowers, normally 

 in many others, and exclusively in at least a few. This last 

 fact seems to indicate that there is no essential rejuvenating 

 action in fertilisation, and makes its evolutionary aspect 

 more prominent. We do not know how general conditions, 

 such as are exhibited in the maize, may be in wild species. 

 The caution of Darwin's statement on out-breeding must 

 be maintained, and its confession of ignorance is still 

 applicable. 



§ 17. The Seed and the Fruit 



The essential result of fertilisation is the initiation in 

 the ovum of the process of development leading to the 

 formation of the new sporophyte, which shortly enters on 

 a state of rest as the embryo of the seed. The critical stage 

 in the transition from rapidly growing embryo to resting 

 embryo is accompanied by a marked loss of water, which 

 falls from round about 70 per cent, to round about 10 per 

 cent. Of the causes governing the change we know little. 

 Kidd (1914, Part II) has recently suggested that the 

 inhibition of growth is connected with the narcotic action 

 of carbon dioxide accumulated by the vigorously respiring 

 embryo. The embryo may be accompanied by an 

 endospermic food store of independent origin. 



Subsidiary Effects of Fertilisation. — The effects of ferti- 

 lisation are not confined to the initiation of growth and 

 division of the ovum ; they are felt, too, in various parts 

 of the flower — that is, in the parent sporophyte. Most 

 intimately connected w^ith the embryo are the changes 

 which result in the integuments of the ovule being converted 



