Ch. VI, 4] NATURE OF FERTILIZATION 



279 



ordinarily wither and fall away, 

 leaving only the ovary on the re- 

 ceptacle. Then this ovary grows 

 into a fruit, the ovule into a seed, 

 and the fertilized egg cell into an 

 embryo plant. In case, however, 

 no fertilization is effected, the parts 

 of the flower usually persist some- 

 what longer than otherwise, though 

 no fruit, seed, or embryo is 

 formed; but presently all parts, 

 including the ovary, wither and 

 fall. This is the way in which 

 flowers are essential to the pro- 

 duction of seed. 



4. The Nature and . Conse- 

 quences of Fertilization 



Fertilization in flowers, as the 

 preceding section has shown, cen- 

 ters in the fusion of the male and 

 female nuclei within the egg cell; 

 for pollination and the growth of 

 the pollen tube are merely the 

 mechanism for bringing the sex 

 cells together. Fertilization occurs 

 in the reproduction of nearly all 

 plants and animals, and while the 

 mechanisms for bringing the sex 

 cells together are as diverse as 

 possible, the central feature of the 

 fusion, especially of the nuclei, is 

 always the same. Thus this fusion 

 act of fertilization runs as a thread of structural and physi- 

 ological identity almost throughout the plant and animal 

 kingdoms, binding plants and animals together in this 



Fig. 192. — The fusion of 

 the sex cells, somewhat gen- 

 eralized from a typical case ; 

 X375. 



A, the end of the pollen 

 tube (D of Fig. 188), contain- 

 ing two sperm nuclei, sk ; B, 

 the same tube in contact with 

 an embryo sac, en; C, a sperm 

 nucleus, sk, has entered the 

 egg cell, the nucleus of which, 

 ek, it has approached ; D, the 

 sperm nucleus, sk, has lost its 

 elongated form and become 

 rounded like the egg nucleus, 

 with which next it fuses com- 

 pletely. (Reduced from Stras- 

 burger.) 



