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 ovarj', 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; the .sex cells, somewhat gen- 



r ,1- ,• 1,1 ,1 ,- eralized from a tvpical case; 



tor polhnation and the growth oi x 375. 



the pollen tube are merely the -^^ the end of the pollen 



, . ^ ... , , " tube {D of Fig. ISS), contain- 



mechamsm for brmging the sex i^g two sperm nuclei, sk- b. 



cells together. Fertilization occurs the same tube in contact with 

 ,, , ,. ,, ,11 an embrvo sac, ™ ; C, a sperm 



m the reproduction of nearly all nucleus, sk. has entered the 

 plants and animals, and while the <?ge cell, the nucleus of which, 



, . ^ , . . , , ek, it has approached ; D, the 



mechanisms for bringing the sex ^perm nucleus, sk. has lost its 



cells together are as diverse as elongated form and become 

 ., , j^, i 1 r J !• .1 rounded like the egg nucleus, 



possible, the central feature of the „^th which next it fuses com- 



fusion, especially of the nuclei, is Pletely. (Reduced from Stras- 



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 



