Reproduction 355 



from the pollen-grain, the other from the embryo-sac in 

 the ovule. In the fusion of these nuclei, usually derived 

 from different organisms or flowers, the characters of two 

 individuals are fused. Two lines of ancestry are brought 

 together in one cell, the fertilized egg, or zygote, which 

 will develop into the embryo of the seed. It is important 

 to bear in mind some further details regarding the fertili- 

 zation process. 



The pollen, as has been noted, is a distinct phase of the 

 plant. It represents upon germination the complete 

 male gametophyte, whose reproductive function is the 

 production of a gamete. In most cases the pollen-grain 

 consists of merely two cells, a smaller cell practically within 

 a larger. The larger produces through germination a vege- 

 tative tube, the germ tube, which (in angiosperms) grows 

 through a differentiated portion of the style, or stylar canal ; 

 thence it penetrates the ovule, commonly through the 

 micropyle, until it comes in contact with the egg-apparatus, 

 and ultimately with the egg-cell. The smaller cell of the 

 pollen-grain is largely nucleus. The latter divides by the 

 time the pollen-tube breaks or ruptures, and one of these 

 two gametic nuclei fuses with the nucleus of the egg-cell, 

 the other gamete ; thus fertilization is effected. The 

 fertilized egg - - this single cell, or zygote - - is the be- 

 ginning of the new individual that is developed within 

 the protecting coats of the ovule (now the young seed), in 

 turn inclosed by the ovule-sac, and often by other parts 

 of the flower which may assist in the development of the 

 fruit. 



207. Universality of fertilization. - - It is remarkable 

 how universal is this phenomenon of fertilization. It 



