NATURE OF PLANTS 137 



phyte. When, however, the spore mother cells appear in the 

 anthers and ovules it is to be noted that these cells in dividing 

 to form the micro- and mega-spores reduce the number of 

 chromosomes by one-half so that the nuclei of these spores and of 

 all cells derived from them contain but twelve chromosomes. 

 For this reason the division of the spore mother cell is referred 

 to as the reducing division. The significance of this difference 

 in the number of chromosomes is not known but it furnishes a 

 basis for the sharp separation of the two generations. 



A sexual and an asexual phase or generation characterize 

 the life history of nearly all the higher plants and the succes- 

 sion of these two phases is called the alternation of generation. 

 In the case of the bean we have as the most important features 

 of the sporophyte or asexual generation, (i) the gametospore, 

 (2) the seed, and (3) the flowering bean plant. The micro- 

 scopic plants that comprise the gametophyte or sexual genera- 

 tion are characterized by (i) the micro- and mega-spores which 

 develop respectively into (2) a tubular growth of three cells 

 and (3) a sac-like growth of seven cells, each of which contains 

 one or more (4) sexual cells or gametes. These two phases are 

 called respectively the sporophyte or asexual generation, and the 

 gametophyte or sexual generation, since in the first phase mother 

 cells are developed which form spores, i. e., this is the spore 

 forming or asexual generation and in the second phase gametes 

 or sexual cells are produced. 



58. Significance of Fertilization. Let us now stop to con- 

 sider the meaning of the complicated process that we have termed 

 fertilization. No satisfying explanation has been offered. Some 

 see in these changes only a process of nutrition and chemical 

 stimulation. The gametes are lacking in certain materials that 

 are essential to their further growth. According to this view 

 the male gamete is the complement of. the female and by the 

 union of the two all the substances are supplied that are neces- 

 sary for growth. 



It has also been suggested that the growth culminating in 

 fertilization is attended with the removal of impurities from the 

 sexual cells. Every cell has its growth, maturity, and senescence 

 10 



