130 THE TWO PHASES IN PLANT LIFE 



This number will be found in all the cells of the embryo and 

 continue to characterize the subsequent development of the sporo- 

 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. The 

 evidence at present indicates that the chromosomes of the male 

 and female gametes retain their identity until the spore mother 

 cells are formed. When these cells divide there is a fusion of 

 the male and female chromatin substance, attended with a re- 

 duction division of the number of the chromosomes. If this 

 be the case it will be necessary to alter our conception of ferti- 

 lization for apparently the real fusion of the nuclear substance 

 of the gametes is effected in the formation of the spores rather 

 than in the union of the gametes and the formation of the 

 gametospore. 



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. 



