NATURE OF PLANTS 123 



their cells contain. The cells of the gametophyte generation 

 contain only one-half the number of chromosomes found in the 

 cells of the sporophyte generation. If the nuclei of male and 

 female gametes contain twelve chromosomes (the number varies 

 in different plants) the gametospore which marks the beginnings 

 of the asexual generation will contain twenty-four chromosomes. 

 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 

 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 chromatic 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 generations characterizes 

 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, (1) 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 (1) 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 



