124 Plant Genetics 



(i) Such a study will test the. theories of inheritance 

 derived from the study of the sporophyte; (2) it deals 

 with less confusing material. 



In the first place, the genetics of gametophytes may 

 furnish a good test of the theories of inheritance derived 

 from the study of sporophytes. The chromosome 

 equipment of a gametophyte is exactly the same as the 

 chromosome equipment of a gamete, in both cases the 

 chromosome number being haploid. This results from 

 the fact that in plants the reduction division occurs in 

 connection with spore formation; it is only rarely that 

 it occurs in connection with gamete formation, as in 

 animals, and then only when spores are cut out of the 

 Ufe-history. Fig. 28 represents the typical life-history 

 of one of the higher plants, so far as the chromosome 

 situation is concerned. At the reduction division, 

 whose Mendelian significance has been considered, the 

 chromosome number is reduced frond 8 to 4; and as a 

 result the spore, the gametophyte, and the gamete 

 all contain 4 chromosomes. At fertilization two sets 

 of 4 chromosomes are brought together by the male and 

 female gametes, and therefore the zygote contains 8, 

 and the resulting sporophyte continues the 8 chromo- 

 somes. At a certain season or, better, under certain 

 conditions cells of the sporophyte are set apart to pro- 

 duce spores (spore mother-cells, each with 8 chromo- 

 somes), and the reduction division follows, each spore 

 receiving 4 chromosomes. This familiar picture of the 

 life-cycle may now be appUed to genetics. 



The fact that the chromosomes are reduced in num- 

 ber from 8 to 4 is not the significant fact; the significant 

 fact is how this reduction takes place, that is, not the 



