No. 463-] STUDIES ON PLANT CELL.— VI. 47 I 



Nat., vol. 38, p. 726, Oct., 1904) there is probably a deep sig- 

 nificance in the fact that two mitoses are almost universally 

 present in the spore mother-cell so that four spores are formed. 

 It is probable that these mitoses are always heterotypic and 

 homotypic, although this fact has only been clearly established 

 in comparatively few favorable forms, and that they are indis- 

 pensable to the mechanism of reduction phenomena. The latest 

 accounts describe the first mitosis as the separation of the two 

 portions of a bivalent chromosome, that is of two chromosomes 

 joined either side by side or end to end, giving it a unique posi- 

 tion among the mitoses of the life history. According to these 

 theories the two mitoses of sporogenesis are features of a 

 remarkable mechanism by which the paternal and maternal 

 chromatin after its union in synapsis may become distributed in 

 proportions that can be expressed by mathematical ratios. The 

 peculiarities of the homotypic mitosis depend on a premature 

 fission of the daughter chromosomes of the heterotypic division 

 as will be explained in the next portion of this section. Thus 

 the four spores are the result of these peculiar mitoses and have 

 morphological significance. We are even justified in suspecting 

 that the groups of four spores when found in the thallophytes, 

 as the tetraspores of Dictyota and the red algae, the four spores 

 formed on the basidium and promycelium and the four spores of 

 nuclei present in the germinating oospore and zygospore of 

 CEdogonium and the COnjugales indicate the presence of reduc- 

 tion phenomena simply because the number four is so constant. 

 Williams (: 04a) for Dictyota and Blackman (: 04b) for types of 

 the Uredinales have discovered clear cytological evidence of this 

 reduction phenomenon but we know nothing of the chromosome 

 history in other types^ ikiMji^tj^^ . 



We have already referred to the fact (Section III, Amer. Nat., 

 vol. 38, p. 743, Oct., 1904), that in the spermatophytes the 

 two mitoses characteristic of sporogenesis are very close to the 

 mitoses which differentiate the gamete nuclei. In the male 

 gametophyte of the Angiosperms there are generally only two 

 mitoses between the events of sporogenesis and gametogenesis 

 and in gymnosperars there is a somewhat larger and variable 

 number. The female gametophyte of the angiosperms usually 



