No. 463.) STUDIES ON PLANT CELL — VI. 471 
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 algze, the four spores 
formed on the basidium and promycelium and the four spores of 
nuclei present in the germinating oóspore and zygospore of 
e Conjugales indicate the presence of reduc- 
CEdogonium and th 
the number four is so constant. 
tion phenomena simply because 
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. 
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 gymnosperms there is a somewhat larger and variable 
number. The female gametophyte of the angiosperms usually 
