ee ae ee ee eee 
EEE Le OE NS eT ee ee ee ee 
1899] ORIGIN OF THE LEAFY SPOROPHYTE 57 
more difficult for a zygote to stop producing one distinct portion 
of the plant body, and to continue producing the other. 
Why in both cases it tends to produce the structure less imme- 
diately related to it, rather than the one which has originated it, 
is a question which cannot be answered at present. Cytology 
may offer certain suggestions, but they are vague as yet. The 
fact that the chromosomes are doubled in number by the process 
of fertilization, and are reduced again in the sporogenous tissue 
may have some bearing on the question. It seems clear that in 
all life histories where the sexual act occurs there must bea cor- 
responding reduction division somewhere. In distinct alterna- 
tion of generations, the ‘“ doubling”’ and the “reduction” are 
associated with the two generations. But before distinct alter- 
nation was established « doubling” and “reduction”? must have 
occurred, and there is no present reason to doubt that in such 
case reduction often, if not generally, occurred in connection 
with the development of spores. When, therefore, the zygote 
was restricted to one region of the body, and the spore to a very 
distinct region, the alternation of « doubling” and “reduction” _ 
might well develop into an alternation of generations. 
The very interesting results obtained by Strasburger and 
Farmer in their Study of Fucus, which show that the reduction 
division in that plant occurs in connection with the development 
of the sex organs, may be correlated with the absence of spores. 
Such an observation emphasizes the fact that reduction must 
occur somewhere, and if sporogenous tissue is not developed, it 
etn seem more likely to occur in gametogenous tissue, repre- 
oe a new cell sequence, than in ordinary nutritive tissue. 
ao an origin of the leafy sporophyte, it would follow 
thus dak ee are not secondary, but primary structures, and 
ledece Las ylls have arisen from the differentiation of foliage 
ing sporangia, a state of things certainly suggested 
ioe ust Primitive pteridophytes known. It would further 
Cw that the Evolution of the strobilus has followed the 
development 
of foliage leaves, a view in accordance with the 
old Be 
re morphology, Such a view would make intelligible the 
