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1899] ORIGIN OF THE LEAFY SPOROPHYTE 53 
sporophyte, then it is evident,as Bower has shown, that the 
leaves of the latter are the result of progressive sterilization, 
and are secondary structures of the sporophyte. But if some 
other origin of the leafy sporophyte is possible, the leaves may 
not have arisen as secondary structures. 
It may be well to trace briefly the origin of gametophyte 
leaves, as exhibited by the mosses, since the sequence of events 
seems fairly clear,and may prove suggestive. Among the 
Riccia forms the thallose body produces sex organs and does 
chlorophyll work with no special differentiation of regions. 
From this condition there is evident a tendency to segregate the 
sex organs into definite regions, so that eventually the region of 
the body devoted to sex organs becomes quite distinct. The 
differentiation of a sex organ region is still further emphasized 
by its separation from the rest of the body by being carried up 
upon a vertical branch, an extreme case being displayed by 
Marchantia. Asa result, the chief chlorophyll work and the 
production of sex organs are distinctly set apart by the organi- 
zation of a gametophore arising from the thallus. 
The gametophore, primarily a sex organ branch, proves to 
be more favorable for the display of chlorophyll tissue than the 
thallus, and the simple leaves of mosses appear, supplementing 
the chlorophyll work of the thallus. In sphagnums the thallose 
body continues associated with the leafy gametophore. In the 
true mosses, however, the chlorophyll work of the gametophyte 
'S more or less given over to the gametophore leaves, and the 
thallus region is reduced to the so-called ‘‘protonema.” Ina 
very true sense, therefore, the gametophyte is always a thallus, 
special vertical or radial branches being developed in liverworts 
as gametophores, and in mosses as leafy gametophores. The 
loose habit of homologizing the leafy ‘‘moss plant” with a liver- 
wort thallus on the one hand, anda fern prothallium on the other, 
's not merely bad morphology, but is apt to be very misleading. 
The suggestion to be obtained from this history is that leaves 
may develop in response to more favorable conditions for their 
work, and such development may result in the great reduction 
