288 BOTANICAL GAZETTE [APRIL 
adopt the land habit, seems to account plausibly enough for the 
emergent archegonium and antheridium of the Marchantia and 
moss type, but it is less convincing as the ancestral form of 
sunken sex organs, particularly those which have “lost” (?) 
stalk, wall, and other sterile cells except the cover. The sunken 
sex organs of all plants develop from an epidermal cell of the 
dorsal surface of the gametophyte, except in ferns and the 
Anthoceros forms, in which they are frequently produced on 
both dorsal and ventral sides. This is true of Anthoceros, but I 
know of no authority that it is the case in Notothylas or Den- 
droceros. 
In speculating upon the early development of the sex organs 
and the significance of the variations noted above, one fact stands 
out prominently, that the cells concerned in the formation of 
the sex organs are more plastic and indifferent than has been — 
believed, and that the region giving rise to the sex organs not 
infrequently displays a latent gametogenous character. One 
might go back further to a stage preceding the emergent pluri- 
locular sporangium and postulate as a point of departure thallose 
algae with indeterminate masses of reproductive cells which pass 
through the plant from- one surface to the opposite. With 
increasing thickness of the thallus, sterilization will be most 
likely to appear at the center of the mass and, working pro- 
gressively from the centef outward in both directions, would 
result in layers of gametogenous or sporogenous cells 
surface. These concrete masses by further sterilization might 
become discrete patches or “organs,” if one chose so to desig- 
nate them. Cover cells would arise from sterilization of the 
superficial cells of these regions. Up to this point there might 
be isogamy, or even the indefinite condition that obtains in 
Ulothrix, where zoospores cannot be distinguished from gametes. 
Differentiation of sex cells could be accounted for by certain 
cells ceasing to divide earlier in their dévelopment than others, 
resulting in eggs, as contrasted with those that by repeated div!- 
sions work out sperms. In this way some of the primitive organs 
in all probability would be bisexual. The emergent sex ore 
could evolve from these sunken forms by pushing the steriliza- 
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