184 Campbell ’ — On the Development of 
When Azolla is compared with the other Pteridophytes, it 
is evident enough that its nearest affinities are with the homo- 
sporous Filices. We have already indicated the very clear 
resemblance to these in the development of the sporangia and 
the indusial nature of the sporocarp wall. When a nearer 
comparison is made it seems probable that it is with the 
lower members of the Leptosporangiate series that its affinities 
are most marked. The form of the indusium recalls some of 
the Cyatheaceae and Hymenophyllaceae, and the leaves in 
their earlier stages resemble those of some of the simpler forms 
in the latter family. 
We may conclude, then, that the two families of the 
Hydropterideae represent the ends of two different lines of 
development Of these the Salviniaceae have been derived 
from the lower members of the Leptosporangiate series, pos- 
sibly from near the Hymenophyllaceae, and that the Mar- 
siliaceae have arisen from forms more like the Polypodiaceae. 
Of the two families, the Salviniaceae have departed less from 
the parent stock in regard to the reduction of the sexual 
generation, but the sporophyte is much less like that of the 
ordinary homosporous forms than that of the Marsiliaceae. 
The two genera of the Salviniaceae differ much more from 
each other than do those of the Marsiliaceae, and it is not at 
all likely that one form has been derived from the other, but 
that the two genera diverged at an early stage in the develop- 
ment of the line. 
