A zo l la filiculoides , Lam . 161 
evidently those of the tapetal cells (Fig. 21). It is evident at 
a glance that these nuclei are directly concerned in the growth 
of the spore, and that they play an important part in the 
formation of the extraordinary appendages of the ripe spore. 
Unfortunately, owing to my absence during the summer, 
I failed to get the later stages in the development of the 
macrospore, but to judge by analogy with the microspores, 
and also by what is known in the case of the Marsiliaceae, 
the outer spore-coverings are derived from the protoplasmic 
mass in which the macrospore is imbedded. This protoplasm 
is evidently active, as is shown by the presence of normal 
nuclei, as well as by its increase in bulk as the sporangium 
increases in size. 
When the sporangium is about half grown, the outer cells 
of the very short stalk grow out into short papillae (Fig. 16 
sp.), which apparently are abortive sporangia, as they show 
divisions which recall the earlier ones in the macrosporangium. 
Their position corresponds to that of the microsporangia, so 
that although formed much later than the macrosporangium, 
it is pretty safe to assume that Azolla is derived from some 
form in which, as in Salvinia , there were several macrosporangia 
in the sporocarp. 
The indusium grows much faster than the sporangium and 
soon completely encloses it. It grows mainly by the activity 
of the marginal row of cells, in which divisions are cut off 
alternately upon the inner and outer sides. After reaching 
above the top of the sporangium the edges converge, and this, 
together with an enlargement of the marginal cells, finally 
completely closes the opening. 
Before the opening closes, filaments of the Anabaena creep 
in and form a. mass filling all the space between the top of 
the sporangium and the opening of the sporocarp. Here the 
cells separate completely, and the plant enters a resting-stage 
to resume its activity on the germination of the macrospore. 
In studying the development of the macrosporangium, 
I could not but be struck by the extraordinary resemblance 
to that of an ovule. The close investment of the sporangium 
