t 
256 BOTANICAL GAZETTE [ APRIL 
The tapetum is formed in this case also out of those layers 
of the sterile cells which border upon the fertile cells. No doubt 
a considerable part of it is derived from the unsuccessful mother 
cells; but as these are the homologues of the trabecular cells of 
the microsporangium, being merely sterile sporogenous cells, the 
homology of tapetum and trabecule in the two sporangia is 
complete. The only difference which it is necessary to notice 
is the greater abundance of the tapetum in the megasporangium. 
Instead of being a single layer it is several layers in thickness 
( figs. 57, 58), and often projects into the sporangial cavity 
in the form of irregular papilla, especially from the base of the 
sporangium. A rounding up of the cells immediately about the 
megaspore mother cells, such as is described and figured by 
Goebel, I was never able to find. 
Though the megaspore mother cells do not lie in contact 
with one another as the microspore mother cells do, but are 
isolated in groups of one or sometimes two, the cavities in which 
they lie become continuous in the older sporangia. This is 
brought about by a very great enlargement of the cavities after 
the formation of the spores. The enlargement seems to be due 
to turgescence, induced probably by the osmotic activity of the 
substances surrounding the spores. It cannot be accounted for 
by mere growth of the wall cells, nor by that of the young 
spores, for these do not completely fill the cavities. I have 
computed the enlargement of the megasporangium after all cell 
divisions have ceased to amount to an increase of three or four 
times in volume. A similar change of size, though less in 
extent, occurs in the microsporangium. 
If the preceding account of the development of the spo- 
rangia, especially of the megasporangia, be compared with the 
account given by Goebel (1) and Sadebeck ( 1), it will be seen that 
the differences are considerable, and of much theoretical impor- 
tance. According to these writers certain cells of the arche- 
sporium divide only by the periclinal walls which serve to 
cut off the primary tapetal cells. In these no anticlinal divisions 
occur. One cell of each of the rows formed in this manner, 
