1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 255 
The young megaspores almost invariably have the tetra- 
hedral arrangement, as in fig. 59. Occasionally the bilateral 
arrangement is found, in which case the divisions so far as 
observed are successive (jigs. 60, 61). 
The further growth of the megaspores, the manner in which 
their walls are laid down, and the storing of reserve material, 
were not investigated. 
The arrangement and subsequent development of the trabec- 
ule and tapetum of the megasporangium offer, as is to be 
expected, a rather close homology to what is seen in the 
microsporangium. The trabecule are formed out of the same 
kind of cells as compose all the other parts of the young sporan- 
gium. I do not discover any grounds for considering them the 
product of a peculiar kind of growth. They are altogether 
unrecognizable in the young sporangium, and their position 
when first outlined seems to be determined by that of the 
mother cells. Not until these have been selected and con- 
siderably enlarged is it possible to distinguish the trabecule, 
which then appear as feebly-staining bands extending from 
front to back across the sporangium midway between the fertile 
cells. 
The cells of the trabecule proper undergo the same process 
of elongation and flattening, attended by elongation of their 
nuclei, that has been described as occurring in the microsporan- 
gium. The only noticeable difference is that in the megasporan- 
gium the trabecule are relatively fewer in number and more 
massive. For example, in one case, an exceptional one, I 
counted 400 cells in a cross section of a trabecula, whereas 1n a 
microsporangium the number of cells in a cross section of a 
trabecula rarely exceed fifty, and is oftener under twenty-five. 
This is only another way of saying that the process of steriliza 
tion has gone much further in the megasporangium than in the 
microsporangium, The total mass of the megaspore mother cell 
in a sporangium is only a small fraction of that of the combined 
microspore mother cells, though doubtless the total volume of 
the mature spores in the two cases is about equal. 
