1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 251 
volume of one mother cell nucleus. Very likely similar reduc- 
tions in the volume of the microspore nuclei occur during the 
tetrad division of other plants, but I have not seen any other 
case where the disparity of size is so great, nor do I remember 
to have read any record of such a reduction. 
The number of spores formed within a microsporangium is 
enormous—much greater than in any other living plant. Insome 
species it is said to exceed a million. But the largest number I 
have found in /. echinospora is 300,000. My estimates place the 
average number from 150,000 to 250,000. 
As is well known, no provision is made for the dehiscence of 
the sporangium wall. The spores are set free only by the decay 
of the tissues enclosing them. 
FURTHER DEVELOPMENT OF THE MEGASPORANGIUM. 
My observations on the development of the megasporan- 
gium differ very much from those of previous investigators, so 
very much, indeed, that I would be loath to present them at all 
had I not confirmed them again and again by long and careful 
study. These differences are concerned not only with the origin 
_ of the archesporium and early growth of the sporangium, 
which have been already spoken of, but they involve also the 
manner of selection of the mother cells and the origin and 
behavior of the tapetum. A discussion of the points at issue will 
be reserved until the general history of the megasporangium has 
been considered. 
One of the first megasporangia which I sectioned presented 
the appearance shown diagrammatically in fig. 67. The two large 
cells Mand M are evidently megaspore mother cells, but what 
is the group of cells a, corresponding to them in outline and 
position ? It consists of six cells in all, three in the section 
under examination, and three others in the adjacent section. A 
little search discovered other similar groups of a variable num- 
ber of cells, sometimes but two or three, often five or six. If 
the number had been constantly four the groups might have 
been regarded as spores resulting from a precocious division of 
