1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 243 
observable difference. The general statement of the text-books, 
following Goebel and Sadebeck, is that they follow the same 
course of development only so far as the formation of the 
archesporium, and thereafter may be distinguished by their 
manner of growth. It is said that in the megasporangium certain 
archesporial cells divide only by periclinal walls, but in the 
microsporangium all the archesporial cells divide both anticli- 
nally and periclinally, and that in this respect the two are dis- 
tinguishable from the archesporial stage on. Such is not the 
case in the forms which J have studied. In these all the arche- 
sporial cells, whether of megasporangium or microsporangium, 
undergo divisions inall directions, and the similarity of the two 
kinds of sporangia continues much beyond the archesporial 
stage. Not only do they agree in origin, but up to a time when 
they are eight or ten cells deep, they agree absolutely in manner 
of growth, and exhibit no histological features by which one may 
determine whether a given sporangium will bear microspores or 
megaspores. 
As an example, consider the sporangium of which jig. 43 
represents a section. It had advanced so far beyond the arche- 
sporial stage as to contain about 8000 cells. From the position 
of its sporophyll we may infer it was destined to become a 
megasporangium. But there is nothing in the arrangement or 
character of the cells or in their mode of division to warrant 
that Prediction, or to enable us to say such a group of cells will 
become a trabecula, and such a group will produce spores. It 
has the characters neither of a megasporangium nor of a micro- 
. Sporangium, but is as yet quite undifferentiated. 
The first changes which occur to mark the microsporangium 
are those which lead to the differentiation of the spore mother 
cells from the trabeculz, sporangium wall, and tapetum. Pre- 
viously there has been no essential difference in the cells as to 
size, form, or contents, excepting the external layer. But when 
the Sporangium is approaching a limit of cell multiplication, that 
's, when the number of cells is 15,000—20,000, certain regions 
begin to lose their power of division and reaction to stains, while 
