1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 245 
or in the size and appearance of their nuclei, nor is there anything in 
their arrangement to suggest a difference in their origin or growth. 
In fact, as Professor Bower has pointed out, there is here a most 
excellent illustration of the sterilization of sporogenous tissue. 
The trabecule at this age show about 15—25 cells in cross- 
section (tangential section of the sporangium), and are more or 
less cylindrical. There is as yet no tapetum. Towards the outer 
and inner sides of the sporangium the trabecule are continuous 
with about three layers of cells which form the sporangium wall 
(jig. 49). That the trabecule and walls are of the same nature, 
both being the result of sterilization of potentially sporogenous 
tissue, is proved not only by the similarity of their cells, and 
their passing uninterruptedly into one another, but also by their 
relation to the tapetum, which is formed out of the layer that 
lies next to the spore mother cells. 
The inner cells of the trabecule, those which become the 
trabecule proper (7. ¢., exclusive of the tapetum), are at first 
isodiametric and in no way different from the outer ones. But 
while the latter are undergoing a transformation into tapetum, 
the former undergo changes which are dependent on the growth 
of the sporangium. As the dimensions of the sporangium 
increase —a change which goes on rapidly at the period when 
the sporogenous cells are multiplying — the trabecule are nec- 
essarily lengthened. This is accomplished, not by division of 
the cells, but merely by their elongation. At the same time 
they suffer a lateral compression from the growing sporogenous 
cells and become flattened (jig. 50). The tabular form of the 
cells doubtless furnishes the ground for the common view, which 
ascribes the form of the cells to the direction of their division 
planes. Such a view is incorrect, however, for divisions have 
entirely ceased in this region before the elongated form of the 
cells is attained. The shape of the cells is easily accounted for 
by their growth in the one direction possible for them while 
yielding to the pressure of the turgescent mother cells. 
In this connection it may be remarked that with the possible 
€xception of the tapetum all the cells of the sporangium, after 
