242 BOTANICAL GAZETTE [APRIL 
of the sporangium.” It has already been said that Hofmeister 
was in error because of failure to notice the lateral extension of 
the sporangium rudiment. Even allowing for this, however, I 
am not able to agree fully with his account. It appears rather 
that the upper tier of cells while giving rise to the velum makes 
some additions at the same time to the sporangium. In other 
words, the velum is a sterilized portion of the sporangium. Some 
sections seem to admit of this interpretation only, though others, 
such as fig. 32, are not unfavorable to the view that the separa- 
tion of velum and sporangium proper is accomplished by the 
first transverse division of the sporangium rudiment. 
Early stages of the velum may be seen in figs. 32-36; it is 
at this time a transverse row of slightly projecting cells. Its 
cells soon become comparatively empty, contrasting strongly 
with the young sporangium. Growth is very rapid and in an 
upward oblique direction; in some cases there is a tendency toa 
downward growth also, such as obtains among terrestrial 
species. 
The velum reaches its full size much sooner than the spo- 
rangium, and is not affected by the changes which determine the 
character of the latter. The cells of the interior become large 
and lose their contents; those of the inner surface layer— that 
adjacent to the sporangium—are smaller and more regular in 
size and outline, and have a semi-glandular appearance. In 
many species of Isoetes many of the cell walls of the velum and 
of the leaf region adjoining the ligule become lignified and take 
on spiral and annular thickenings. J. echinospora and J. Engel- 
manni offer no exception in this respect, the thickenings being 
much more pronounced in the latter species. The change os 
appears in proximity to the ligule, and spreads thence into the 
remoter parts of the velum and of the leaf. The thickened cells 
never have any connection with the vascular bundle (figs. 23; 24): 
FURTHER DEVELOPMENT OF THE MICROSPORANGIUM. 
In origin the two kinds of sporangia are identical, and for 
a considerable period. of their development they exhibit no 
