Fawner. — On Isoetes lacnstris, L. 
49 
which is to be accounted for by the slight increase in length of 
the cells of this region. The arrangement both of xylem and 
phloem is also here obscured, and the curvature makes it 
almost impossible to determine their exact relation, though it 
is improbable that they differ from that already described for 
the upper part of the leaf. 
The Sporangium . — The development of the sporangium has 
been so admirably worked out by Goebel 1 , who has corrected 
and extended the observations of other investigators, that I 
have nothing to add to his results, which all my own work 
has confirmed. A comparison with some of the younger 
stages of /. velata and /. Hystrix has not yielded any new 
matter of importance, though it is not improbable that a more 
extended comparative research may throw light on the ques- 
tion as to the uni- or multi-locular nature of the sporangium 
in this genus. The facts are, briefly, that each hypodermal 
archesporial cell has a separate and individual growth ; that 
certain of these form the sterile trabecular tissue, whilst the 
others form the sporogenous tissue. The exact development 
of the latter varies according to the macro- or micro-sporangial 
nature of the body in question. The tapetum in both cases 
is derived partly from the sporogenous archesporial cells, 
partly from the trabeculae, and, according to Goebel, partly 
also from the cells at the back of the archesporium : my 
observations have made me a little uncertain on the latter 
point, but it is in any case of no great morphological import- 
ance, as is shown by the variation which exists in this respect 
in Phanerogams. The trabeculae, then, really appear to serve 
two functions ; first to provide a large amount of the tapetal 
nourishment, and secondly to provide a support for the outer 
wall of the somewhat bulky sporangium, and indeed from the 
close manner in which the leaves are packed together on the 
stem and the mutual pressure they must exert, it is probably 
a matter of some importance that some such protection should 
be supplied. The formation of sterile tissue, which is homo- 
logous with the actual sporogenous tissue, is well known in the 
1 Goebel, Beitr. z. vergl. Entw. d. Sporangien, Bot. Zeit. 1880. 
E 
