416 SPHENOPHYLLALES. B. PSILOTACEAE 
is attained. But the tissue of the normal septum is of common origin 
with the sporogenous tissue itself, and in the earliest stages does not differ 
from it in position or structure. The normal cevelopment, in fact, would 
be compatible with the view that the septum is formed from potential 
sporogenous tissue sterilised: the unilocular condition would then follow 
from reversion of that sterile tissue to its original fertile state again. 
This question is intimately connected with that of the origin of the 
sporangiophore, and opportunity will be taken later to discuss it. 
The detailed study of the sporangiophore or synangium of Pszlotum by 
means of sections is more difficult than that of Zmeszpferis on account of 
its trilocular character. In radial sections through the terminal bud the 
a B 
Fic. 231. 
Tinesipteris tannenss, Bernh. A, median section through synangium, showing the 
tissue where the septum normally is developing as sporogenous cells (s). ¢=tapetum. 
B, part of the contents of a similar synangium, rather older. x shows the line where 
the septum should normally be, while a chain of fertile cells stretches continuously across 
it. This drawing also indicates how cells distributed through the sporogenous tissue 
become disorganised. 100. 
young sporophylls are found to present a general outline and structure 
similar to those of Zmesipteris. Fig. 232 a shows one such: the cell (x) 
is believed to represent the organic apex of the sporophyll, though it is 
doubtful whether it be this initial which gives rise to the whole mass of 
the tissue. The sporangiophore appears as an outgrowth of the upper 
surface of the sporophyll, while the tissue on the abaxial side of it is 
already growing out into a bulky projection. But it has not been possible 
to trace the development of the essential parts of the loculi of the 
synangium from the superficial cells of the adaxial side of the sporophyll 
in this case with the same certainty as in Zmesépzeris: this is chiefly 
owing to the stalk being here narrower, and to the fact that only one 
loculus can be cut in a median direction in any one section; supposing 
this to be the median plane of the whole sporophyll, then it will be the 
