180 
Dehiscence of the Sporangium of Adiantum pedatum.* 
By FLORENCE MAY LYON, 
The sporangia of Adiantum pedatum are fixed by slender 
stalks to the under surface of the pinnule whose reflexed margin 
forms the indusium. Each sporangium consists of a flattened, 
somewhat obovate sac. Its walls, consisting of a single layer of 
cells, are—with the exceptions given below—composed of pure 
cellulose. Extending vertically about two-thirds around the 
sporangium is the annulus. The walls which separate adjacent 
cells of the annulus, together with the floor of each cell, are much 
thickened and lignified, while the external wall, which curves over 
the edge of the sporangium in such a manner as to form a half 
cylinder, is a thin membrane of cellulose. In the remaining one- 
of the circumference, about midway be- 
tween the end of the annulus and the 
stalk, are two narrow and elongated cells 
with thick lignified walls. It is between 
these two cells, along a definite line, that 
3 dehiscence always begins. That this is 
the point of greatest weakness is evident 
from the fact that if an unopened ripe 
sporangium be put into Schulze’s mac- 
erating fluid, almost immediately, and 
before any other rupture is seen, these 
two lip-cells gape apart. The same result 
may be produced by strong potash. 
Ripe sporangium after © Moreover, the margins of the slit are 
ie clear cut, and never jagged, as if torn. 
These two cells seem to have been overlooked hitherto, or to 
have been considered of no importance, for they are rarely rep- 
resented in the cuts of fern sporangia, and dehiscence is described 
as taking place transversely, somewhere or anywhere between 
the end of the annulus and the neck. The constancy of this 
structure in the sporangia of the true ferns seems to indicate its 
importance. In the ferns mentioned below, I have found these 
modified cells, with slightly varying details of structure, and have 
* Read before the Botanical Club, A. A. A. S., New York, Aug., 1887. 
