240 Campbell — On the development 
cell has a small amount of granular content. The central part 
of the antheridium shows plainly the original bipartition, the 
sperm-cells being arranged in two groups. Numerous cases were 
observed(Fig. 9 <2) which looked as if sometimes a sterile cell were 
formed within the central cell, although this appearance may 
have been due to the formation of a large intercellular space. 
At maturity the sperm-cells are much crowded, and so nearly 
fill up the antheridium, that at first sight there appear to be no 
peripheral cells ; but a careful examination shows that they 
are not in immediate contact with the endosporium, but are 
separated from it by a more or less evident space. The cap- 
cell is also somewhat difficult to detect at this stage, but just 
before the antheridium opens it absorbs much water, and 
becomes then very conspicuous (PI. XIII, Fig. 14 c). Owing to 
the destruction of the starch through heating, the whole pro- 
thallium appears much more transparent than in life. 
The mechanism of opening seems to be the same as in other 
Pteridophytes. The parietal cells absorb a great deal of 
water, becoming in consequence very turgid. At the same 
time the mucilaginous walls of the ripe sperm-cells also absorb 
water, increasing their volume thereby, and at the same time 
freeing the separate cells. The tension finally becomes so 
great that the wall of the antheridium is ruptured and the 
sperm-cells are forced out. The opening usually occurs be- 
tween the cap-cell and the second parietal cell. As the 
internal pressure is removed, the parietal cells, in case they 
have not been ruptured, become much distended, and in the 
few cases where it was possible to free the empty antheridium 
from the spore, the parietal cells were found to project into 
the cavity of the antheridium, nearly filling it. Owing to 
the thinness of the walls the empty antheridium usually 
collapses, so that it is not easy to follow the outlines of the 
cells. 
The presence of parietal cells in the antheridium of Pihdaria 
corresponds with the later investigations in regard to other 
heterosporous Pteridophytes. Belajeff 1 demonstrated the 
1 Belajeff, in Bot. Zeit., 1885, pp. 793-809. 
