1898] THE LEAF AND SPOROCARP OF PILULARIA Ba 
is continuous with this hypodermis laterally and ventrally (jigs. 
33,34). Fora narrow space in the dorsal region, however, the 
outer hypodermis from the dorsal wall of the capsule is seen in 
sagittal section to continue on down into the stalk, making a 
sharp bend and becoming much thickened just opposite the 
basal pit, while the similar layer of the basal wall laps over on to 
this above and abuts against the thin inner hypodermis (fig. 33). 
Horizontal sections (fig. 35) show that the region of overlapping 
is a very narrow one, and that this arises from the transverse 
division here of cells which are elsewhere undivided. Whether 
this division gives rise to an open slit is difficult to determine, 
but I believe that at maturity there is an actual opening here 
which may have the function, attributed by Russow to the simi- 
lar structure in Marsilia, of allowing an interchange of air 
between the capsule and the air canals of the stalk. 
Russow noticed the thinness and the bulging outward of the 
outer hypodermis just above the basal pit, as shown in horizontal 
section (fig. 34), but was unable to study it thoroughly from 
lack of material. The course of the light line at the point of 
overlapping is worthy of special notice. It moves toward the 
inner surface of the hypodermis (//, figs. 33, 34) and finally 
passes over into the basal wall (figs. 33, 35), and the thickening 
of the walls of the cells in these layers is seen to be definitely 
related to this line, the cell cavity increasing in size with the dis- 
tance out from this line toward the end of the cell (figs. 34, 
35), while where two layers are present (fig. 35), the cells of 
the outer one, in which the light line is wanting, have very 
slightly thickened walls. At the stomata also the light line is 
seen to bend outward to the guard cells, so that we may con- 
clude that this line indicates the distribution of some material 
which makes the hypodermis impervious to air or moisture, and 
is therefore present only where needed for this purpose. 
At the base of the capsule the inner hypodermal layer is 
wanting, and is replaced by several layers of brown-walled cells 
which form the inner portion of the basal wall ( fig. 33). Next 
to the prismatic outer layer of this wall these cells are closely 
