Leaf and Sporocarp in Marsilia quadrifolia , L. 137 
In the development of the sporangial cells just described, 
the plerome of the acroscopic part of the basiscopic half of 
section VI has played an important part. The cells derived 
from this ( pa. b., Figs. 35, 36) push in back of the swelling 
macrosporangium mother-cell and between the placental cells 
(pa., Figs. 37, 41) derived from the primary microsporangium 
mother-cells. Most of these cells derived from section VI 
form the middle portion or axis of the placenta, but a row of 
them next to the base of the macrosporangium (pa. b., 
Fig. 36), and running the whole length of the sorus (pa. b., 
Figs. 32, 33, 44), develop the vascular bundle of the placenta, 
while at a point about opposite the middle of the sorus these 
same cells become modified across the whole width of sec- 
tion VI (pa. br., Figs. 31-33, 42, 44), to form the placental 
branch connecting the placental bundle with the lateral 
bundle. 
During this activity of the other cells of the sorus the 
indusial cells have been developing also. The acroscopic part 
of section V and the acroscopic marginal cell have each split 
by a transverse anticline (Figs. 35, 36) to form, in connexion 
with the cells of section II, the complete outer layer of the 
indusium (o. ind ., Figs. 35-37, 41, 43). The inner indusial 
cells derived from the basiscopic marginal cell and the basi- 
scopic portion of section V (i. ind., Figs. 32-37, 41, 43) 
complete the inner layer also. Each of these layers remains 
one cell in thickness throughout ; but by growth of the cells 
in a direction parallel to, and division by walls perpendicular 
to the surface of the indusium, the latter pushes out so as 
to accommodate the growing sporangia. During the growth 
of the indusium intercellular spaces appear at many points 
between the two layers (Fig. 41), and other larger ones 
between the outer layers of the indusia of adjacent sori, both 
laterally and along the median wall (i-s.c , Figs. 41, 42). 
By the increase in size of the latter spaces the indusia of 
adjacent segments become entirely separated, and the sori 
of each side of the capsule push into the furrows between 
the sori of the opposite side (Fig. 42). At a time a little 
