i 38 Johns on. — On the Development of the 
before this happens the sori may appear opposite each other, 
though they are really alternate in origin, as we have seen 
above (Fig. 34). 
Finally we come to speak briefly of the last six or seven 
pairs of segments of the capsule, beyond the youngest soral 
segments. In these there is no single dorsal bundle, as this 
divides to two just beyond the origin of the lateral bundles 
of the last pair of sori. These two divisions run along nearly 
parallel to each other near the dorsal wall of the capsule 
(Fig. 44}, and each gives off three or four branches which 
arise, like those in the soral segments, from the plerome of 
sections III and IV, and are joined like those also with their 
fellows of the same side near the ventral margin. The exact 
region of origin of the two divisions of the dorsal bundle was 
not made out satisfactorily. All the plerome of this region, 
except the little devoted to the dorsal and lateral bundles, 
is apparently devoted to the formation of the gelatinizing 
tissue of this part of the capsule. 
Summary and Conclusions. 
The leaves of Marsilia arise in two rows on the stem each 
from a cell quite near the growing-point. The two-sided 
apical cell formed in this leaf mother-cell cuts off fifteen pairs 
of segments, and these are divided by radial anticlines into 
six main divisions, five sections, and an ultimate marginal cell 
of the sixth grade. Four of these divisions on each side take 
part in the formation of the axial bundle of the petiole, while 
all of them help to form the mesophyll and epidermal tissues. 
One quarter of the vascular tissue contributed by section II 
develops without further division to the large trachea of its 
side of the bundle, which has its oblique end-walls always 
inclined in the same direction. Fourteen air-canals are formed 
between the 'mesophyll and hypodermis of the petiole, and 
a single longitudinal row of the mesophy 11-cells gives rise 
to both the longitudinal and transverse partitions between 
