586 F. O. BOWER, 
to the section are formed at the peripheral part of the stem. 7. e. 
nearer the shaded portion of the figure, which represents the 
region of greatest vegetative activity at the base of the leaf. 
On comparing the description now given for the older plant 
with that before given for the younger plant, we must conclude 
that the vascular arrangement corresponds in the two cases, and 
that no fundamental change appears in the mutual vascular con- 
nections as the development of the plant proceeds. The chief 
difference lies in the fact that the bundles of the leaf trace do 
not all proceed for an equal distance into the stem in the older 
plant ; and this has an important bearing (1) upon the mode of 
increase in number of the bundles in the leaf as its development 
proceeds, and (2) on the arrangement of the bundles in the stock. 
in the older plant. 
It is an obvious fact, which may be observed on comparing 
plants of different ages, that the plumular leaves increase in 
width as they grow older, and that this is accompanied by an 
increase in the number of their vascular bundles. The question 
therefore arises—How and where the fresh bundles are formed ? 
The course of the first two bundles of the plumular leaf was 
described in my former paper (cf. fig. 15, 4). Comparing this 
with our fig. vir, we see that at first, at all events, the number 
of bundles is increased by the successive development of fresh 
bundles near to the margins of the leaves. In the case in question 
we have two pairs of such secondary bundles, but this mode of 
increase in number of the bundles is not continued in the later 
stages of development ; this is proved by such an appearance as 
that presented in fig. vi, 6, 7. On carefully examining such 
sections as the former of these no trace of young marginal bundles 
in course of development is to be found. On the other hand, 
there appear between the older bundles of the leaf trace younger 
bundles, as shown in fig. v1, 7, which are evidently freshly inter- 
calated between the pre-existing bundles of the leaf trace. In 
fig. x11 we have further evidence of this intercalation of fresh 
bundles, which seems to be more common in older than in 
younger plants. These younger intercalated bundles run parallel 
to the older series, but do not extend so far into the stem; in 
fact, those bundles which are youngest follow the plane of the leaf 
for the shortest distance into the stem. We have, then, an 
explanation afforded us of the difference of the bundles in this 
respect, and we may presume that the same rule applies to the 
bundles of the leaf trace throughout, that is, that the oldest 
bundles are those which extend furthest into the stem in the 
plane of the leaf, while those which extend successively a less 
distance are younger. And this leads us to the second point, 
viz. the bearing of these conclusions upon the arrangement of the. 
