Presidential Address to Section K. 
347 
thickening characteristic of the endodertnis in the root, or by the 
presence of starch in its cells. He shows that such a sheath 
surrounds the vascular cylinder in a very large proportion of 
Dicotyledons and in a majority of the Monocotyledons, while among 
Gymnosperms it occurs but rarely; and since the Angiosperms in 
which this bundle-sheath is obscure or wanting are commonly 
closely related to species in which it is perfectly well-defined he 
concludes that its absence in such cases must be attributed to 
reduction. Allowing that such a layer is as general among Angio¬ 
sperms as Schoute believes, doubt may still exist as to its homology 
with the endodermis of the root, which is defined, not only by its 
thickened walls, but also by the fact that the cells form the inmost 
rank of the series of radial files distinguishing the inner cortex, 
while in the stem the inner cortex cells are very rarely arranged 
radially. 
As regards the second class of evidence, that drawn from the 
history of the tissues in the individual plant, we have already seen 
that the differentiation of plerome from periblem is far less definite 
at the growing-point of the stem than at the root apex, and doubts 
have even been thrown on the identity of plerome and periblem with 
stele and cortex respectively. But we must now follow the 
development of the tissues of the embryo into those of the seedling. 
The normal seedling of all Phanerogams consists at first of coty¬ 
ledons, hypocotyl, and root, the plumular bud being still rudimentary. 
The hypocotyl is commonly the first part of the embryo to lengthen, 
and then its xylem is lignified a little earlier than that of the root 
or even of the cotyledon ; but when, as in many Monocotyledons, 
the base of the cotyledon lengthens first, lignification begins in that 
region and advances through the hypocotyl to the primary root. 
The investigation of the anatomy of the seedling at this epoch 
becomes extremely important when the vascular system of the root 
is compared with that of the stem, for in the seedling we have a 
complete and simple vascular skeleton which at one end belongs to 
the primary root of the plant and at the other to its primary stem ; 
hence there must be an intermediate region in which stem structure 
passes into root structure, and the method of transition should at 
least suggest, if it does not precisely determine, the relation in which 
they stand to one another. For this reason great value has been 
attached by anatomists to the transitional region of the main axis. 
Van Tieghem showed that there are several types of transition 
between root and stem, in all of which the xylem and phloem 
bundles of the root are continued into the cotyledons or plumule; 
on their way through the hypocotyl they may divide or be displaced, 
and the xylem bundles “ rotate ” (that is, they turn on their axes 
until the protoxylem is internal), but all the elements present in the 
root are continued upwards in regular succession and are simply 
re-arranged in the upper part of the seedling. Hence Van Tieghem 
considered that the steles of root and stem are completely 
homologous. Gravis and others, however, consider that there is no 
morphological continuity in the hypocotyl between the vascular 
systems of root, stem and leaf: their traces are merely in contact 
sufficiently intimate for physiological purposes, but there is no true 
homology between the central cylinder of the stem and that of the 
root. The third view is that of Chauveaud, who agrees with Gravis 
