PRESIDENTIAL ADDRESS. 697 
But morphology is not merely descriptive. If we suppose that the stem-stele 
in remote ancestors of the Phanerogams was as well defined as that of the root 
and clearly comparable to it, we may attach a real morphological meaning to 
the term when applied to modern Phanerogams, provided we can show cause 
to believe that what we call the stele in their stems represents the ancestral 
stele. Its tissues will then have a history distinct from those of the cortex, 
though not clearly separated from them. The burden of proof, however, 
certainly lies with those who assert that an apparently continuous and uniform 
tissue can be separated into two parts of distinct origin. 
The evidence advanced is of two kinds—one founded on the comparative 
anatomy of stems, and the other on the history of the tissues in the individual 
plant. Dr. Schoute has argued the case with great skill from the first point 
of view in his ‘Stelartheorie. Depending to a large extent on his own 
researches, he has collected a great body of evidence to show that in the stems 
of Angiosperms a specialised layer is commonly distinguished from adjacent 
tissues either by the peculiar thickening characteristic of the endodermis 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 the Dicotyledons 
examined, and in a majority of the Monocotyledons. Among Gymnosperms it 
occurs but rarely. Observing that the Angiosperms in which this bundle-sheath 
is obscure or wanting are commonly closely related to species in which it is per- 
fectly well defined, Dr. Schoute concludes that its absence in such cases must 
be attributed to reduction. 
Allowing that such a layer is as general among Angiosperms as Dr. Schoute 
believes, grave doubts may still exist as to its homology with the endodermis of 
the root. The latter is defined not only by its thickened walls, but also by the 
position of its cells. They form the inmost rank of the series of radial files 
which distinguish the inner cortex, and the morphological endodermis—the 
phleoterma, as Strasburger calls it—can usually be distinguished by this purely 
morphological character, even when its walls are unthickened. In the stem, 
however, the cells of the inner cortex are not radially arranged, except in rare 
cases, such as Hippuris. Thus, there is no morphological criterion to distinguish 
the phleoterma, or inmost cortical layer of the stem, from adjacent tissues. The 
bundle-sheaths distinguished by their thickened walls or by the presence of 
starch in their cells are physiologically similar; they play a definite part in the 
economy of the stem, but the presence of either character must depend mainly on 
the demands of the conducting or assimilating system, and need not imply the 
morphological identity of such layers with each other, or with the layer per- 
forming a similar function in the root. 
Turning now to 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. Doubts have even been thrown on the identity of plerome and 
periblem with stele and cortex respectively. But we have not yet followed the 
development of the tissues of the embryo into those of the seedling. 
The normal seedling* of all Phanerogams consists at first of cotyledons, 
hypocotyl, and root, the plumular bud being still rudimentary. The primary 
root lies as a rule in a straight line with the primary stem, or hypocotyl. 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 that of the coty- 
ledon. 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 anatomy of the seedling at this epoch has lately been investigated by 
many independent observers. They constitute, indeed, the third school of 
embryology to which I have referred as completing the work of two earlier 
schools—namely, morphologists of the type of Irmisch and students of early 
embryology like Hansiein and his school, But though the subject is limited to a 
short period in the history of the plant, and to one in which its vascular structure 
* By this qualification I mean to exclude cases in which the young seedling 
is very greatly reduced, ; ; 
