PRESIDENTIAL ADDRESS. 695 
suspensor and embryo, the general development of both, and the appearance 
of external and internal differentiation in the embryo before germination. 
Some of Hanstein’s general conclusions as to internal anatomy. have become 
the common property of text-books; for instance, the early differentiation of 
dermatogen in the embryo, and its subsequent development into the epidermal 
system. He was less successful in demonstrating the initial independence of 
plerome and periblem and their relation to the vascular cylinder of the mature 
stem. 
The early differentiation of plerome and periblem from the internal tissues 
of the embryonic axis, and their continued formation at the growing points of 
stem and root respectively, are processes which demand the most careful investi- 
gation, on account of their bearing on the stelar hypothesis. 
Dr. Schoute’s work on the exact relationship of plerome and periblem at the 
growing-point to the central cylinder and cortex as differentiated in the older 
regions of the same axes, whether stem or root, is very important. He accepts 
Professor Van Tieghem’s definition of the stele as the solid cylinder of root 
or stem enclosed within the endodermis. The endodermis itself, of course, is 
considered as belonging to the cortex, because in the root its cells are opposite 
the radial files of the inner cortex, and, indeed, form the inmost rank of those 
files. This is assumed to indicate a common origin by repeated tangential 
division. The cells of the pericycle—the outermost layer of the stele—alternate 
with those of the endodermis. As a rule there is no corresponding radial 
arrangement in the cortical tissue of the stem, but where such exists—as in the 
stem of Hippuris—the endodermis is again included in it and terminates it. 
Using the microtome as an instrument of precision, Dr. Schoute in 1903 
published the most careful observations on the growing-points of roots. His 
aim was to determine whether the limit between plerome and periblem (Hanstein) 
corresponded with that between stele and cortex (Van Tieghem). For this 
purpose Dr. Schoute was, of course, obliged to choose roots in which the plerome 
is clearly distinguished from the periblem at the growing-point. In the end he 
obtained precise results in three species: Hyacinthus orientalis, Helianthus 
annuus, and Linum usitatissimum. In each of these the periblem passed into 
the cortex, its inner layer becoming the endodermis, and the plerome gave rise 
to the stele only. 
Owing to difficulties of observation, arising chiefly from the insertion of 
leaves close up to the growing-point and displacements in the original stem- 
structure consequent on this habit, Dr. Schoute was not equally successful in 
his work on stems. Hippuris vulgaris was the only species to give definite 
results. In this species he found that the plerome gave rise not only to the 
stele, but also to the endodermis, and to the two or three layers of cortex 
immediately beyond it. If these results are well founded the limit between 
plerome and periblem does not correspond with that between stele and cortex 
in the stem of Hippuris. Moreover, doubt is thrown on the assumption made 
by all previous observers that rows of cortical cells arranged in radial files must 
be of common origin. 
Observations on a single species, however well attested, form a slender basis 
for conclusions regarding stems in general. Nor have Dr. Schoute’s observations 
escaped criticism. Dr. Kniep has since examined the growing-point of Hippuris, 
and believes that he can identify plerome with central cylinder, and periblem 
with cortex, even in this test case. However this may be, no one denies the 
obscurity of stem anatomy in this respect compared to that of the root, nor the 
cause of that obscurity. The continuity of the stem stele is perpetually inter- 
rupted by the insertion of the leaf-traces, just as the symmetry of the stem 
growing-point is destroyed by the formation of leaf rudiments close up to its 
apex. 
The stelar hypothesis is essentially an assertion of the real homology between 
the vascular systems of stem and root throughout all vascular plants. This was 
pointed out to me more than twenty years ago by Dr. D. H. Scott, and it has 
been the sheet anchor to which I have since clung through much stress of morpho- 
logical weather. No difficulty arises so long as we are dealing with roots only, 
or with the stems of those Vascular Cryptogams in which the vascular system 
