Presidential Address to Section K. 
345 
theories based on its early history; indeed, it was hardly to he 
expected that a small mass of meristem, developing within a con¬ 
fined space and feeding parasitically on the tissues of the mother- 
plant, should preserve ancestral features. Still, Hanstein and his 
successors did good service in elucidating the growth of the pro¬ 
embryo from the fertilised egg-cell ; its division into suspensor and 
embryo ; the general development of both, and the appearance 
of external and internal differentiation in the embryo before 
germination. 
While some of Hanstein’s general conclusions as to internal 
anatomy have become the common property of text-books—for 
instance, the early differentiation of the dermatogen and its sub¬ 
sequent development into the epidermal system—he was less suc¬ 
cessful in demonstrating the initial independence of plerome and 
periblem, and their relation to the vascular system of the mature 
stem. The early differentiation of plerome and periblem from the 
inner tissues of the embryonic axis and their continued formation at 
the growing-points of root and shoot are processes which demand 
the most careful investigation on account of their bearing on the 
stelar hypothesis. 
Schoute’s work on the exact relationship of plerome and peri¬ 
blem at the growing-point to the central cylinder and cortex as 
differentiated in the older regions of the same axis, whether stem or 
root, is very important. He accepts Van Tieghem’s definition of 
the stele as the solid cylinder of root or stem enclosed within the 
endodermis. The endodermis itself 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 these 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. Schoute in 1903 got 
precise results in species of Hyacinthus, Helianthns, and Linuin, in 
the roots of which the periblem passed into the cortex, its inner 
layer becoming the endodermis, and the plerome gave rise to the 
stele only ; but owing to difficulties arising chiefly from the insertion 
of leaves close up to the stem apex and displacements in the original 
stem structure owing to this habit, Schoute found definite results 
only in Hippuris, where the plerome gave rise not only to the stele 
but also to the endodermis and to the two or three inner layers of 
cortex immediately beyond it. If Schoute’s results are well founded 
the limit between plerome and periblem does not correspond with 
that between stele and cortex in the stem of Hippuris, and doubt is 
also thrown on the assumption made by all previous observers that 
rows of cortical cells arranged in radial files must be of common 
origin. 
The stelar hypothesis is essentially an assertion of the real 
homology between the vascular systems of stem and root throughout 
all vascular plants. 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 is a closed cylinder without gaps at the 
insertion of the leaf-traces: in such plants the vascular cylinder is 
