244 
be four or five years before it flowers, while the 
annual has to complete its whole life-cycle in a single 
season. 
Nor is the division of the subject into two parts, 
the first ending with the embryo in the ripe seed, a 
natural one, even if the time of maturation be in- 
cluded in that first period. The structure of the 
embryo cannot be completely grasped by reference to 
its past only. The observer must expect adaptive 
characters of three kinds: first, those impesed upon 
the embryo in the past by its development within the 
embryo-sac while it is still parasitic on the parent 
plant; secondly, certain adaptations to the process of 
germination itself; and, finally, characters which will 
be useful after germination. Before the utility of the 
characters included in this third class can be fully 
understood, the development of the seedling must be 
followed for some time. In short, the structure of 
the embryo is dependent on its future, as well as on 
its past; and a division of the subject which excludes 
that future is, as Balfour says, purely artificial. Thus 
the work done of late years on the anatomy of the 
seedling has not only completed Irmisch’s work on 
its external morphology, but has also thrown light on 
the problems of early embryology attacked by Han- 
stein and his immediate followers. 
These problems are of two kinds, relating to the 
internal anatomy or the external morphology of the 
embryo. Hanstein himself was chiefly interested in 
the former. It is curious to realise when reading his 
paper that up to the date of its publication botanists 
were prepared to find an apical cell in the embryo of 
Angiosperms. They acknowledged, indeed, that no 
such cell existed in the growing-points of the mature 
plant.1. There each new portion of tissue was formed 
by the activity of a group of similar and equivalent 
cells. But it still seemed possible that the embryo 
might possess an apical cell in ‘the earlier stages of 
its growth—a reminiscence of its Cryptogamic an- 
cestors. Hanstein’s work disposed once for all of 
this possibility. It was conclusive even against the 
great authority of Hofmeister, who had described an 
apical cell in the embryo of orchids. 
One general result of the work on the embryo since 
Hanstein’s time has been to discredit phylogenetic 
theories based on its early history. Indeed, it was 
scarcely to be expected that a small mass of meristem, 
developing within a confined space and feeding para- 
sitically on the tissues of the mother-plant, should 
preserve ancestral features, and one is surprised to 
find a morphologist with the experience and the wide 
grasp of Hanstein attaching so much importance to 
the succession of divisions within such a body. The 
conscientious student finds it a laborious task to 
follow the work done in plant embryology during the 
period which succeeded the publication of Hanstein’s 
great paper. No wonder that when the end is seen 
to discredit rather than crown much of that work, 
when he realises how little has been gained as a 
result of so much patient toil, he is apt to renounce 
the whole subject in disgust. Yet in science we dare 
not rule out the unexpected, perhaps even less in 
morphology than elsewhere. Hanstein and his suc- 
cessors did good service when they described 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. 
Some of Hanstein’s general conclusions as to 
internal anatomy have become the common property 
of text-books; for instance, the early differentiation 
1 Korschelt in 1884 revived the hypothesis that the growing points of 
some Angiosperms at any rate increased by means of an apical cell. He 
worked chiefly on aquatic plants. His views have not heen accepted. 
NO. 2295, VOL. 92] 
NATURE 
[OcToBER 23, 1913 
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 investigation, 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 Prof. Van Tieghem’s 
definition of the stele as the solid cylinder of root 
or stem enclosed within the endodermis. The endo- 
dermis 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 endo- 
dermis. 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 ob- 
servations on the growing-points of roots. His aim 
was to determine whether the limit between plerome 
and periblem (Hanstein) corresponded with that be- 
tween 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 Linwmn 
usitatissimum. In each of these the periblem passed 
into the cortex, its inner layer becoming the endo- 
dermis, 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 regard- 
ing stems in general. Nor have Dr. Schoute’s ob- 
servations 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. How- 
ever 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 interrupted by the inser- 
tion 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 
