246 
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 performing 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 cotyledon. But when—as in many Mono- 
cotyledons—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 embryo- 
logy 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 
Hanstein 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 is compara- 
tively simple, yet it has been attacked from different 
sides, and the attempt to give a concise account of 
the results attained beset with difficulties. For 
the present, however, I propose to consider only their 
bearing on the stelar hypothesis. 
Indeed, seedling anatomy becomes extremely im- 
portant when the vascular system of the root is com- 
pared 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. 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 each other. For this 
reason great value has been attached by anatomists to 
the transitional region of the main axis. Jt was not 
completely investigated, however, until the microtome 
was introduced into botanical practice, for the change 
of structure is often very abrupt, and cannot be 
studied in detail unless all possible sections are present 
in their proper order. 
In this, as in other branches of modern anatomy, 
Prof. Van Tieghem was first in the field. In his 
memoir of 1872, “Sur les Canaux Secréteurs des 
Plantes,” he described the course of the bundles of 
the hypocotyl of Tagetes patula, an example of the 
second type of transition given in his textbook (1886). 
The three types were, indeed, already identified in 
1872, for the first and third are defined in a footnote 
appended to the description of Tagetes. 
Tagetes patula was, of course, examined in 1872 
with the aid of hand-sections only. Two traces enter 
the hypocotyl from either cotyledon, and form in the 
end a diarch root. The plane passing through its 
is 
_ 3 By this qualification 1 mean to exclude cases in which the young s-ed- 
ling is very greatly reduced. 
NO. 2295, VOL. 92] 
NATURE 
[OcToBER 23, 1913 
xylem poles is the median plane of the cotyledons. 
In the upper part of the hypocotyl this plane bisects 
the space which separates the two bundles entering 
each cotyledon. So far the description of Tagetes 
given in 1872 is identical with the generalised account 
of type 2 in the text-book (1886). But a detail of 
some importance is mentioned in the description of 
Tagetes which does not reappear in the definition of 
type 2. In each of the spaces just mentioned—called, 
for convenience, xylem spaces, because they lie above 
the xylem poles of the root--lies an isolated xylem 
element, the direct continuation of the most external 
element in one of the root poles, and this element 
comes to an abrupt end higher up. 
Thus Prof. Van Tieghem has tacitly assumed that 
Tagetes is exceptional in this respect, and this view 
was also adopted by Prof. Gérard in his laborious and 
accurate paper of 1881. He describes the transitional 
phenomena of a number of Dicotyledons, among them 
Tagetes erecta. Not only is the transition in this 
species exactly the same as that in T. patula, but the 
author records a similar isolation of primitive xylem 
elements in Raphanus niger, Ipomaca versicolor, and 
Datura Stramonium, still treating the arrangement as 
exceptional. 
These details are important, because if certain 
protoxylem elements belonging to the root are not 
continued upwards in regular succession into the 
cotyledonary or plumular bundles, but end abruptly 
in hypocotyl or base of cotyledon, there is not that 
complete correspondence between stem- and _ root- 
structure which is assumed in Van Tieghem’s three 
types. In all of them 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 own axes until 
the protoxylem is internal. But all the elements pre- 
sent in the root are continued upwards in regular suc- 
cession, and are simply rearranged in the upper part 
of the seedling. This is one of the main arguments 
advanced by Prof. Van Tieghem to support his view 
that the steles of root and stem are identical. 
According to most later observers, however, such 
temporary prolongation of the root-poles upwards as 
that described by Profs. Van Tieghem and Gérard in 
a few instances, and considered by them as excep- 
tional, is really of general occurrence. ‘Lhe protoxylem 
elements, indeed, are not commonly isolated from the 
main xylem of the cotyledonary traces as in Tagetes, 
but are in more or less complete contact with them 
on either side. Such contact is approached in 
Raphanus niger, where it is very clearly suggested in 
Prof. Gérard’s figures. 
There is then a real difference of opinion on a 
question of fact between Prof. Van Tieghem and his 
school, on the one hand, and certain modern embryo- 
logists on the other. Three distinct views are now 
held as to the interpretation of the isolated xylem 
elements in the hypocotyl of Tagetes. I shall try to 
state them as fairly and concisely as possible. a" 
Profs. Van Tieghem and Gérard treat Tagetes and 
the genera which resemble it as exceptional, because 
part of the external xylem of the root is continued 
upwards between the cotyledonary traces, and dies out 
in the base of the cotyledon. They consider that the 
remainder of the external xylem turns on itself and 
becomes internal in the usual way. 
Prof. Gravis and his pupils think that a similar 
prolongation of the xylem poles of the root into the 
hypocotyl or cotyledon is the rule, and that they 
terminate there abruptly. But in most cases this 
vestigal root-xylem is not isolated; it is in contact 
on either side with the early xylem of the cotyledonary 
