OcTOBER 23, 1913] 
NATURE 
247 
traces, and is therefore apt to be confused with it. 
The characteristic shape of so many cotyledonary 
traces arises in this way. They are often called double 
bundles, but according to Prof. Gravis they are more 
than double, for each really consists of two traces 
in close contact with the last vestige of root-xylem. 
The latter always disappears higher up in the cotyle- 
don, and the two traces may then unite into a midrib, 
with or without lateral branches. As a consequence 
of this view, Prof. Gravis considers that there is no 
morphological continuity in the hypocotyl between the 
vascular systems of root, stem. and leaf.t Their traces 
are merely in contact sufficiently intimate for physio- 
logical purposes. There can, therefore, be no true 
homology between the central cylinder of the stem 
and that of the root. 
The third view is that of M. Chauveaud, who has 
been engaged for upwards of twenty years in following 
the development of the vascular elements in the hypo- 
cotylar region and its neighbourhood. He agrees with 
Prof. Gravis that the presence of external xylem is 
the rule in the hypocotyl and in the base of the 
cotyledon. But he considers that this external xylem 
belongs to the primitive structure of hypocotyl and 
cotyledon as well as to that of the root. We have 
already said that the vascular system of seedlings is 
first differentiated in the hypocotyl, base of cotyledon, 
and base of primary root. In all these regions M. 
in which the stele of the hypocotyl—at that time the 
only representative of the stem—is developing on 
exactly the same lines as the stele of the primary 
root, and is, in fact, continuous with it. At that 
epoch each cotyledonary trace is also developing on 
the same plan. It belongs to the same phase of evolu- 
tion, and in many species of Dicotyledons the insertion 
of the cotyledons is the simplest imaginable. The 
original stele of the hypocotyl divides below the cotyle- 
donary node, and one-half goes to each cotyledon.® 
In species where this formation is clearly developed 
there cannot be said to be any transition between 
stem- and root-structure. Stem-stele and _ root-stele 
are continuous: their steles are developing in the 
same way. Even the leaf-traces of the first two 
leaves are on similar lines, and their insertion, there- 
fore, does not modify the structure of the stele. 
How, then, does the structure we associate with 
the stem of Phanerogams appear. In the transitonal 
region of the hypocotyl the first xylem elements 
perhaps only two or three at each pole—alternate with 
the phloem groups. The elements next differentiated 
lie within them, for development is still centripetal, 
but in two diverging groups. The xylem-ray is then 
shaped like an inverted V. Each arm of the V 
approaches the adjacent phléem group as it travels 
inwards, until the last-formed elements lie on the 
same radius as the centre of the phloem group, but 
Chauveaud thinks the primitive stele to be root-like 
in his own phrase it belongs to the “disposition 
alterne.”’ The xylem alternates with the phléem, and 
its development is centripetal. This primitive forma- 
tion, however, is permanent only in the root, and 
commonly in the lower part of the hypocotyl also. 
In the upper part of the hypocotyl and in the base of 
the cotyledons the first xylem elements are fugitive. 
They disappear so early that, as a rule, they are 
missed completely by the anatomist, who is apt to 
prefer well-differentiated tissues, and theretore to 
choose seedlings which are past their first youth. 
In considering the theory of stelar evolution in which 
M. Chauveaud has correlated his own long series of 
observations with the results of other embryologists, 
I shall confine myself strictly to the question now 
under discussion—namely, the extent to which the 
stele of the young stem in Phanerogams can be con- 
sidered to represent that of the root. Prof. Wan 
Tieghem, as we have seen, considers them completely 
homologous, while Prof. Gravis denies that they are 
homologous at all. j 
M. Chauveaud occupies a middle position. If I 
understand his views rightly, he considers that there 
is an early phase in the development of the seedling 
4 A. Gravis, ‘Recherches . . . 
del Acad. royal. . 
_: Sur le ‘ Tradescantia virginica,’ J/ém. 
+» Tome lvii., Bruxelles, 1898. See account of hypo- 
cotyl( pp. 28-32), including insertion of cotyledon (pp. 31-32). Also memoir 
by same author on Uvrtica dioica (1885), footnote on p- 117. Cf also Mr. 
k. H. Compton's paper in Vew Phylologist, xii., p. 13, 1912. 
NO. 2295, VOL. 92] 
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The next elements are differentiated 
well within it. 
on that radius, but are directed towards the phléem : 
development has become centrifugal. These succes- 
sive xylem formations are called by M. Chauveaud the 
alternate, the intermediate, and the superposed. They 
are distinguished in the diagram by dotted lines for 
the alternate elements, thin lines for the intermediate, 
and thick lines for the superposed. 
The alternate elements are fugitive in this transi- 
tional region; they commonly disappear as the super- 
posed elements become conspicuous. The inter- 
mediate xylem persists. But higher up in the hypo- 
cotyl the intermediate elements, too, disappear as the 
seedling grows older. They vanish in the traces of 
the cotyledons also, and in the cotyledons themselves. 
Thus in seedlings of a certain age we have endarch 
| bundles at the top of the hypocotyl, forming a stele 
of the stem type, and an exarch stele lower down, 
which passes unchanged into the root. The connec- 
tion between the two is maintained by the inter- 
mediate xylem of the transitional region. 
Although M. Chauveaud has been publishing his 
researches since 1891, yet he has only lately (1911) 
put his results into a connected form, and they are 
| therefore less familiar than might otherwise be ex- 
| 
pected to anatomists who are not also embryologists. 
They clearly have a direct bearing on the theory of 
5 Chauyeaud, ‘‘ L'Appareil Conducteur des Plantes vasculaires," 1911. 
See description of Wercurialis annua on pp. 216, 217, and figs. 62, 63. 
