700 TRANSACTIONS OF SECTION K. 
simplest imaginable. The original stele of the hypocotyl divides below the 
cotyledonary 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, therefore, 
does not modify the structure of the stele. 
How, then, does the structure we associate with the stem of Phanerogams 
appear? ‘In the transitional 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. 
Kach arm of the V approaches the adjacent phl6em group as it travels inwards, 
until the last-formed elements lie on the same radius as the centre of the phléem 
group, but well within it. The next elements are differentiated on that radius, 
but are directed towards the phléem: development has become centrifugal. 
These successive 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 transitional region: they com- 
monly disappear as the superposed elements become conspicuous. The inter- 
mediate xylem persists. But higher up in the hypocotyl 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 connection 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 expected to anatomists who are 
not also embryologists. They clearly have a direct bearing on the theory of the 
stele. Before, however, entering on this subject, I ought to say something on 
the question of fact. In my opinion M. Chauveaud’s figures and descriptions 
represent the vascular development in the hypocotyl and cotyledons of Angio- 
sperms more accurately than any others with which I am acquainted. He has 
dealt more fully with Dicotyledons than Monocotyledons, but I have been able 
to verify his account of the latter to some extent by reference to my own pre- 
parations, which include a number of species closely allied to those which, he has 
cut. Among Dicotyledons I have had the great advantage of consulting the 
5 Chauveaud, L’Appareil Conducteur des Plantes vasculaires, 1911. See 
description of Mercurialis annua on pp. 216, 217, and figs. 62, 63. 
