348 The British Association at Birmingham. 
that the presence of external xylem is the rule in the hypocotyl and 
in the base of the cotyledon, but considers that this external xylem 
belongs to the primitive structure of hypocotyl and cotyledon as 
well as to that of the root. 
As already stated, the vascular system of seedlings is first 
differentiated in the hypocotyl, base of cotyledon, and base of 
primary root. According to Chauveaud, in all these regions the 
primitive stele is root-like, the xylem alternating with the phloem 
and its development being centripetal; but this primitive formation 
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 and 
disappear so early that as a rule they are missed completely by the 
anatomist, who is apt to prefer well-differentiated material and 
therefore to choose seedlings which are past their first youth. 
Chauveaud therefore considers that there is an early phase in the 
development of the seedling 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, continu¬ 
ous with it. At that epoch each cotyledonary trace is also developing 
on the same plan: it belongs to the same phase of evolution. In 
many Dicotyledons the insertion of the cotyledons is the simplest 
imaginable—the original stele of the hypocotyl divides below the 
cotyledonary node and one half goes to each cotyledon. Where 
this formation is clearly developed there cannot be said to be any 
transition between stem and root structure, since stem stele and 
root stele are continuous and their steles are developing in the same 
way, while 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. The structure we associate with the stem of Phanerogams 
appears as follows. 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. Each arm of the V approaches the adjacent phloem group as 
it travels inwards, until the last-formed elements lie on the same 
radius as the centre of the phloem group, but well within it. The 
next elements are differentiated on that radius, but are directed 
towards the phloem—development has become centrifugal. These 
successive xylem formations are termed by Chauveaud the alternate, 
the intermediate, and the superposed. The alternate elements are 
fugitive in this transitional region: they commonly disappear as the 
superposed elements become conspicuous. The intermediate xylem 
persists ; but higher up in the hypocotyl the intermediate elements 
also disappear as the seedling becomes older. Hence 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 being maintained by the intermediate xylem of the transitional 
region. 
Chauveaud believes the stem cylinder in the upper hypocotyl 
of a fairly old seedling to be a true stele, but one belonging to a 
later phase of evolution than that of the root, and not strictly 
