PRESIDENTIAL ADDRESS. 701 
preparations of Miss Thomas and of Mr. R. H. Compton. Neither of these 
botanists made their preparations to illustrate M. Chauveaud’s theory; indeed, 
they attacked the subject, as I did, with aims distinct from his. There has, 
therefore, been nothing like complete verification in any single species, but so 
remarkable a correspondence with his figures in similar stages of the material, 
that I am satisfied of M. Chauveaud’s fidelity. 
Assuming, then, that his account of the vascular development in a young 
seedling is substantially correct, what are we to conclude as to the homology of 
the central cylinder of the stem with that of the root? 
M. Chauveaud himself 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, therefore, strictly homologous with it 
in the sense in which the earliest vascular formations in cotyledon and hypocotyl 
respectively were homologous with each other. He considers the successive 
vascular formations which we have just followed in these regions—formations 
marked by the appearance of alternate, intermediate, and superposed xylem in 
turn—to represent three successive phases of stelar development. The root-stele 
corresponds to the first of these phases only. 
But questions of phylogeny are strictly historical, and the only precise mean- 
ing that can be attached to the expression ‘successive phases of stelar develop- 
ment * in the seedling of an Angiosperm is that at some past period a group of 
plants in the direct line of descent of Angiosperms possessed a stele resembling 
that which is now a mere stage in the life of the individual. Thus the alternate 
formation found throughout the very young seedling implies an ancestral group 
with an exarch stele in stem as well as root, and a leaf-trace of corresponding 
structure. 
There is nothing at all improbable in this hypothesis, since groups with 
exarch steles in stem as well as root are found among living and extinct plants. 
But if adopted several important consequences follow. The seedling while it 
consists of cotyledons, hypocotyl, and primary root only—the plumule present 
as a mere bud—must represent a past period in race-history when its ancestors 
possessed an exarch stele in stem and root alike; when the stem-stele belonged to 
the stem only, and the insertion of leaf-traces hardly modified its structure; 
when it entered the root without change, and therefore no transitional region 
occupied and puzzled the anatomist of the period. 
This early stage in the development of the seedling is succeeded by that in 
which the epicotyl begins to grow, and as a rule the epicotyl is quite undoubtedly 
modern.* Its vascular skeleton is built: up of leaf-traces, which are endarch from 
the first. At the cotyledonary node they are inserted on the vascular cylinder 
of the hypocotyl, which has become endarch at the top. This transition has been 
etfected lower down in the hypocotyl, as described already, by the formation 
first of intermediate, and then of superposed xylem, together with the gradual 
disappearance of the original alternate xylem. 
Thus the cotyledonary node may be considered to mark the interval between 
two acts in the drama of evolution—an interval the length of which cannot yet 
be estimated, but is clearly to be reckoned in geological epochs. 
The race-history of the Phanerogamic stem-cylinder is at present unknown. 
How did the ancestral stele lose its exarch character, and what intermediate 
stages led up to its present construction from endarch leaf-traces? Possibly the 
development of the hypocotyl may give a clue as suggested by M. Chauveaud, 
and the change have been effected by the development of intermediate xylem. 
Or Professor Jeffrey may be right in deriving the leaf-traces from a siphonostele 
which has been gradually more and more broken up by the appearance of foliar 
“ The epicotyl in the Vicieee has been described as containing centripetal 
xylem. The facts are given by Mlle. Goldschmid (1876), Prof. Gérard (1881), 
M. Chauveaud (1911), and Mr. Compton (1912). The theoretical interpretation 
is discussed by M. Chauveaud (/.c., p. 348) and Mr. Compton (i.c., pp. 93-95). 
There seems every reason to think, as Mr. Compton suggests, that the character 
is adaptive, depending on the twining habit. He points out that Vicia Pada, 
which does not twine, has a normal epicotyl. 
