PRESIDENTIAL ADDRESS. 699 
There is then a real difference of opinion on a question of fact between Pro- 
fessor Van ‘l'ieghem and his school, on the one hana, and certain modern embryo- 
logists on the ucher. ‘L’hree distinct views are now held as to the interpretation 
o1 the isolated xylem elements in the hypocotyl of Z'agetes. 1 shall try to state 
tnem as fairly and concisely as possible. 
Protessors Van ‘Lieghem and Gerard treat Z'agetes and the genera which 
resemble 1t as exceptional, because part of the external xylem of the root is con- 
tinued upwards between tne cotyleaonary traces, and dies out in the base of the 
cotyledon. ‘Iney consider that the remainder of the external xylem turns on 
itselt and becomes internal in the usual way. 
Professor Gravis and his pupils think that a similar prolongation of the 
xylem poles of the root into the nypocoty! or cotyledon is the rule, and that they 
terminate there abruptly. but in most cases this vestigial root-xylem is not 
isolated ; it is in contact on either side with the early xylem of the cotyledonary 
traces, and is theretore apt to be contused with it. ‘ne characteristic shape of 
so many cotyledonary traces arises in this way. ‘They are often called double 
bundles, but according to Professor Gravis they are more than double, for each 
really consists of two traces in close contact with the last vestige of root-xylem. 
‘he latter always disappears higher up in the cotyledon, and the two traces may 
then unite into a midrib, with or without lateral branches. As a consequence 
ot this view, Professor Gravis considers that there is no morphological con- 
tinuity in the hypocotyl between the vascular systems of root, stem, and leaf.* 
‘heir traces are merely in contact sutiiciently intumate for physiological purposes. 
‘1nere can, therefore, be no true homology between the central cylinder of the 
stem and that of the root. 
‘he 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 
hypocotyiar region and its neighbourhood. He agrees with Professor Gravis 
tnat the presence of external xyiem is the rule in tue hypocotyl and in the base 
ot 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 difterentiated 
in the hypocotyl, base otf cotyledon, and base ot primary root. In all these 
regions iM. Chauveaud thinks the primitive stele to be root-like—in his own 
phrase it belongs to the ‘disposition alterne.’ ‘the xylem alternates with the 
phioem, and its development 1s centripetal. This primitive tormation, however, 
+8 permanent only in the root, and commonly in the lower part of the hypocoty1 
also. in the upper part of the hypocotyl and in the base of the cotyledons the 
nrst xylem elements are fugitive. ‘lhey disappear so early that as a rule they 
are missed completely by the anatomist, who 1s apt to preter well-ditferentiated 
tissues, and therefore to choose seedlings which are past their first youth. 
in considering the theory of stelar evolution in which M. Cnauveaud has 
correlated his own long series of observations with the results of other embryo- 
logists, 1 shall confine myself strictly to the question now under discussiou— 
namely, the extent to which the stele of the young stem in Phanerogams can 
be considered to represent that of the root. Protessor Van Tieghem, as we 
have seen, considers them completely homologous, while Professor Gravis denies 
that they are homologous at all. 
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 m 
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 ot 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 evolution, 
and in many species of Dicotyledons the insertion of the cotyledons is the 
* A. Gravis, Recherches . . . sur le ‘ Tradescantia virginica, Mém. de l’Acad. 
royal . . . Tome lvii., Bruxelles, 1898. See account of hypocotyl (pp. 28-32), 
including insertion of cotyledon (pp. 31-32). Also memoir by same author on 
Urtica dioica (1885), footnote on p. 117. Cf. also Mr. R. H. Compton’s paper 
in New Phytologist, xi., p. 13, 1912. 
