Theories of Transition from Root to Stem. 
2 3 
these three groups, median and lateral, which may be fitly called a 
“ triad ” : the “ disposition intermediaire ” is omitted at the higher 
levels and we pass directly from the alternate to the superposed 
phase—or in the older language from centripetal to centrifugal 
differentiation. 
This frequent omission of the intermediate phase in the xyletn 
is the chief weakness in the otherwise consistent and illuminating 
theory of recapitulation. It may be conceded that such “ short- 
circuits ” constantly occur in ontogeny, the organism forgetting 
(to use the mnemic metaphor) certain chapters of its evolutionary 
history. But in any case the “ phase intermediate ” seems to be of 
quite subordinate importance in vascular phylogeny : it exists as an 
independent disposition only as an often fugitive stage in the 
transition from alternate to superposed arrangement ; and it 
apparently never forms the final term of an ontogenetic vascular 
series. In fact it appears to be mainly of physiological significance, 
no doubt serving as an aqueduct from the still functional “ vaisseaux 
alternes ” of the absorbing region of the axis to the “ vaisseaux 
superposes ” of its upper parts : and its frequent absence in situations 
where there is no such need (as in the case of triads) is consonant 
with a physiological rather than with a phylogenetic interpretation. 
If we limit the number of phases to two, the alternate and the 
superposed, it is no new thesis that the former is primitive relatively 
to the latter. A long phylogenetic series from centripetal to 
centrifugal as the prevalent type of xylem has been outlined by 
Scott: and it may be remarked that Poroxylon Edwardsii, one of 
the members of Scott’s phylogenetic series, is cited by Chauveaud 
as exhibiting his own three ontogenetic phases (though direct 
evidence is lacking here). The demonstration that the theoretical 
phylogenetic sequence is realised in the ontogeny of the hypo¬ 
cotyledonary axis of Phanerogams, together with the notion of 
basifugal acceleration, are the most noteworthy original general¬ 
isations in Chauveaud’s work. 
The theory of the nature of the anatomical transition from 
root to stem proposed by Chauveaud differs from others previously 
considered by reason of its phylogenetic content. The obvious 
affinity with Bonnier’s scheme is only superficial: for according to 
Bonnier we have a gradual repulsion of protoxylems from the 
periphery towards the centre of the stele as we ascend the hypo¬ 
cotyledonary axis ; while according to Chauveaud the protoxylem 
of the “ disposition superposee ” belongs to a later phase of vascular 
