5 8 9 
Structure of Acer Pseudoplatanus . 
the { alterne ’ grouping show a characteristic alteration in relative position. 
In this phase, showing what Chauveaud terms the ‘disposition intermediate 
the xylem vessels arise in a tangentially extended series to the right and left 
respectively of the plane occupied by those of the ‘ alterne 5 phase, whilst the 
new phloem elements develop in a plane roughly parallel with the earlier 
ones, but are situated nearer the xylem. In continuity with the latest formed 
xylem elements of the second phase (‘ disposition intermediate ’) arise other 
elements developed in a centrifugal series, whilst the new phloem elements 
lie still nearer the xylem and thus lie on the same radii as the centrifugal 
vessels. As a result of this, two collateral bundles are formed, one on either 
side of the original plane of xylem differentiation. This third phase is 
termed the ‘ disposition superposee ’. 
The xylem of all three phases may be represented at one time, or the 
development of each of the later phases may be accompanied by the dis¬ 
appearance of the preceding one. 
In the absence of the first phase the elements of the second appear as 
two separate groups of vessels inclined to one another like the arms of the 
letter V, whilst the third phase, in the absence of the two earlier ones, takes 
the form of two quite separate collateral bundles. Chauveaud has shown 
that in the ontogeny of the seedling vascular system not only the root, but 
also the hypocotyl and even the basal portion of the cotyledon, may exhibit 
the c alterne ’ grouping of the xylem and phloem. In the root this vascular 
grouping is persistent, and though further conducting elements possessing 
the ‘ intermediate 5 arrangement may augment the original units (e. g. Phaseo- 
lus vulgaris ), they do not obscure its character. In the hypocotyl and 
cotyledon, however, it is far otherwise, since the development of each new 
phase is followed by the partial, or total, resorption of the vascular elements 
of the phase preceding it. The result is that in these organs the early 
stages in vascular development are only to be observed during the initial 
stages of germination and those which immediately follow it. Chauveaud 
considers that seedlings exhibit in their vascular development an acceleration 
from below upwards, so that whilst the primitive exarchy is retained in the 
root it is transient in the aerial parts of the seedling, this transient character 
being shared to a less degree by the intermediate elements. Where this 
acceleration is accentuated it leads to the total elimination of the earlier 
phases from the ontogeny in the upper portion of the hypocotyl and in the 
whole of'the cotyledon and to their early resorption at lower levels. It will 
thus be seen that, according to Chauveaud, the differences in orientation 
exhibited during the transition from the radial distribution of the xylem and 
phloem in the root to their collateral distribution in the stem are simply the 
result of the persistence of only the phyletically more recent stages at 
successively higher levels of the hypocotyl and cotyledons. 
Whilst Chauveaud’s theory supplies a consistent explanation of the 
