598 Davey . — Seedling Anatomy of certain A me nti ferae. 
centre ‘ dies out J in ascending the hypocotyl, and actual connexion cannot 
be demonstrated. 
Similar distribution of the intercotyledonary vascular units between 
cotyledons and plumular leaf has been recorded as obtaining in many 
Leguminosae by Compton, who describes it as a phenomenon of replacement 
in which the cotyledons are being supplanted by plumular leaves* 
The variation of the level to which the intercotyledonary poles per- 
sist upwards is correlated with the rate at which the successive phases 
of vascular differentiation supersede one another as the hypocotyl is 
ascended. 
The succession of phases which has been conclusively demonstrated by 
Chauveaud, and receives the support of Compton and of Thomas, accounts 
also for the discontinuity which sometimes exists between the central 
protoxylem in the base of the cotyledons and that present at lower levels. 
In the youngest stages, development of the xylem proceeds centripetally 
from the first centres of differentiation (‘ elements alternes ’ of Chauveaud). 
This is followed by development in a tangential or lateral direction on 
either side of the centre, and this in turn by centrifugal differentiation, both 
primary and secondary. 
As the seedling elongates, successive increments of growth will have 
their vascular differentiation initiated in continuity with the phase which 
is active in the preceding increment. In the hypocotyl the earliest differen- 
tiation takes place in the region of the collet and of the cotyledonary 
node. When there is much intercalary elongation between these two 
regions a succession of phases may be found in ascending from the collet 
and to a less extent in descending from the node, and thus there will be 
lack of continuity between, for example, the ‘ alterne * elements or central 
protoxylem of the cotyledonary petiole and those of the root pole (e. g. in 
Carpinus ). 
Although, in general, the massive hypogeal seedlings possess the larger 
number of root poles, there are very striking instances of the absence of 
a definite relation between size or habit and the number of root poles 
present. Thus tetrarchy is equally characteristic of the largest hypogeal 
seedlings examined (species of Juglans) and of the slender epigeal seedlings 
of Casuarina (as instanced by Thomas ! ), Myrica , A Inns cor difolia > Carpinus , 
&c. The relatively slender epigeal seedling of Fagus sylvatica, while resem- 
bling in external habit the diarch and tetrarch species of Calycanthus 
described by Chauveaud and by Thomas, nevertheless exhibits diagonal 
octarchy. 
1 Thomas, Ei N. : A Theory of the Double Leaf-trace founded on Seedling Structure. New 
Phyt., 1907, p. 85. 
