94 MR. R. H. COMPTON: AN INVESTIGATION OF THE 
einen Mitteleharakter zwischen den streng centripetalen Wurzel- und den 
centrifugalen Stengelstriingen bekunden." She offers no theory to explain 
the presence of the centripetal xylem in the epicotyl, nor does she describe 
its connection with the root xylem, Gérard (1881, p. 351) examined Lrvum 
Lens and other Viciez, and came to the conclusion that the centripetal mass of 
xylem is the upward prolongation of the deeper part of the triarch root star, 
the outer portions of which alone enter the cotyledons and the first leaf, He 
considered that this *root-strueture? in the epicotyl is an example of a 
transition prolonged beyond its ordinary limits. Tourneux (1910, p. 325) 
considered that the centripetal wood should not be regarded as a residue from 
the root ; and suggested that it is a relic of the primitive condition of the 
xylem found in the stems of many Gymnosperms and Cryptogams. This 
theory is the only one hitherto advanced which attempts to explain the 
centripetal xylem in a phylogenetic fashion, and it is in many ways attractive. 
The description by Scott (1902) of the gradual change from exarchy to 
endarchy in the wood of the Pteridospermi, taken together with Chauveaud's 
demonstration (1911, p. 259) that the protoxylem of stem and leaf in the 
Angiosperms is not primitive in the same sense as that in the root, leads to 
an interpretation. of the stem, hypocotyl, and root as differentiated parts 
of the same primitive axis with exareh development of the xylem. On this 
view the appearance of centripetal xylem in a stem would be merely a 
phenomenon of reversion to a more primitive type of axis, a type now normal 
only in the root and to some extent in the hypocotyl. 
A great objection to the adoption of this view in the case of the Vicieæ 
lies in the highly advanced character of the group, and the absence of other 
signs of primitiveness, Moreover, in no other more primitive Angiosperm 
has true centripetal xylem been discovered in the stem. Thus it appears 
difficult to aseribe phylogenetie significance to the presence of centripetal 
xylem in the stem of the Vicieæ. It is more probable that the structure in 
question is cenogenetic, and that an explanation should be sought in the recent 
needs of the plant rather than in an assumed reversion to 
condition, 
an ancestral 
It appears to the present author that a more probable explanation of the 
central xylem core сап be found. In the first place, the development of this 
core is complementary to that of the lateral xylems of the central cylinder ; 
these lateral xylems only appear as distinct entities when the central xylem 
core is absent, This is found to be the ease when the lower and upper inter- 
nodes of the same species are examined (see, e. 9-, Mlle. Goldsmith’s plate v. 
figs. 15 & 16), and also in the first internode of Vicia Faba, which has a large 
pith. It is thus suggested that not only are the two kinds of xylem comple- 
mentary but they are identical, and that the solid xylem core characteristic of 
the majority of the Viciew has arisen simply through the lateral compression 
