yio 
Hill and de Fraine . — On the 
by a rotation of the xylem in order that the protoxylem may be situated in 
an exarch position. Finally, the opposing phloem-masses unite in pairs ; 
thus a diarch root-structure obtains. 
9. There is, however, some variation. There is frequently no definite 
rotation of the protoxylem, the exarch position being gained rather by the 
movement of the metaxylem elements, e. g. Taxus , Juniperus virginiana , 
J. Cedrus , Cnpressus obtusa , C. pisifera Libocedrus , and Callitris . 
The remaining plants considered exhibit a more definite rotation of the 
protoxylem. 
These variations differ in degree in the several plants, and they merge 
one into the other. Further, it may happen that the trace of one cotyledon 
shows practically no movement, of a definite nature, of the protoxylem, 
while the same tissue of the other seed-leaf may exhibit a well-defined 
rotation e. g. J.procera . 
There is also a well-pronounced variation, more especially in the Cupres- 
sineae, in the level at which the transition begins, and this not only in 
different species of the same genus, but in individuals of the same species. 
The transition-phenomena may start in the cotyledons themselves ; or, in 
the topmost part of the hypocotyl, before the traces have reached the central 
region ; or, finally, it may be further postponed and take place at different 
levels of the hypocotyl. It is obvious that, in the first two cases, a root- 
structure obtains practically throughout the whole of the hypocotyledonary 
axis. 
10. The transition-phenomena in some of the polycotyledonous forms 
described ( Cnpressus tondosa , C. macrocar pa> and Sequoia gig ante a) follow 
the same general course as is summarized above, but showing certain 
variations which are due to the increased number of seed-leaves. The 
consideration of these leads to the inference that some of the cotyledons 
probably represent the halves of single pre-existing seed-leaves ; in other 
words, that the dicotyledonous condition is the more primitive, and that 
polycotyledony has been derived from it. 
11. Podocarpus , and possibly also other plants of the Podocarpeae, 
differ from the other members of the Taxaceae and the Cupressineae, 
inasmuch as each of the two cotyledons have two vascular bundles, which 
together form one pole of the primary diarch root. 
Root. 
12. An assise de soutien occurs in the roots of Cephalotaxus , Taxus , 
and Juniperus. 
13. The following table shows the relations, in the plants examined, 
between the number and nature of the cotyledons and the root-structure. 1 
1 Van Tieghem (loc. cit.) states that the primary root of the Taxaceae and Cupressineae is 
diarch, and is in agreement with the number of cotyledons. Occasionally, when the number of 
seed-leaves is three, the tap-root is triarch. 
