A Theory of the Double Leaf-Trace. 87 
form. The latter is generally associated with extremely definite 
and constant features, which suggest a certain stereotyped rigidity 
unfavourable to the evolution of new forms. The tetrarch type, on 
the other hand, particularly as seen in Gymnosperms, gives an 
impression of plasticity and variability, in the grouping of the 
bundles (two central, or one central and one lateral), and in the 
occasional appearance of diagonal tetrarchy, etc., which gives 
ample scope for the derivation of all the modifications seen. 
The vestigial theory certainly appeals to one as a more plausible 
explanation of the intermediate form seen in Liriodendron tulipifera, 
Clematis Hendersoni, and curiously enough in some Composites, 
than the possible view which would regard it as a step in the 
progressive direction. It is difficult to see what functional advantage 
would accrue from such a very abortive attempt to form the 
intermediate poles of a tetrarch root, while it can very readily be 
understood, if regarded as an ancestral feature not completely 
eliminated. 
Further, it is to be remembered, that where the root changes 
from tetrarch to diarch, it is always the basal hypocotyledonary 
end, i.e., the older first formed portion, which is tetrarch, while the 
later apical end becomes diarch. It is reasonable to suppose that 
the modifying influences, due to the correlation of function 
with external conditions, would operate on the root and on the 
cotyledons, and affect least the hypocotyl, which is as a fixed 
point between two forces, only moved when these are in the same 
direction. Thus we find that in Althaea rosea and in many of the 
Leguminoseae the four bundles of the cotyledon lamina become 
approximated to form a central group in the petiole, but the 
hypocotyl and root are nevertheless tetrarch. In Mirabilis, on the 
other hand, the cotyledon bundles are separate, and although the 
apex of the root is diarch, the hypocotyl remains tetrarch. Even 
in the most characteristically diarch families, if the laterals are 
persistent and independent, the top of the hypocotyl shows a more 
or less definite tetrarch stage ( Liriodendron tulipifera, Clematis 
Hendersoni, Lunaria biennis, Inula graveolens). 
But the hypocotyl is liable to certain modifications in response to 
its own particular function—that of support—and this modification 
is seen in both types. Where the cotyledons are epigeal, and the 
hypocotyl early bears a weight of foliage and has to provide for 
the insertion of numerous leaf-traces—as in many tree forms—the 
hypocotyl is stem-like in structure through the wide separation of 
