3 1 6 Guynne- Vaughan. — -On Polystely 
existence in any particular region would thus depend upon 
the physiological conditions to which that region is exposed. 
In P. Auricula and P. Palinuri , although the leaf-trace 
produces no internal vascular bundles at any point in its 
foliar course, yet they are invariably present in the cauline 
portion, appearing as soon as the leaf-trace turns downward 
in the stem. 
The Leaves of the Seedling. 
The leaves of the seedlings of these plants exhibit, as 
regards their structure, an interesting series of gradations 
through which the leaf-traces become gradually less and less 
complex as the cotyledons are approached. In those species 
in which the petiole of the adult form of leaf is polystelic 
(P. japonica, P. denticulata ), the first step that takes place is 
the reduction of the steles to meristeles throughout the whole 
length of the petiole, which takes place at about the seventh 
leaf. Then the leaf-trace ceases to give off lateral branches, 
so that the petiole contains one meristele only. Finally, this 
meristele itself diminishes in size until it appears simply as 
a single small collateral bundle. In P. involucrata and 
P. Auricula similar changes take place: but since the traces 
in the adult form of leaf itself are only meristeles, the first 
step that takes place in descending to the earlier leaves in 
the former is a reduction in the number of branches given 
off by the median trace, and in the latter a reduction of the 
leaf-traces given off from the steles of the stem. In P. Auricula 
the traces are at first reduced to three, and then, at the first 
or second leaf above the cotyledons, the leaf-trace in both 
species is represented by a single small bundle only. 
These gradations indicate progressively the various arrange- 
ments employed by the individual plant to provide the amount 
of vascular tissue necessary for the steadily increasing needs 
of the leaves. And, as far at least as the later steps are 
concerned, they probably represent phylogenetically the 
several devices by which the ancestors of the plants en- 
