204 WorsdelL — The Vascular Structure of 
cylinder, and the centripetally-formed xylem of the bundles 
composing the latter. These are, I think, ancestral characters 
which, in the vegetative part of the stem, have been lost 
owing to special modification of its tissues for functional 
purposes, and which in the peduncle, as having undergone 
much less modification in structure, have been retained. This 
more primitive structure of the peduncle has been emphasized 
by Solms-Laubach 1 as a result of his observations on the 
course of the vascular bundles in this organ, and also by 
Scott 2 , who first discovered the presence of centripetal 
xylem here. 
Precisely as in the case of the peduncle, we shall find that 
the sporophyll , the normal foliar appendage of the former, has 
also retained certain primitive characters which are not found 
in the foliage-leaf. But in the following pages two factors 
have to be considered in the structure of the former — the 
presence of these primitive characters, and the modification 
of its general structure in connexion with the sporangiferous 
function. 
In order to throw light upon the structure of the sporophyll 
I shall proceed to compare it with that of the foliage-leaf. 
In the base of this latter organ, which is the first -formed 
portion, the vascular bundles have a purely endarch structure ; 
higher up in the petiole, however, a mesarch structure of the 
bundle obtains, the centripetal xylem gradually increasing, 
and the centrifugal xylem gradually decreasing, in quantity 
from below upwards, until the former is almost entirely pre- 
dominant. The number of the bundles is almost uniform in 
every part of the petiole. The difference in structure between 
the bundles in the leaf-base and those in the remaining 
portion of the petiole is explicable from the fact that a 
transition occurs in the bundles of the lower region of the 
petiole from the structure peculiar to the stem to that which 
is peculiar to the leaf. 
When the structure of the bundle-system in the sporophyll 
1 Die Sprossfolge der Stangeria und deriibrigen Cycadeen, Bot. Zeit. 1890. 
2 Loc. cit. 
