Factors in the Evolution of Solenostely. T49 
hence the ventral portion of the xylem-ring would be broader than 
the dorsal, as is actually the case ” in these types. 
Another objection which may be urged against the belief that 
the stelar types characteristic of the Lindsayeae are immediately 
derived from those found among the Gleichenias, is the fact that 
while the latter belong to the lowest soral grade—the Simplices, 
the former are typical Mixtas. It is however perfectly possible to 
suppose that in some cases the evolution of a “ mixed ” type of 
sorus took place with comparative rapidity from a primitive 
condition, while the stelar system was relatively little modified, and 
this interpretation is strengthened hy the fact that transitions from 
one type of sorus to another are known within the limits of a single 
mentis in more than one instance. 
The evolutionary series indicated is also very strongly supported 
by evidence from the anatomy of the young plants of the more 
advanced Leptosporangiate ferns, a topic with which it will be more 
convenient to deal separately in a later lecture. 
Factors in the Evolution of Solenostely. 
Having thus traced the evolution of the solenostele from the 
protostele through a very complete series of intermediate forms, it 
remains to enquire as to the factors which have brought about such 
a course of evolution. 
The first point we have to notice is that the solenostele is 
normally associated with the arched type of leaf-trace, i.e. with a 
type which itself resembles a solenostele incomplete on one side. 
It is a striking fact that this type of leaf-trace is also found in many 
protostelic forms. We saw in the second lecture that the C-shaped 
type of petiolar strand is common in the protostelic Botryopterideae 
though the leaf-trace itself is isodiametric or nearly so. We also 
saw that there is a very widespread if not a universal tendency to 
what I have called “ basipetal evolution ” of the vascular system, 
based no doubt on the fact that the evolution of the leaf itself is 
the main determining factor in the changes occurring in the 
vegetative body. The C-shaped type of petiolar strand is correlated 
with a flat frond possessing lateral pinnae, and whatever the exact 
method of origin it is certainly well adapted to the vascular supply 
of such a frond. Increased demands on the part of the leaf will 
certainly tend to the carrying down of the C-shaped structure to 
the point of junction of the stele of the rhizome, and this is the 
condition found in such a form as Gleichenia Jlabellata. The effect 
