6 
A. G. Tansley. 
fold, the lower or dorsal arc of the pinna-trace being derived front 
the dorsal margin and the upper or ventral part ( i.e ., the two 
ventral arcs), together with the ventral limbs of the lateral loops, 
arising from the ventral (upper) margin of the lateral fold of the 
petiolar system (Fig. 97). The vascular system of the pinna has 
d, v, l, ditto of secondary pinna system. It will be seen that the latter departs 
from the two edges of the gap bordering on L, and that each part arises from 
the corresponding part of the parent axis. 
the same general form as that of the petiole, though it is of course 
on a smaller scale. It is difficult to resist the conclusion that the 
lateral folds of the petiolar system have been elaborated in relation 
to the insertion of numerous large pinnae. It will be seen that they 
are developments of the edge of a gap bearing very much the same 
relation to the pinna-trace as the leaf-gap in the cauline stele does 
to the leaf-trace, and when we remember that the elaboration of 
the internal accessory system of the stem is connected in these forms 
with the elaboration of the leaf-trace, the analogy becomes very 
fairly complete. Thus we seem able to extend the “basipetal evolu¬ 
tion” of the vascular system, which as we have clearly seen, obtains 
in the evolution of the cauline stele, back into the frond, and to base 
it upon the increase in size and complication of the laminal system. 
It must of course be remembered that the evolution of the vascular 
system in the frond still has to be worked out in detail. The 
available facts are much too scanty to permit us to do more than 
suggest the main lines upon which this evolution has proceeded. 
In the Marattiacese the internal accessory strands of the 
