Anatomy of Solenostelic Ferns. 723 
Upon the whole, therefore, I am inclined to regard this 
type of leaf-trace as relatively primitive, and as one from 
which the various more complex forms found in the Cya- 
theaceae and Polypodiaceae may be derived. Further, I agree 
with Bertrand and Cornaille in their important conclusion 
that, except in Acrosticum tenuifolium and A. sorbifolium (in 
which the petiole seems to possess accessory peripheral strands 
not derivable from the ordinary system ) 1 , every modification 
undergone by the vascular system of the petiole in these two 
orders takes place in pursuance of a single main design. As 
the petiolar bundle increases in size the curvature of the xylem 
is followed by the vascular strand as a whole. The strand 
thereby takes up the form of an arc or an f| (Fig. 26), and 
as it increases further in size and curvature it comes to 
assume different forms of gradually increasing complexity, 
which when seen in section give rise to various different 
outlines, suggesting an arch, horse-shoe, amphora, &c. Later 
on the single strand may break up into separate portions, but 
these always remain so related to each other that it is still 
possible to determine the sectional outline of the strand from 
which they were derived. It has already been stated that 
a single cordate petiolar bundle is generally related to stems 
with the Lindsay a- type of stele. The petioles of solenostelic 
stems also usually contain a single strand, but of varied and 
often complicated outline. In stems with advanced dictyo- 
stelic structure the petiolar bundle is generally broken up 
into two or more portions ; although when the structure is but 
little removed from solenostely a single strand is often to be 
found. 
According to Bertrand and Cornaille (loc. cit, cap. iii) 
there are two main regions to be distinguished in the vascular 
system of a Fern petiole. In that of Cyathea Brunonis , for 
instance (Fig. 15), there is a large folded curve extending 
abaxially from c to c , and there are two adaxial arcs, one 
on either side, extending from c to b. In petioles of a sim- 
pler structure, such as those of Davallia tenuifolia (Fig. 25) 
1 1. c., p. 133 ( Polybotrya Meyeriana and Lomariopsis fraxinifolia ). 
