284 Bower . — Studies in the Phytogeny of the Fi lie ales. 
of phloem on either side of the xylem presents no characters for remark. 
The xylem varies in its thickness, thinning off especially where a leaf-trace 
is about to depart. It consists of tracheides mixed with parenchyma. 
The tracheides are of uniform character, and, as is often the case in the 
Cyatheaceae, there appears to be no protoxylem. Not only is this absence 
of protoxylem seen structurally, but it is also shown in sections of young 
stems by the fact that the tracheides mature almost simultaneously over the 
whole transverse area. 
Though the axis is often very bulky, no medullary vascular system has 
been seen. 
Anatomy of the Leaf. 
The leaf, which is slightly dilated at the base, is continued upwards 
with an almost cylindrical stipe, marked on either side by pale lines 
similar to those in the Bracken, and often accompanied by dark streaks of 
sclerenchyma. These are interrupted, just as in the Bracken, at the first 
pinnae, which are inserted in large leaves at two or three feet from the base. 
The leaf trace has been seen to come off as a simple meristele, which 
soon assumes a complicated horseshoe outline, with lateral involutions. 
In small leaves it may remain throughout as a single continuous strand ; 
but in large leaves it commonly divides close to the leaf-base into three 
distinct portions, which in very large leaves may be widely separated 
(PI. XXXIV, Fig. 9). In leaves of smaller size the margins of these several 
portions may remain in close relation, or the strands may even fuse with 
one another from time to time. But sooner or later they finally join again 
to form a single band. This was found to be completed in a specific case 
within nine inches of the base of the leaf (Fig. 13 bis , a , b). 
Very soon after leaving the axis protoxylem-groups make their appear- 
ance in the vascular strand, which had previously been without them. They 
are characterized in the usual way, but are very numerous, forty-seven being 
counted in a specific case. Each protoxylem acts as the centre of a ‘diver- 
gent’, and lies apparently nearer the periphery of the section than the meta- 
xylem ; but in actual fact the protoxylem elements, accompanied by cavity 
parenchyma, are really on the adaxial face of the xylem of the meristele. 
Since this arrangement impresses itself on the whole contour of the strand, 
a characteristically corrugated outline results (PI. XXXIV, Fig. 13 bis ; 
PL XXXV, Fig. 14). This condition is continued with little change in form 
of the trace, or even in number of the protoxylems, up to the level of the 
first pinna. 
Opposite the involutions of the horseshoe-shaped strand a break in the 
continuity of the band of sclerenchyma which bounds the section may be 
seen on either side. The epidermis is there thin, with numerous stomata, 
while a lacunar ventilating tissue lies below it. This structure constitutes 
those lateral lines already observed from without (Fig. 13 bis). 
