*5 
Vascular System of Pteris. 
Figs. 36—57. Pteris aquilina. Selected sections from a transverse series 
through the piece of rhizome represented in Fig. 35, shewing the 
departure of two successive leaf-traces. Vascular strands and scleren- 
chyma represented as in previous figures, c.s., compensation-strands, 
see below. 
Fig. 5S. Pteris aquilina. Diagram of the course of outer vascular strands 
of piece of rhizome in F'ig. 35 represented as spread out in a plane, the 
dorsal strand in the middle. Sections of the leaf-trace strands are 
shewn where they pass out of the plane of section, those derived from 
the inner strands of the rhizome black. An inner strand (c.s.) passes 
into the outer circle as a “compensation-strand” and helps to close 
each leaf-gap. 
petiolar system on the basiscopic side may arise directly from the 
inner strands of the rhizome behind the fork and run independently 
of the branch system, though the rest of the leaf-trace arises 
directly from the latter. This is a frequent case. Finally, if the leaf 
is inserted actually in the angle of the fork, its trace arises from the 
rhizome strands behind the fork quite separately from the branch. 
The last two types are apparently covered by Mettenius’ first case. 
It is noteworthy that the vascular system of the petiole, when 
it arises in the actual angle of the dichotomy, does not in this 
species ever become so important in relation to that of the rhizome 
as to form a direct continuation of the latter, the branches of the 
dichotomy being nipped off from its sides, as in the case of Pteris 
incisa var. integrifolia. On the other hand, though we have not 
found cases shewing intermediate vascular structure, it is easy to 
see how the weaker branch of the dichotomy which in this type is 
developed late and is much smaller than the stronger one, could, by 
a still further delay in its development, be carried up onto the 
petiole and so lead to the type in which a bud arises from the 
posterior side of the petiole base. In this way it is possible to 
suppose that this type of branching, not uncommon in ferns, may 
have been derived from primitive dichotomy in which a leaf arose 
on the inside of one of the branches. This appears to have been 
Stenzel’s view (’61, p. 40 and footnote). In any case it is significant 
of some genetic connexion that where in this species there is 
branching in the neighbourhood of leaf-insertion, the relation of 
position between the stronger branch, the weaker branch and the 
leaf is constant. The whole subject of the morphology of branching 
in ferns and its relation to vascular structure—a subject which 
occupied a considerable amount of attention about the middle of 
last century—deserves re-investigation. 
Fig. 58 is a representation of two nodes and the intervening 
internode, the cylinder formed by the outer rhizomic system being 
spread out on a plane ; the inner strands (black in section) are 
