7 o8 Gwynne- Vaughan — Observations on the 
course of which is under dispute. To return to the stem ; the 
internal strands are small and round, and about twenty or 
thirty of them are scattered in the ground-tissue within the 
ordinary stelar cylinder. At each leaf-insertion four of them 
approach the margin of the leaf-gap and join on to it exactly 
at the points of departure of certain of the leaf-traces 
(Figs. 15 and 16). The first pair join on to the traces a, a, 
the next pair divide each into two branches, which join on 
to the traces b, b and c, c, respectively. In their course from 
the leaf-gap margin down the stem the internal strands run 
first of all towards the centre, and then, turning more directly 
downwards, they travel obliquely towards without, diminishing 
as they do so, and finally ending blindly without coming into 
contact with the external stelar cylinder. The leaf-trace 
protoxylems are all endarch, but they gradually become 
mesarch as they pass down the stem ; those of the leaf-traces 
that abut upon internal strands are continued down the 
internal strands ; those of the others run down the margin 
of the leaf-gap, joining on to each other as they do so, and 
rapidly disappearing after the leaf- gap has closed. In other 
species (C. arbor ea and C. glauca) Tr^cul has shown that 
matters are much more complicated and obscure, because 
the number of internal steles related to each leaf-insertion 
is greater, and those leaf-traces that abut upon the internal 
strands often stand away from the leaf-gap margin, remaining 
connected with it only by a short horizontal strand, or they 
may even be altogether free from it. 
Considerable light is thrown upon the nature of the central 
strands of the petiole by the structure of Dicksonia Baro- 
metz. There are no internal vascular strands at all in the 
stem of this Fern, but only the ordinary stelar cylinder. 
The leaf-gaps are very small and close up rapidly, neverthe- 
less they occasionally overlap each other, and therefore the 
structure must be regarded as dictyostelic, although it is very 
near solenostely. The leaf-trace departs as a single piece, 
but sooner or later it breaks up into a large number of 
separate strands. The point at which the disintegration 
