Vascular System of Pteris. 
7 
become open to the exterior so as to form two deep lateral bays of 
the stele, and the whole structure increases greatly in size. 
Meanwhile the vascular ring has broken in the mid-dorsal line 
(Fig. 8). The central ground tissue now has the form of an 
irregular H lying on its side, closed in all round, except in the mid¬ 
dorsal line, by the vascular ring. The dorsal limb of the H is 
rather longer than the ventral one. 
The right-hand end of this dorsal limb, which has retained the 
waviness of outline of the original stele, is now nipped off as a closed 
ring and becomes the stele of one of the branches of the stem 
(Fig. 9, St. It is contributed to by a small vascular branch 
nipped off as a closed ring from the right-hand end of the ventral 
limb of the H. This quickly becomes attached to the ventral side 
of the branch stele, but retains its individuality for some distance 
(Fig. 9), eventually fusing completely with the stele to form one of 
the bays of its wavy margin. 
The left-hand end of the dorsal limb of the H is now nipped off 
in the same way (Fig. 10) and becomes the stele of the other branch 
(St. Bv.. 2 ) ; like the stele of the former branch it is also contributed 
to by a strand arising from the left-hand end of the ventral limb of 
the H. This strand breaks up into two before joining the stele of 
the branch (Fig. 11.). The third strand, lying on the ventral side 
of this stele (Fig. 12), has taken its origin previously from the outer 
side of the internal fold (A) of the original solenostele (Fig. 7). The 
three strands now join the ventral side of the branch stele and form 
the three central projections of the wavy ventral side (Fig. 12). 
The abbreviated H (Fig 12) is now the petiolar meristele. It 
is to be noticed that the ventral side of its ventral limb and the 
dorsal side of its dorsal limb (left white in the figures), i.e. the 
parts corresponding to the arch and the free edges of the primitive 
curved petiolar strand from which it must have been phylogenetic- 
ally derived, are continuous with parts of the solenostele of the 
stem below, while the deep lateral bays A ] and B 1 (black in the 
figures) are continuous with the folds (A and B) which arose in the 
solenostele as the node was approached. 
The structure of the vascular system at this type of node, in 
which the leaf arises between the two branches of what is 
practically a dichotomy, is at first sight difficult to bring into 
harmony with a node at which the leaf-trace is directly inserted on 
the stele of an axis that is continued as such beyond the origin of 
of the petiole. 
