Polycycly. 195 
from one to three separate concentric vascular strands present in 
the pith of the stele, and never coming into connexion with the 
main cylinder except at the nodes. At each node a single internal 
strand approaches the free margin of the leaf-gap, and gradually 
fuses with it, until the two xylems are perfectly continuous, giving 
an appearance exactly as in D. adiantoides. As soon as the leaf- 
trace has departed the internal strand separates from the cylinder 
and passes as a free strand into the internode beyond. Distinct 
protoxylem groups in a mesarch position are found in these internal 
strands, but they do not appear to be in any way related to those of 
the leaf-trace. 
The frequent development of lateral shoots from the bases 
of the petiole, with a vascular supply originating from the 
base of the leaf-trace, may provide, in these plants, the immediate 
stimulus to the development of the internal strands of the main 
stele. Separate vascular strands sometimes run from the margin 
of the leaf-gap, from the point where the vascular cylinder is 
reinforced by the junction of the internal strand, directly to the tissue 
supplying the cylinder of the lateral shoot (Fig. 58, A). 
A more elaborate system of internal accessory vascular strands 
is to be met with in P ter is elata (Fig. 59), which has an erect or 
oblique rhizome, with the leaves arranged radially. The vascular 
system is perfectly solenostelic and the leaf-gap is confined to the 
Fig. 59. Ptcris data var. Karsteniana. An internal accessory perforated 
cylinder connects by an ascending branch with the edge of the leaf-gap (here 
confined to the base of the trace). An additional delicate strand is seen inside 
this cylinder and connected with it just in front of the point of closure of the 
gap made by the departure of the ascending branch. After Gwynne-Vaughan. 
base of the trace. The structure of the internal accessory system 
varies according to the dimensions of the plant, and according to 
the point at which the section is taken. In a rhizome of average 
thickness (Fig. 59) the internal system usually consists of a large 
gutter-shaped strand or of a completely closed cylinder, the latter 
being generally present for some distance below each node. As the 
