Polycycly. 193 
states that there is no internal endodermis, but Gwynne-Vaughan 
finds that the plant is dictyostelic. In A. plantagineum both these 
authors agree that there is no internal endodermis, though the 
“ pith ” is histologically unlike the cortex. A little internal phloem 
is present on the ventral side of the stele (Fig. 57, B). In the genus 
Vittaria similar phenomena are met with. In V. stipitata the 
“ decurrent ground-tissue ” forming the pith decreases downwards 
through the internode, and in the specimens examined by Gwynne- 
Vaughan, is not continuous with that belonging to the node below. 
In this respect the plant resembles Wibelia pinnata (see p. 146), but 
there are facts which suggest reduction. Thus there are two strands 
of the leaf-trace, one departing from each side of the foliar gap. The 
stele is very small, the xylem-ring has a thickness of one or two 
tracheids only, and, unlike Wibelia, is equally narrow on all sides. 
Further, the pith is not surrounded by internal phloem, which is 
present in very small quantity, and is absent from the ventral side 
of the stele. The ground-tissue pith, as it dwindles, is replaced by 
tissue like that of the pericycle, suggesting a replacement of internal 
phloem in the course of phylogeny. 
Facts of this kind naturally lead to the belief that the Schizcea - 
stele also has originated by reduction from solenostely (see p. 154), 
and this is certainly a possible view, though there is absolutely no 
evidence in Schizcea of internal phloem, and it is at least equally 
possible that here there has been a direct derivation from a solid 
protostelic form. 
Polycycly. 
In a good many Ferns the vascular tissue is not confined to a 
single cylinder of haplostelic, solenostelic, or dictyostelic con¬ 
struction, but appears as two or more concentric cylinders or 
systems of strands. To all such structures the term polycyclic may 
be applied, as opposed to the much more frequent, and certainly 
primitive monocyclic condition. 
The polycyclic condition is sometimes found as a special 
development in certain species of an otherwise monocyclic genus, 
while in other cases, notably in Matoniaceae, Cyatheace^e and 
Marattiaceae, it is characteristic of a whole family or group. It is 
no doubt a response to a need for an increased vascular supply when 
for one reason or another this is not obtained by a mere increase in 
the diameter of the stele. 
