194 
A. G. Tansley. 
Solenostelic Types. 
It is to Gwynne-Vaughan’s beautiful researches on the soleno¬ 
stelic ferns' that we owe the discovery and elucidation of the first 
beginnings of the polycyclic condition in a series of species 
belonging to the genus Dennstcedtia, a discovery which provides the 
morphological key to the great majority of cases of polycycly, 
whether occurring in solenostelic or in dictyostelic types. 
The simplest case is found in Dennstcedtia apiifolia. I his fern 
possesses a typical solenostele, but with a very decided thickening 
of the xylem of the leaf-gap margin, which is often twice as thick 
as other parts of the xylem cylinder. In D. adiantoides (Fig. 58, A) 
Fig. 58. Evolution of polycycly. A. Dennstcedtia adiantoides. The 
internal thickening of the leaf-gap edge can be seen projecting into the pith 
and continuing into the internode, l.t., leaf-trace ; l.sh., lateral shoot. 
B. Dennstcedtia rubiginosa. Two distinct free internal strands can be 
seen, one connected with the edge of the leaf-gap. After Gwynne-Vaughan. 
the thickening of the xylem of the leaf-gap margin projects markedly 
towards the axis of the stem and further is not confined to the 
region of the leaf-gap, but is continued as a ridge along the internal 
surface of the vascular cylinder, throughout the whole length of 
the internode from one leaf-gap margin to the next. The additional 
xylem elements become separated as a distinct strand of xylem in 
the internode, and this is sometimes surrounded by a distinct 
phloem sheath of its own. The detachment reaches its maximum 
towards the upper end of the internode, while the additional xylem 
is most completely fused with that of the main vascular ring 
towards the upper end of the leaf-gap. In Dipteris conjugata there 
are rather similar thickenings of the leaf-gap margin, hut these are 
practically separated from the tissue of the vascular cylinder by 
ground-tissue. In Dennstcedtia rubiginosa (Fig. 58, B) there are 
1 Gwynne-Vaughan (’03). 
