Anatomy of Solenostelic Ferns . 695 
leaf-gaps remain open long enough after the departure of the 
leaf-trace they will eventually overlap ; again, the same result 
will also be obtained if the leaves be crowded sufficiently close 
together, although the leaf-gaps may close up comparatively 
rapidly. 
In some of the intermediate forms the leaf- gaps only over- 
lap now and then, so that the stem, to a certain extent, still 
remains in a solenostelic condition. In others again the over- 
lapping is more general, and a complete stelar cylinder is 
only to be found at rare intervals. It will therefore be under- 
stood that cases sometimes arise where it is impossible to say 
definitely of the vascular system of the plant as a whole 
that it is either solenostelic or dictyostelic. 
The investigation of these forms shows that the distinction 
drawn by De Bary 1 between the dorsiventral and the radial 
type of vascular arrangement in dictyostelic Ferns is one of 
considerable value, because it will be seen that the structural 
features of the two types depend upon the different methods 
by which they are derived from the solenostele. 
The transition from solenostely to dictyostely in a dorsi- 
ventral rhizome with two rows of leaves, one on either side 
of the upper surface, will be first considered. A reference to 
the diagrams (Figs. 7 and 8) will show the effect of closely 
crowded leaves, or of long persisting leaf-gaps upon the 
solenostele. It is seen that the dorsal internodal portion of the 
solenostele has become so reduced that it is now no more 
than a mere strand running across between each leaf-insertion 
from one margin of the large ventral portion of the solenostele 
to the other. A structure such as this may be found in the 
rhizomes of Nothochlaena Marantae (Fig. 7), trichomanoides , 
ferruginea, Pellaea rotundifolia (Fig. 8), andromedae folia, A dian- 
turn trapeziforme, Kaulfussii, and Gymnogramme vestita. A 
similar type appears also to be present in A n trophy urn reticu- 
latum , but it is a little exceptional and will be referred to again 
later on. Transverse sections of the stems of these Ferns 
will in most cases exhibit a single large gutter-shaped 
1 Comparative Anatomy, pp. 284 and 287 (Engl. ed.). 
3 B 
