Anatomy of Solenostelic Ferns . 717 
Lindsay a cultrata , is very small and thin, and, moreover, the 
leaf-trace departs as two separate strands and not as a single 
piece. 
Tansley has suggested that the two types of stele exhibited 
by Davallia repens and D. pinnata represent structures inter- 
mediate between protostely and dictyostely. I thoroughly 
agree with this, and consider that the type of stele found 
in D. aculeata may now be added to this series. The almost 
exactly parallel stages passed through by the vascular system 
in the young plant, even in such an advanced dictyostelic 
Fern as that described on p. 710, appear to me to give the 
suggestion a high degree of probability. As I understand 
the facts, the idea is that as the leaf and the leaf-trace in- 
creased in importance relative to the stem, the phloem lying 
on the adaxial side of the leaf-trace became extended down- 
wards into the substance of the xylem of the protostele. 
Gradually reaching further down through the internode this 
internally decurrent phloem at length came into contact with 
that decurrent from the leaf-trace below, and a continuous 
solid core of phloem was thus formed within the stele. Then 
the ground-tissue lying in the adaxial concavity of the leaf- 
trace also began to extend downwards into the stele, forming 
at first a prolongation that ended blindly in the core of 
phloem, but eventually it reached down from one leaf-trace 
until it met with that decurrent from the leaf-trace below. In 
this manner an internal strand of ground-tissue was formed 
which is continuous throughout the stem, and the stele has 
become a solenostele. Now if such a series of changes were 
to take place in a dorsiventral rhizome with the leaves inserted 
only on the dorsal surface, it is extremely probable that the 
phloem and ground-tissue decurrent from the leaf-traces 
would not at first occupy the very centre of the stele, but 
would lie nearest to the dorsal surface on which the leaves 
are inserted, and hence the ventral portion of the xylem-ring 
would be broader than the dorsal, as is actually the case 
in Davallia repens , D. pinnata , and D. aculeata. What is 
not so easy to understand is why the xylem-ring should 
