148 
A. G. Tansley. 
LECTURE V. 
The Evolution of the Solenostele.—The Schiz^eacE/E. 
[Figs. 47—50.J 
The series of types described in the last lecture clearly forms 
a beautifully complete chain connecting the protostele with the 
solenostele, and there is the strongest reason to believe that the 
latter type of vascular structure has arisen from the former through 
similar phylogenetic stages, though it is quite possible that the 
solenostele or some similar type of structure may in other cases 
have arisen somewhat differently. It may of course be suggested 
that the Wibelia- and Lindsaya -types are derived by reduction from 
the solenostelic condition. There is however nothing whatever to 
suggest such reduction, and in Gwynne-Vaughan’s opinion (’03, p. 
718), “ the increased thickness of the lower region of the xylem-ring 
forms an insuperable objection to the general application of any 
reduction hypothesis to this series.” As this author points out (’03 
p. 717), “ if such a series of changes ” [as would be brought about 
by the gradual extension first of the phloem, and then of the 
sclerenchymatous ground tissue, or by both of these simultaneously, 
from the concavity of the leaf-trace into the xylem of the stele, till 
connexions were formed with the same tissues from the next leaf- 
trace below] “ 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 
