Anatomy of Solenostelic Ferns. 
739 
instance, the first appearance of internal parenchyma might 
take place in the centre of the xylem of the protostele. The 
xylem-ring thus formed might subsequently become interrupted 
by the departure of the leaf-traces, and finally a structure 
resembling a solenostele might be attained by the gradual 
differentiation of phloem and endodermis through the leaf-gaps 
and all round the inside of the xylem-ring. Again, an internal 
endodermis might be differentiated within the internal paren- 
chyma before the xylem-ring is interrupted, in which case 
the phloem alone would have to extend around the inside in 
order to bring about the above-mentioned structure. Finally, 
the internal phloem and endodermis might both be dif- 
ferentiated within the internal parenchyma before ever the 
xylem-ring is interrupted, and then the formation of leaf-gaps 
would do no more than set the internal tissues in continuity 
with the external. Although the first of these methods alone 
has been ascribed to the Cyatheaceae and Polypodiaceae 
above, it is very probable that the others may occur in other 
orders. Indeed Boodle 1 has already made some suggestions 
on similar lines in relation to the Schizaeaceae. 
In a vascular system arising according to the two last 
methods it is evident that there can be no two opinions 
regarding the stelar origin of the central parenchyma. In 
the first two cases, however, it is possible to conceive of the 
intrusion of the cortex into the stele in the manner previously 
explained. It is difficult to see where any conclusive evidence 
upon this point, one way or another, is to be sought for. 
It is, however, reasonable to suppose that if the young plant 
of such a Fern as Davallia pinnata be examined at different 
stages in its development, some light may be thrown on the 
matter by the comparison of the state of affairs at those regions 
of the apex where the distinction between stele and cortex 
first becomes evident : or, perhaps, even by the comparison 
of the same region in the mature plant with that in a typically 
solenostelic Fern. 
1 On the anatomy of the Schizaeaceae, 1 . c., p. 407, and Further observations 
on Schizaea, Annals of Botany, vol. xvii, no. lxvii, p. 530, 1903. 
