502 Tans ley and Chick . — On the Structure 
as the first definite advance on the protostele with solid xylem. 
This is the condition found in Lindsay a. With further dilation 
the extra-stelar pouch appears at the node, just as in Schizaea , 
only here it replaces phloem instead of intra-stelar parenchyma. 
The meeting and fusion of the pouches, which is in process of 
happening in Microlepia pinnata , will then result in the forma- 
tion of the typical ‘amphiphloic siphonostele 1 of Jeffrey or 
c solenostele ’ of Gwynne- Vaughan 1 . 
The alternative, and as some will perhaps think, the more 
natural hypothesis, is that the Schizaea-stzlQ has been derived 
by reduction from a siphonostelic type, the occasional internal 
endodermis being a vestige of this former condition. There is 
no a priori objection to such a view. It must be remembered, 
however, that no vestiges of internal phloem have been found 
in a considerable amount of material, and that the ectophloic 
siphonostele is a rare type in Ferns, no evidence of its existence 
being forthcoming in any of the allies of Schizaea. Further, 
the considerable amount of persistent central intra-stelar 
parenchyma, with its occasional tracheids, is difficult to under- 
stand on this hypothesis. If Schizaea were really derived by 
degeneration from an ectophloic siphonostelic type we should 
rather expect the remains of the internal endodermis to be 
always close to the xylem-ring. Thus it appears that the 
hypothesis we have put forward is really the simpler and more 
natural one, provided the general view of the factors govern- 
ing the formation of physiologically extra-stelar tissue within 
the stele be accepted. 
The Anatomy of the Leaf-trace and Leaf. 
The leaf-trace (Figs. 3 and 4) consists simply of a section of 
the stele of the stem which gradually passes off through the 
cortex at a slight angle with the long axis of the stem. It is 
therefore strictly collateral and remains so throughout the leaf. 
It is made up of a short band of phloem, separated by a single 
sometimes incomplete row of parenchyma from a short band 
of xylem, the whole surrounded by a peridesm composed of 
a single layer of rather large cells — those on the outer side 
1 Cf. Tansley and Lulham, op. cit. 
