Filicales. 
237 
solenostele by the over-lapping of successive gaps left in the latter by 
the departing leaf-traces. There seems no reason to doubt that the 
dictyostely of Aneimia Phyllitidis and Mohria was evolved in this 
way from solenostely. As regards the phytogeny of the Schizcea- 
stele it is difficult to speak with any certainty. Mr. Tansley and 
Miss Chick contend that as the protostele of Lygodium appears to 
be primitive among the Ferns, it is natural to suppose that “. ... in 
Schizcea the central tracheides of such a stele are normally replaced 
by parenchyma, just as is the case inmost species of Lepidodendron, 
whose structure is known ” (35). They point out that such a 
change might result from an increase in the size of the stele, or a 
decrease in the demand for water-conduction. They contend that 
this view is supported by the small size and shade-loving habit of 
Schizcea ; but they admit that the greatest objection to the view 
that the stele of Schizcea represents an advance on protostely, and 
is not a reduced siphonostele, lies in the difficulty of seeing a use in 
these internal endodermal structures in their present condition. 
They argue that this objection may be partly met by the suggestion 
that if the demand for water-conduction were lessened, this should 
affect the vascular tissue. They maintain that the proportion of 
intra-stelar parenchyma present in Schizcea, where there is no 
internal endodermis, considerably exceeds the proportion ordinarily 
found in fern-steles, and that such a central parenchyma will tend 
to become functionless for conducting purposes. They continue : 
“The mass of intra-stelar parenchyma is greatest at the point of 
departure of a leaf-trace, and here, consequently, we get the 
beginnings of its replacement by a physiologically extra-stelar tissue, 
which is definitely non-conducting” (35). This tissue is the internal 
endodermis and the endodermal pockets. Mr. Tansley and Miss 
Chick contend that so long as the stele does not increase in size the 
physiologically extra-stelar tissue produced within the stele remains, 
as in Schizcea, small in amount and inconstant in occurrence. They 
admit that their view is not the only tenable one, and suggest the 
alternative that the stele of Schiziea has been reduced from a 
siphonostele, and that the occasional internal endodermis is vestigial. 
They claim, however, that no traces of internal phloem, indicating 
reduction from a solenostele, have been found in a considerable 
amount of material, and that the ectophloic siphonostele is rare in 
ferns. They argue that the tracheides occasionally present in the 
central parenchyma would be hard to understand if Schizcea were 
reduced from an ectophloic siphonostele, and that in that case we 
