500 Tans ley and Chick . — On the Structure 
even relatively to the diameter of the stele, makes it ex- 
ceedingly likely that such a change would take place if we 
suppose an ancestor of the genus to have had a stele of 
the type which we find in the allied genus Lyg odium. 
But these phenomena are distinctly complicated by the 
occurrence of the internal endodermal pouches and rods in 
connexion with the leaf-traces, bringing Schizaea into relation 
with solenostelic forms. Are we to regard these endodermal 
structures as a progressive modification, constituting an ad- 
vance on the medullated protostelic type, or as vestiges of 
a former solenostelic condition ? 
The main difficulty of the former hypothesis is to see how 
these endodermal structures, in their present condition, can be 
of any use to the plant. The same difficulty occurs in con- 
sidering the downward extension of an internal endodermis 
from the node of Gleichenia x and Lindsay a 1 2 . Perhaps it 
may be partly met by the following suggestion. Suppose 
the diameter of a protostele be increased for any reason while 
the demand for intra-stelar (i. e. conducting) tissue is not 
increased ; or suppose, as we are doing for Schizaea , that 
the diameter remains constant or is only slightly diminished, 
while the demand for intra-stelar tissue is considerably 
diminished. The lessened demand for conducting-tissue 
should affect the intra-stelar parenchyma as much as it 
will affect the xylem and phloem, since we have reason 
to believe that the main function of the parenchyma is to 
conduct carbohydrates. Certainly the proportion of intra- 
stelar parenchyma in Schizaea , in places where no internal 
endodermis is present, considerably exceeds the proportion 
ordinarily found in fern-steles. The central parenchyma here 
may be directly compared with that found in the roots and 
stems of some Phanerogams where this central parenchyma or 
pith is relatively narrow, and does not differ very markedly 
from the small-celled active tissue immediately connected with 
1 Cf. Boodle, The Anatomy of the Gleicheniaceae, Ann. of Bot., Dec. 1901. 
2 Cf. Tansley and Lulham, On a new type of Fern-stele, &c., Ann. of Bot., 
March, 1902. 
