of Schizaea malaccana . 501 
the vascular bundles — in other words, where there is no very 
clear differentiation between the ‘ internal ’ and c external con- 
junctive 5 of Flot. Such a central parenchyma will tend to 
become functionless for conducting purposes. In Phanero- 
gams, when the stele is further dilated, this central tissue 
takes on the characters of typical ‘ pith , 5 i. e. its cells are short 
and broad, frequently entirely passive to all appearance, and 
sometimes even destroyed by the rapid growth of the sur- 
rounding tissues of the vascular ring. Such a pith obviously 
forms no functional part of the stele, though it is not separated 
from the vascular ring by a differentiated endodermis. But 
the corresponding tissue in Ferns, which is either used for the 
storage of starch or is sclerotic, is always separated from the 
vascular ring by a definite endodermis, and it is the origin of 
this state of things that is perhaps illustrated in Schizaea. The 
mass of the 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. This takes the form 
of what has been described as an c intrusion 5 of cortical tissue 
into the stele, or, as we should prefer to say, of the develop- 
ment of a strand of tissue in connexion with the cortex, pene- 
trating into the stele ; and since the boundary of extra- and 
intra-stelar tissue is always marked by an endodermal layer, 
such a strand is always bounded by an endodermis, or may 
even consist of a mere rod of endodermal cells. In this way 
the balance of the different intra-stelar tissues is readjusted. 
So long as the stele does not increase in diameter we have 
these first beginnings of the development of physiologically 
extra-stelar tissue within the stele, remaining in the inconstant 
and irregular condition met with in Schizaea. If the stele were 
to increase in diameter, however, these pouches of extra-stelar 
tissue would increase in diameter with it, would meet and open 
into one another at the nodes, and an ‘ ectophloic siphonostele 5 
(Jeffrey) would be formed. The ectophloic siphonostele is, 
however, the exception in Ferns. The more usual course of evo- 
lution apparently involves the development of internal phloem 
M in 2 
