498 Tansley and Chick. — On the Structure 
In other cases, and far more frequently, there appears, also 
embedded in the central parenchyma, an internal endodermis. 
Such an endodermis may consist of a ring of as many as 
thirteen endodermal cells (Fig. 3), having all the characters of 
those belonging to the external endodermis, and enclosing one 
or more cells considerably smaller than, but otherwise possess- 
ing the characters of the cells of the cortex. On following such 
an internal endodermis upwards (towards the apex of the 
stem), it is found to be continuous with the ordinary external 
endodermis at the ‘ axil ’ of the next leaf-trace, while the 
enclosed cells are continuous with those of the cortex, in 
exactly the fashion characteristic of a solenostelic Fern. 
Traced downwards, the internal endodermis ends blindly, 
usually in the region of the node next below the one to 
which it belongs. In one case only was a blind termination 
found also at the upper end. In that case the internal endo- 
dermis had no connexion whatever with the exterior of the 
stele. Frequently, however, the internal endodermis, for the 
whole or for part of its course, encloses no cells, but simply 
consists of a solid strand, which may be reduced to a single 
row of endodermal cells. Many of the leaf-traces, however, 
have no internal stelar endodermis in connexion with them, 
the endodermis of each being continuous at the ‘axils ’ simply 
with the external stelar endodermis (second node in Fig. 33). 
Fig. 33, p. 499, gives a diagrammatic view of a median longi- 
tudinal section through an actual stem, showing the behaviour 
of the endodermis at the insertion of three successive leaf- 
traces. [These insertions are represented as distichous so that 
they may all three be shown in one plane. In reality they are 
not so. See p. 496.] When, as is not unfrequently the case, 
one or more strands of central tracheids coexist with an 
internal endodermis, the two have no connexion whatever. 
Phylogenetic significance of the Stelar Anatomy. 
The bearing of these new facts on the question of the phylo- 
genetic position of the Sckizaea-stdie are not entirely clear, 
and must be considered at some length. 
