548 FILICALES 
are finely scalariform, and are scattered round the periphery of the xylem- 
core, which is itself composed of tracheides, intermixed with parenchyma 
(Fig. 306). The petiole is traversed by a single strand, which comes off 
from the protostele with only superficial disturbance of it: in fact the 
mature plant maintains the simple relation of the protostele and trace 
which is seen in its seedling (Fig. 307). The foliar strand in the climbing 
petiole is an almost cylindrical body, with bays of phloem protruding 
into the xylem: it is probably a derivative form of the more usual flattened 
type, contracted in accordance with the climbing habit. In Schizaea the 
obliquely ascending or erect stock shows a medullated stele: endodernfal 
Sek 
PRI aaa: 
Z 
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ects 
Ae 
igh 
Fic. 306. 
Transverse section of the rhizome of Lygodium dichotomum. X50. e=endodermis; 
ph=phloem; +=xylem. (After Boodle.) 
pockets are often present at the nodes, or an isolated internal endodermis 
is occasionally seen, but no internal phloem. Internal tracheides occur 
in the medullary region, sometimes isolated, sometimes almost bridging 
across the central pith (.S. mod/uccana). It has been clearly demonstrated 
that as the stele in the seedling expands, no internal phloem appears: 
these facts favour a theory of amplification of the stele in ScAzzaea rather 
than one of reduction. In Anemia Phyllitidis and most other species, 
as also in A@ohria, the mature stem is dialystelic, having a hollow reticulate 
vascular cylinder similar, except for the leaf-traces being only a single 
strand, to that of Mephrodium filix mas: each mesh is a leaf-gap and the 
leaf-trace is inserted at its base. But in A. mexicana and other creeping 
species the dialystelic state is replaced by closed vascular ring or solenostele. 
The simpler type of Zygodium is probably the more primitive, and as 
