406 Boodle . — Anatomy of the Schizaeaceae . 
p. 245). In the stems of many Monocotyledons a function 
similar to that of the endodermis is probably performed by 
the sclerotic sheaths of the bundles, which would prevent 
accidental ruptures in the ground-tissue from continuing the 
intercellular spaces right up to the tracheae. In Dicotyledons, 
where intercellular spaces occur in the pith, the perimedullary 
zone may perhaps have a similar function. But the spoiling 
of a few tracheides would be comparatively unimportant in 
plants which form new ones by secondary thickening. In the 
Ferns, on the other hand, the endodermis seems to be an 
important layer, though often assisted in its function in the 
mature plant by adjacent sclerotic tissue. The vascular tissue 
is therefore normally completely enclosed by an endodermis, 
especially towards any loose parenchyma. In solenostelic 
forms the connexion of inner and outer endodermis at the 
leaf-gap is of course necessary from this point of view, if 
a leaf-gap is to be formed. We will now assume the hypo- 
thesis mentioned previously, that the horseshoe-shaped ‘stele’ 
is morphologically only part of a circular or oval stele, and 
further that in the polystelic (or dialystelic) type the limit of 
the stele is also circular and includes the separate ‘ steles ’ 
and the central ground-tissue. The following might then 
be a physiological description of what has taken place. The 
primitive solid stele has become much enlarged by increased 
cell-division in the cylinder of cells which were set apart to 
produce it. This increase in diameter may be connected with 
the attachment of the tangentially broad leaf-traces of large 
leaves 1 , especially if they are polystichous. A solid mass of 
tracheides of the increased diameter is not necessary for the 
water-supply, and consequently the central cells do not attain 
to tracheide-formation, but remain parenchymatous or become 
sclerotic as the case may be. At this stage one would have 
a ring of xylem surrounded by phloem, and enclosing a central 
pith. This is found in Schizaea. In S. digitata the pith is 
1 Jeffrey (’00, p. 38 ) connects the siphonostelic type of structure with mechanical 
strengthening of the axis to enable it to bear large leaves ; this may be one factor, 
but sclerotic tissue is the chief supporting tissue in Ferns. 
