192 ANATOMICAL EVIDENCE 
developmental point of view is still required before a complete elucidation 
is possible. The case for Medudlosa must also remain for the present 
unexplained ; but at least it seems almost certain that it is not a polystelic 
condition arising from overlapping of leaf-gaps, as in Ferns. 
Among Phanerogams the polystely in Auszcula has been investigated 
by Gwynne-Vaughan, and the origin of it is again by a resolution of a 
primitive monostele. Perhaps the same may be the case for the more 
complex condition of Gunnera, but that is still uncertain. Of these 
isolated cases of polystely which exist among the vast majority of mono- 
stel's forms the Ferns are the most important: and as their case has been 
shown to be a result of amplification of the monostele, the existence of 
occasional exceptions elsewhere, not yet fully explained, cannot be held as 
a valid objection to the acceptance of the monostele as the fundamental 
type of structure in the sporophyte at large. 
Another mode of amplification of the protostele is exemplified by 
Lycopods, and possibly occurs also elsewhere: it is by the progressive 
conversion of a central tract of the xylem-core into parenchymatous tissue 
of the nature of a pith. This is probably related to another modification 
of the stelar structure found in stems, viz. that designated the schizostelic, 
or by some the aséedic, state. Here the several vascular strands are not 
collectively surrounded by an endodermal ring, but are independent, and 
may have a special endodermal sheath surrounding each. This structure 
is found in some species of Eguésetum and in certain Phanerogams. It 
seems to be generally admitted that in the stem this condition is derivative 
from the ordinary monostelic state, a conclusion which would naturally 
follow in the case of the genus Lguisefum from a comparison of its 
different species (Fig. 96). If this be so, then both the marked excep- 
tions in vascular structure in the stem are referable in origin to the usual 
monostele, and the conclusiofi seems justified that in the axis of Vascular 
Plants there is only one fundamental stelar type, and that is the mono- 
stelic type. The morphological importance of any character is held to 
be in accordance with its constancy in a large series of allied organisms : 
the general occurrence of a monostelic structure, or of arrangements 
"derivative from it in Vascular Plants at large gives the monostele a 
place in the first rank as an internal morphological feature of the axis. 
The prevalent bifacial character of the leaf is appareht in the simplest 
forms of Vascular Plants, where its comparatively small expanse is traversed 
by a single vascular strand. This structure is found in such primitive types 
as the Lycopods, Equiseta, /svefes, etc. Where the leaf is larger the 
vascular system is expanded in various ways: numerous strands may traverse 
it, diverging from one another towards the margin, but converging towards 
the base, where, with or without fusions, they may form a curved series 
as seen in transverse section (Fig. 97). The orientation with the proto- 
xylem tending adaxially is a constant feature. Each strand is surrounded 
by a definite sheath throughout its individual course, but on fusion two 
