192 ANATOMICAL EVIDENCE 



developmental point of view is still required before a complete elucidation 

 is possible. The case for Medullosa 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 Auricula 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- 

 stelic 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 astelic, 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 Equisetum 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 Equisetum 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 conclusion 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 

 place in the first rank as an internal morphological feature of the axis. 



The prevalent bifacial character of the leaf is apparent 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, Isoetes, 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 



