33 o SCIENCE PROGRESS 



Turning to the Angiosperms, Gwynne-Vaughan (25) was one 

 of the first to test the validity of Van Tieghem's theory by the 

 examination of the development of the polystelic condition 

 obtaining in certain primulas. 



The seedling of Primula Auricula is what may be termed 

 monostelic ; which condition obtains until four or five leaves have 

 been formed, for the outgoing leaf-traces leave but a small gap 

 in the central cylinder. Afterwards it is found that these foliar 

 gaps are of some size, and the endodermis develops in the 

 concavity, thus separating the pith from the vascular tissue. 

 Before any one particular leaf-gap is repaired another is formed 

 by the giving off of another foliar bundle from the convex side 

 of the central stele ; hence the latter becomes divided into two 

 portions. Further subdivision of the vascular zone takes place 

 in connection with the succeeding leaves ; that is to say, a 

 polystelic condition is attained. 



It is clear that, as Gwynne-Vaughan points out, the leaf- 

 traces are the all-important factors in determining the organisation 

 of the stele, a point overlooked by Van Tieghem. 



The work of the former was carried much further by Jeffrey, 

 whose contributions entirely altered the aspect of the subject. 

 He examined various polystelic Angiosperms, viz. species of 

 Primula and Gunnera and Parnassia palustris. The comparison 

 of these with types of vascular structures of other Angiosperms 

 and Ferns led to conclusions, which will be dealt with later on, 

 at variance with the theory of Van Tieghem. 



From the above account of the development of the vascular 

 systems of mature plants, it is obvious that the polystelic 

 condition does not arise by the successive branching of the 

 protostele, and the earlier work of Jeffrey (37) shows that not 

 only is the polystele not derived from the protostele in that 

 manner, but further that (1) the astelic type is not formed from 

 the medullated monostele by the separation of the latter into 

 bundles or meristeles, and the enclosure of each by an endo- 

 dermis ; for in young astelic axes, e.g. in that region of 

 seedlings situated immediately above the insertion of the seed- 

 leaves or cotyledons, there is a collateral stelar tube lined 

 internally and externally with an endodermis and having foliar 

 gaps ; there is not a series of meristeles, as would be required 

 by Van Tieghem's hypothesis; (2) the three types, viz. poly- 

 stelic, astelic, and medullated monostele, are modifications of one 



