190 ANATOMICAL EVIDENCE 



It is indicated by the presence in the transverse section of two or more 

 vascular masses, each being constructed and delimited in the same way 

 as the single stele of normal axes. At first it was thought by Van Tieghem 

 that to produce this condition the stele, originally simple, underwent a 

 branching, notwithstanding that the axis in which this would take place 

 remained simple. This suggestion seemed inherently improbable, and it 

 has since been shown by direct examination of specific cases that the 

 real origin of the polystelic state as it occurs in Ferns is by the formation 

 of large leaf-gaps below the bases of insertion of the successive leaves : 

 the steps of increasing complexity of stelar structure in Ferns have been 

 tentatively outlined by Gwynne-Vaughan as follows : The most primitive 

 type of vascular construction was probably the single protostele, with 

 uninterrupted central xylem, and this is met with in some of the early 

 Fern types in the mature stem, but it is also seen in polystelic types of 

 Ferns at the very base of the young plant. Internal parenchyma then 



makes its appearance about the peri- 

 phery of the protostele at points just 

 above the departure of the leaf-traces : 

 this advances gradually inwards from 

 these points until the most central 

 region of the stele is affected. A struc- 

 ture resembling a cylinder or, as it is 

 styled, a " solenostele," may then be 

 FlG attained by the gradual differentiation 



Dicksoniafunctilola. Diagram ot vascular f Phloem and endodermis through the 



system of rhizome, including a node and th leaf-gaps and all round inside of the 



base of a leaf-trace. The upper surface of the 



rhixome would face the observer. (After xylem-ring. Below the insertion of 



(jwynne-Vaugnan.) * 



each leaf a large leaf-gap occurs in the 



solenostele (Fig. 95) : in transverse section at such a point the stele will 

 appear as an incomplete ring. If, then, the arrangement of the leaves be 

 a close one, two or more of these gaps would occur in a single transverse 

 section, and the result would be an appearance as of several steles arranged 

 in a ring. These originate, however, not by branching, as Van Tieghem 

 thought, but by resolution of the stele, first into a cylinder and then into 

 a cylindrical network. The result of this mode of amplification would, 

 therefore, be more correctly styled "a dictyostele " rather than a polystelic 

 state, and the parts " meristeles " rather than a plurality of distinct steles, 

 since the whole is a result of amplification, not of branching, of the 

 original monostele. 



But polystely is also found in the genus Selaginella. There is little 

 doubt that the monostelic type is the original one for this genus also, 

 since it exists in many species. The origin of the more complex state 

 is, however, connected rather with the branching of the axis than with 

 the insertion of the minute leaves. It has certainly taken place within 

 the genus, but the comparative study of the illustrative species from the 



