THE STELA R THEORY. 143 



first leaf contains five separate bundles each with a separate, 

 though feebly suberised, endodermis. Hence it was 

 treated by Van Tieghem, in the Traits, as astelic. In 

 Botrychium Lunaria, whose stem is also monostelic at the 

 base, the endodermis, after the departure of the first leaf 

 trace, does not close round each separate bundle but 

 becomes as it were invaginated into the cylinder, so that 

 the vascular tissue forms on transverse section a horse- 

 shoe bounded by the endodermis. The free edges of the 

 horseshoe meet, as we pass up the stem, and the inner 

 portion of the endodermis becomes entirely separated from 

 the outer, so that we have an equivalent of the second or 

 gamodesmic condition found in the stems of Eqtiiseta. 

 Higher up the inner endodermis loses its thickenings, just 

 as in some Equiseta, and this gives us an apparently 

 monostelic condition. In accordance with his revised 

 view, Van Tieghem considers that OpJiioglossum has an 

 astelic-dialydesmic stem, while those of Botrychium and 

 Helminthostachys are astelic-gamodesmic. 



THE STATUS OF THE STELE CRITICISM. 



It will be most convenient to introduce here a critical 

 investigation of the stelar theory as thus modified by its 

 author, and so far as it depends upon the morphological 

 interpretation of the arrangement and relations of vascular 

 tissue in the adult organs of vascular plants ; deferring for 

 the present a consideration of the developmental facts 

 bearing upon the theory. 



There is no need to discuss at any length the funda- 

 mental conception of the stele arrived at in the period which 

 we have called the first phase of development of the idea. 

 It depends upon the tracing into the stem of the root 

 cylinder, and upon the demonstration that its characters as 

 a cylinder are maintained in the latter. This demonstration, 

 begun, as we have seen, in 1872, eventually led to the 

 explicit recognition of the fact that the system of bundles 

 forming the central cylinder possesses morphological charac- 

 ters much more constant than those of the vascular bundle, 



