XIV] SELAGINELLA 55 



centre of the axis of the tubular stele. The cylindrical stele 

 consists of xylem with external and internal phloem (p): 

 cuticularised endodermal cells occur at e and e. 



Keference has already been made to the descending naked 

 branches given off from the points of ramification of the foliage 

 shoots of Selaginella. It has been shown by Harvey-Gibson ^ 

 that these branches, originally designated rhizophores by 

 Nageli and Leitgeb, as well as the dichotomously branched 

 roots which they produce below the level of the ground, 

 possess a single vascular strand of monarch type. It is in- 

 teresting to find that in some species the aerial portion of the 

 rhizophore has a xylem strand with a central protoxylem, 

 an instance of endarch structure like that in certain portions 

 of the shoot-system of S. spinosa. The root-anatomy of 

 Selaginella and the dichotomous habit of branching afford 

 points of agreement with the subterranean organs of Lepido- 

 dendron and Sigillaria. 



Leaves. The leaves of Selaginella'^ usually consist of a 

 reticulum of loosely arranged cells, but in some cases part of 

 the mesophyll assumes the palisade form. The single vascular 

 bundle consists of a few small annular or spiral tracheae and at 

 the apex of the lamina the protoxylem elements are accompanied 

 by several short reticulated pitted elements. Both foliage 

 leaves and sporophylls are characterised by the possession of a 

 ligule, a structure which may present the appearance of a 

 somewhat rectangular plate (fig. 130, 4, I, and fig. 131, E — G,Z) 

 or assume a fan-shaped form with a lobed or papillate margin. 

 The base, composed of large cells, is sunk in the tissue of the 

 leaf close to its insertion on the stem (fig. 131, E, I) and 

 enclosed by a well-marked parenchymatous sheath. The 

 sheath is separated from the vascular bundle of the leaf by one 

 or more layers of cells, and in some species these become trans- 

 formed into short tracheids. The ligule is regarded by Harvey- 

 Gibson^ as a specialised ramentum which serves the temporary 

 function of keeping moist the growing-point and young leaves. 



Cones. The terminal portions of the branches of Selaginella 

 usually bear smaller leaves of uniform size which function as 

 1 Harvey-Gibson (02). - ihid. (97). ^ ibid. (96). 



