THE VASCULAE CRYPTOGAMS 



11 



From each angle of the stele in S. spinosa, where 

 the protoxylern is situated, a slender bundle runs out 

 to a leaf, which it traverses from end to end without 

 branching. 



As we have already mentioned, the structure of this 

 species is exceptional in the genus ; its interest lies 

 chiefly in the resem- 

 blance to so many IL 

 fossil forms,from which \M w >f//r-/. 



we may probably infer 

 that it is a very 

 primitive type of 

 structure. A great 

 many Selaginellas, like 

 S. spinosa, have only a 

 single vascular cylinder, 

 or, in other words, are 

 monostclic ; but most 

 commonly the single 

 stele has a simpler 

 structure. 



If we now return to 

 the species, S. Kmus- 

 siana, with which we 

 started, we find a total- 

 ly different arrange- 

 ment. In this species, 

 the stem is traversed 

 by two parallel steles, 

 each of which has a single protoxylem-group. 



G . 7. Selar/inella Kraussiana; dia- 

 grammatic transparent view of stem. 

 st, the two steles, anastomosing at base 

 of branches ; l.t, leaf- trace bundles, only 

 shown in upper part. (After Harvey 

 Gibson.) 



The 



structure of these steles, their course through the 

 stem, and their relation to the leaves, are sufficiently 

 indicated in Figs. 7 and 8. In other species the steles 



