THE VASCULAR CRYPTOGAMS 



11 



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

 the protoxylem 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 

 fossil forms, from which 

 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. Kraus- 

 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. 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 



FIG. 7. Selatjinclla 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. ) 



