12 



STRUCTURAL BOTANY 



are more numerous and are sometimes fused together 

 in a complicated manner. The anatomical peculiarities 



of the stem of the 

 genus Selaginella may 

 be summed up as 

 follows : 



(1) The stele con- 

 tains no pith. 



(2)Thexylemand 

 phloem of the stele 

 are not arranged in 

 separate bundles. 



(3) The xylem is 

 usually developed 

 centripetally. 



(4) In many 

 species there is more 

 than one stele. 



As regards the 

 details of the tissues, 

 it is only necessary to add that the tracheides of the 

 protoxylem are annular or spiral, as is usually the case. 

 The other tracheides usually have long transverse pits, 

 and are hence called scalariform (see Fig. 23, p. 47), from 

 the ladder-like appearance which these pits give to their 

 walls. We shall find this form of tracheide very general 

 among the higher Cryptogams, and shall study it more 

 fully in the Ferns. In one or two species of Selaginella 

 true vessels, arising by cell-fueion, occur in the wood. 



b. The Leaves 



The leaves of Selajinella are of excessively simple 

 structure ; each leaf, as we have seen, receives a single 



FIG. 8. Selaginella Kraussiana ; part of 

 transverse section of stein showing one 

 stele, x, the wood ; px, protoxylem ; ph, 

 phloem ; pe, pericycle ; en, endodermal 

 cells forming the whole or part of 

 trabeculae ; c, inner layers of cortex. 

 Magnified about 100 diameters. (After 

 Harvey Gibson.) 



