THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 271 



evolutionary interest. In Equisetum arvense (Fig. 1956), in all 

 probability one of the most highly specialized members of the 

 group, an external endodermis alone is for the most part present, 



an internal boundary of 

 this nature being found 

 only in certain instances 

 in the region of the nodes. 

 In E. silvaticum, which may 

 be considered a somewhat 

 less specialized type, only 

 an external endodermis ap- 

 pears in the aerial stem, 

 which accordingly presents 

 a condition very similar to 

 that found in E. arvense. 

 In the subterranean stems, 

 however, and in primitive 



FIG. 195 a, b, and c. Diagrams illustrating the distribution of the endodermis 

 in the genus Equisetum. 



regions of the aerial axis an internal endodermis is likewise seen. 

 In other species such as E. hiemale (Fig. 1950), etc., both internal 

 and external limiting layers are developed throughout, while in 

 E. limosum (Fig. 195^) each bundle is surrounded by an individual 



