254 



THE ANATOMY OF WOODY PLANTS 



the larger stems, but these in no case are related to outgoing foliar 

 traces. The general topography of a stem of the type found in the 



Psilotaceae is presented in Fig. 180. 



In the Lycopodiaceae 



in general the leaf traces 

 are ordinarily mesarch in 

 their organization, a 

 condition more or less 

 characteristic of the 

 Lycopsida as a whole. 

 An endodermis can 

 usually be distinguished 

 about the foliar strands, 

 although this limiting 

 layer is ordinarily con- 

 spicuous by its absence 



FIG. i So. Transverse section of the stem of in the Stem in most spe- 



Psilotum. cies of Lycopodium. In 



other representatives of the 

 two families under discus- 

 sion an external endoder- 

 mis is usually found in the 

 stem, and, as has been 

 shown above, an internal 

 endodermal zone is seen in 

 the tuberous subterranean 

 stem of Phylloglossum. 



In the genus Selaginella 

 the fibrovascular tissues of 

 the axis are distinguished 

 by considerable variety in 

 topography. In some spe- 

 cies the stele is a single 

 mass, separated from the 



FIG. 181. Transverse section of stem of 

 Selaginella laevigata, showing a siphonostelic 

 central cylinder. 



cortical tissues by an air-containing region representing the endo- 

 dermis. In other species the fibrovascular system becomes divided, 

 and the protostelic condition as a result passes into that known as 



