XIII 



lYCOPODlNEM 



so? 



internally by the endodermis which limits the central cylinder. 

 Miss Ford finds that with proper treatment, the endodermis 

 can be readily differentiated, although ordinarily its presence 

 is not evident. 



The central cylinder, or stele, has its axis composed of a 

 mass of sclerenchyma about which the radiating xylem-masses 

 form a more or less regular star-shaped mass, when seen in 

 transverse section. The number of xylem masses varies from 

 3 to lo. The protoxylem, composed as usual of narrow spiral 

 tracheids, occupies the points of the star-shaped section, the 

 larger secondary tracheids being developed centripetally. The 

 latter are scalariform. The phloem is very poorly differenti- 

 ated, and its boundaries are impossible to determine exactly. 

 Larger elements, probably representing sieve-tubes, are present 



Fig. 293. — ^A, Section of the stem of Psilotum triquetrum, X^^o; B, part of the central 

 cylinder, X150; C, section of the stem of Tmesipteris tannensis, X20; D, part of 

 the central cylinder, X150. 



but neither well-defined sieve-plates nor callus could be dem- 

 onstrated. Between the endodermis and protoxylem are sev- 

 eral layers of pericycle cells. In Psilotum the leaves have no 

 vascular bundle; in Tmesipteris a single bundle traverses the 

 leaf, as in Lycopodiiim. 



The structure of the stem in Tmesipteris (Fig. 293, C) is 

 much like that of Psilotum, but is simpler. There are 3 to 5 

 xylem-masses which are much less symmetrically arranged 

 than in Psilotum. The leaves, however, possess a w'ell-devel- 



