XIII 



LYCOPODINEAi 



507 



internally by the endoderniis whicli limits the central cylinder. 

 Miss Ford finds that with proper treatment, the endoderniis 

 can be readily differentiated, altlioni^li ordinarily its presence 

 is not evident. 



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

 mass of sclerenchyma ahont whicli tlic radi.ating xylem-masses 

 form a more or less rei^ular star-sha])ed 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 i)oints 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 Psilotiiin tnquctrum, X20; B, part of the central 

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

 the central cylinder, Xiso. 



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 Psilofum the leaves have no 

 vascular bundle ; in Tiiicsipfcris a single Inindle traverses the 

 leaf, as in Lycopodiuni. 



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

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

 xylem-masses which are much less symmetrically arranged 

 than in Psilotiun. The leaves, however, possess a well-devel- 



