174 



XIII. THE STEMS OF PTERIDOPHYTES. 



strictly comparable to that which we see in the rhizome of the 

 Fern. 



Stele of Pteris. In Pteris aquilina the general relations of 

 the steles are the easiest to make out, although the abundant 

 sclerenchyma makes the material somewhat difficult to cut. 

 Alcohol material is undoubtedly preferable, and cross-sections -may 



FIG. 69. Cross-section through a stele from the leaf-stalk of Pteris aquilina. sc, 



scalariform vessels ; sp, protoxylem ; in the scalariform vessel sc*, a piece of a ladder- 

 like wall is broken through ; Ip, wood-parenchyma ; v, sieve-tubes ; s, phloem cells ; 

 pr, protophloem ; pp, starch sheath ; e, endodenuis (x 240). 



be cut from the rhizome not too far behind its growing end, or 

 from the leaf-stalk of a leaf that is still young. In such sections 

 the steles will be found already fully developed, while the charac- 

 teristic (sclerenchymatous) thickening of the ground-tissue is still 

 wanting. The structure of the stele is the same in rhizome as 

 in leaf-stalk, and the accompanying Fig. 69, which shows us the 

 cross-section of a stele from the base of a leaf-stalk, will serve to 



