THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



569 



phloem and the xylem is centripetal, that is, from the outside towards the 

 interior. The protoxylem, which consists of spiral and annular tracheids 

 of small diameter, will thus be seen at the outer edge of each xylem mass, 

 that is to say, in the exarch position. 



^'^*— Per/cyc/e 

 Phloem 

 Metaxylem 

 Protoxylem 



Inner cortex 

 Outer cortex 



Fig. sjy.^Lvcopodium claratiim. Transverse section 

 of the stem showing the xylem plates with 

 external protoxylems. 



Anatomy of the Leaf 



The leaves are arranged in a close spiral. They are small, evergreen and 

 lance-shaped, with a distinct point or av^-n, and tend to turn upwards from 

 the creeping stem. In Lycopodiuni complanatiim and in one or two other 

 species the leaves are arranged in four rows, two large and two small, as in 

 Selaginella. The margin may be slightly serrated and the lamina is traversed 

 by a midrib which does not quite reach the apex. This midrib is composed 

 of a small number of very thick-walled spiral tracheids, surrounded only at 

 the base of the leaf by a rather irregular row of sieve tubes (Fig. 578). 



Stoma 



Mesopbyll 



Vascular bundle 



Fig. 578. — Lycopodiuni clavatum. 

 of the leaf. 



Transverse section 



19A 



