THE PTERIDOPHYTA : LYCOPSIDA, ETC. 6ii 



oiPsilotum is interpreted as due to a primary dichotomy of the sporangiophore, 

 one hmb of which develops a sporangium, while the other again divides, 

 each end forming immediately another spor- -t 



angium. Abnormal cases are known in which 

 this branching may be seen fully developed. 



Anatomy of the Stem 



The stem and the rhizome both grow from 

 three-sided apical cells. Both branch by di- 

 chotomy and the apex of the stem cuts off, 

 additionally, three rows of leaf initials from its 

 three faces. In the early stages the sterile leaves 

 and the sporangiophore shoots are alike, and in 

 the latter case the synangium appears as a 

 hump of tissue on the adaxial face of the 

 initial only after it has reached about half its 

 full size. 



In section the stem shows a strongly cutin- 

 ized epidermis with stomata in the grooves 

 of the surface (Fig. 622). The cortex develops 

 three zones. The outermost consists of cells 

 which appear rectangular in cross-section, 

 but which are very irregular in outline when seen longitudinally, with air 

 spaces between them. This is an assimilatory tissue and contains abundant 



Fig. 621. — Tmesipteris tannemis. 

 Fertile shoot showing syn- 

 angia attached to bifid leaves. 





_ Protoxylem 



Fig. 622. — Psilotum triquetnim. Transverse section of 

 aerial stem showing thickened cortex and stele with 

 central core of sclerenchyma and several external 

 protoxylems. 



