482 



MOSSES AND FERNS 



CHAP. 



Ferns, but less regular in its divisions, and it is not possible to 

 trace back all the tissues with certainty to this single cell. 

 The branching is a true dichotomy, but is not brought about 

 by the division of the original apical cell, but this becomes 

 obliterated previous to the formation of the two branches, and 

 two new initial cells are formed quite independently of it.^ 



The tissues are very simple. In the subterranean stems 

 the bulk of a section is composed of parenchyma, while the 



Fig. 251. — Psilotuvi triquetrum (Sw.). A, Fragment of a subterranean shoot with a young gemma 

 {k"), X 120 ; B, longitudinal section of the apex of a subterranean shoot, X 185 ; C, transverse 

 section of the apex of a subterranean shoot in the act of forking ; x, x, the apical cells of the two 

 branches, X 185 (all figures after Solms-Laubach). 



vascular bundle is very similar to that of the roots of the 

 Ferns. The bundle is diarch, and the two xylem masses are 

 confluent at the centre. In the aerial shoots the cross-section 

 of the vascular cylinder shows a central mass of thick-walled 

 lignified cells about which the triarch to octarch xylem forms 

 a continuous ring." The phloem is poorly developed, and the 

 xylem is mainly composed of small thin-walled scalariform 

 tracheids. In Psilotuvi the leaves have no vascular bundle, in 

 Tmesipteris a single bundle traverses the leaf, as in Lycopodium. 



^ Solms-Laubach (i), p. 154. ^ Russow (i), p. 131. 



