506 



MOSSES AND FERNS 



CHAP. 



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 of the Psilotaceae are quite simple (Russow ( i ) , 

 Pritzel ( i ) , Ford ( i ) ) . The most recent account is by Miss 

 Ford, who has made a very complete study of the tissues of 

 P silo turn triquetrum. 



The surface of the aerial shoot is strongly ribbed (Fig. 293, 

 A) in the stouter portions, but nearly triangular in section 



B. 



A. 



FIG. 292 Psilotum triquetrum. A, Fragment of a subterranean shoot with a 

 young gemma (k), Xi2o; B, longitudinal section of the apex of a subterranean 

 shoot, Xi8s; C, transverse section of the apex of a subterranean shoot in the act 

 of forking, x, x, the apical cells of the two branches, Xi8s (all figures after 

 Solms-Laubach) . 



nearer the apex. Within the epidermis, in which are numerous 

 stomata, there is a zone of outer cortical cells, containing nu- 

 merous chloroplasts, and constituting the principal assimilating 

 tissue. The cells of this zone are irregular in outline, with 

 numerous intercellular spaces, like the mesophyll of many 

 leaves. Inside this assimilative cortex is a zone of scleren- 

 chyma forming the principal mechanical tissue of the shoot. 

 Within this zone is a mass of thin- walled parenchyma, bounded 



