334 



BOTANY OF THE LIVING PLANT 



of the cell in roots, in most stems, and in some leaves [Osmimda) is that of a 

 three-sided pyramid , but where the organ is flattened, as in some stems 



Fjg. 273. 

 Transverse section of a root of a Fern {Pellcsa) ( x 150). Outside lies the sclerotic 

 cortex, limited internally by a definite endodennis. There are two groups of pro- 

 toxylem ; a very broad pericycle, of 3 or 4 layers, strrrounds the \-ascular tissues. 



(Pieyidium), and almost all leaves, it has two convex sides, and is shaped like 

 half of a biconvex lens. In the former case the segments are cut off in regular 

 , succession from the three sides 



(Fig. 274,276), in the latter alter- 

 nately from the two sides (Fig. 

 275). The ftirthcr subdivision 

 01 the segments to form the 

 tissues is represented in surface 

 view for the case of Osmuuda 

 in Fig. 274 : and Fig. 275 shows, 

 in the surface view of a young 

 XeziolCcratoplciis liowthe whole 

 member may be built up from 

 such segments. In roots the seg- 

 mentation is complicated by the 

 origin of the root-cap. This is 

 provided by a segment cut off 

 from the Irontal face of the 

 pyramid, after each cvcle of 

 three has been cut off from its 



