334 



BOTANY OF THE LIVING PLANT 



of the cell in roots, in most stems, and in some leaves (Osmunda) is that of a 

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



FIG. z73- 



Transverse section of a root of a Fern (Pel'.tea) ( x 150). Outside lies the sclerotic 

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

 toxylem ; a very broad pericycle, of 3 or 4 layers, surrounds the vascular tissues. 



(Pteridium), 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 further subdivision 

 of the segments to form the 

 tissues is represented in surface 

 view for the case of Osmunda 

 in Fig. 274 : and Fig. 275 shows, 

 in the surface view of a young 

 leaf of Ceratopteris how the 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 



FIG. 274. 



Apex of stem of Osmunda regalis, seen from above, 

 showing the three-sided apical cells (x) of stem, (/) of leaf ; 

 /' I', older leaves. The successive segments of the apical 

 cell form the whole of the apical cone. ( x 83.) F. O. B. 



from the frontal face of the 

 pyramid, after each cycle, 'of 

 three has been cut off from its 



