492 



BOTANY OF THE LIVING PLANT 



shape ; it shows continued though slow growth, and segments are cut off 

 from its sides in definite succession. As the whole tissue of the stem, 

 leaf, or root is derived from such segments, the whole of each part is referable 

 in origin to its apical cell, which maintains its identity throughout. The form 

 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 

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



Fig. 382. 



Transverse section of a root of a Fern (Pellcea) ( 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. 



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

 succession from the three sides (Figs. 383, 385), in the latter alternately from 

 the two sides (Fig. 384). The further subdivision of the segments to form 

 the tissues is represented in surface view for the case of Osmunda in Fig. 383 : 

 and Fig. 384 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 from the frontal face of the pyramid, after each cycle of 

 three has been cut off from its sides (Fig. 385). Thus every fourth segment 

 goes to form the protective cap, and renews it from within. Not only does 

 the leaf also show continued growth and apical segmentation from its two- 

 sided apical cell, but the lateral wings or flaps originate by the activity of 

 rows of marginal cells. There is also a definite segmentation seen in the 

 origin of the sporangia. Thus Ferns have not stratified meristems like 



