54 



STRUCTURAL BOTANY 



in one important respect from that in the higher 

 plants. In Ferns each rootlet arises, not from the 

 pericycle, but from the endodermis, and, in fact, its 

 origin can always be traced to a single endodermal cell, 

 lying opposite one of the groups of protoxylem. The 

 cells destined to give rise to rootlets can be distinguished 

 by their larger size. The cell in question divides up 



p'e en 



FIG. 27. Apex of root of a Fern in longitudinal section, showing 

 triangular apical cell, pi, stele; pe, pericycle; en, endodermis; 

 p, cortex ; r.c. root-cap. The dark lines mark out the cell- 

 groups, each formed from a single segment. Magnified 120 

 diameters. (After Van Tieghem.) 



by inclined walls, so as to form at once a pyramidal 

 apical cell, by means of which the further development 

 of the rootlet is carried on (see Fig. 25). The young 

 root, as it makes its way through the tissues of the 

 parent organ, is at first enveloped in a digestive sac, 

 derived from an inner layer of the cortex, and serving 

 to absorb the tissues which have to be penetrated. 



In the Ferns, the pericycle has nothing to do with the 



