THE VASCULAR CRYPTOGAMS 



53 



cell divides in regular order by walls successively 



parallel to each of its three sides. The cells thus cut 



off are called segments (see 



Fig. 26). By the growth and 



repeated subdivision of the three 



rows of segments all the tissues 



of the stem are produced. The 



stem figured is monostelic ; 



here the first tangential walls 



formed in the segments mark 



the limit between the central 



cylinder and the surrounding FlG - 26. Apex of stem of a 



T IT Tern (stolon of Ncplirolepis} 



Cortex. Ill a polystellC Stem, i n longitudinal section, n, 



SUCh as that of the Male Fern, apical cell; s i} s, segments ,; 



c, cortex ; p, stele ; r, cell 



the steles are not marked out 



from which a root will arise. 

 Magnified 80 diameters. 

 (After Van Tieghem.) 



until after more numerous 

 divisions have taken place. It 

 is probable that each leaf owes its origin to the out- 

 growth of cells derived from a single segment. 



13. The Pioot 



The root, like the stem, carries on its apical growth 

 by means of a single cell, which here also has the 

 form of an inverted three-sided pyramid. The essential 

 difference between the divisions in the apical cell of the 

 root and in that of the stem, is that, in the former, cell- 

 walls are not only formed parallel to the three sides, but 

 also parallel to the base of the pyramid. The segments 

 thus cut off from the outer end of the apical cell (see Fig. 

 27.) go to form the root- cap; those cut off laterally build 

 up the tissues of the root itself, in much the same way 

 as in the case of a mom stelic stem. 



The mode of branching of the root in Ferns differs 



