THE VASCULAR CRYPTOGAMS 



53 



FIG. 26. Apex of stem of a 

 Fern (stolon of Neplirolepis) 

 in longitudinal section, a, 

 apical cell ; s lt s. 2 , segments ; 

 c, cortex ; p, stele ; r, cell 

 from which a root will arise. 

 Magnified 80 diameters. 

 (After Van Tieghem.) 



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 

 cortex. In a polystelic stem, 

 such as that of the Male Fern, 

 the steles are not marked out 

 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. 



0. The Eoot 



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 monostelic stem. 



The mode of branching of the root in Ferns differs 



