THE VASCULAR CRYPTOGAMS 



53 



FIG. 26. Apex of stem of a 

 Ferii (stolon of Nephrolepis) 

 in longitudinal section, a, 

 apical cell ; s ly 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. 



@. 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 a 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. The segments cut off from the 

 base of the apical cell (see Fig. 27) go to form the root- 

 cap ; those formed at its sides 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 



