336 



AIOSSES AND FERNS 



CHAP. 



and Van Tieghem (5), who has made a very exhaustive study 

 of the subject, states that they ahvays arise from an endodermal 

 cell. This divides into a basal cell and a terminal one, and by 

 the former the young root is directly connected with the xylem 

 of the stem bundle. In the outer cell the three walls defining 

 the pyramidal apical cell now arise, and the latter at once be- 

 gins its characteristic divisions. 



The segmentation in the apex of the roots of the Lepto- 



sporangiatae is exceedingly regular. 

 Corres])onding to each set of lateral 

 segments an outer segment forms 

 as well. Van Tieghem does not 

 apparently recognise the root-cap 

 as distinct from the epidermis, but 

 all other observers consider the root- 

 cap as a distinct structure. The 

 first division wall in the lateral seg- 

 ments is the sextant wall, which is 

 perpendicular to the broad faces of 

 the segment and curves somewhat 

 so as to strike one of the lateral 

 walls a little above the base, and 

 thus makes the two sextant cells of 

 unequal size (Fig. 188, C). The 

 next wall is transverse and sepa- 

 rates an inner from an outer cell, 

 and with this divides the plerome or 

 stele from the cortex. After this 

 in the outer of the primary cells 

 there is a separation of an outer 

 from an inner cell, the former giving rise either directly or l)y a 

 subsequent division to a single layer of cells upon the outside 

 of the root, which is usually regarded as the epidermis, and the 

 inner cells from the cortex. The inner layer of the cortex, 

 which can be traced back almost to the summit, is the endo- 

 dermis. 



According to Strasburger (10) in Ptcris Crctica the cap 



cells divide only by perpendicular walls, and the older layers of 



the cap remain but one cell in thickness. Van Tieghem states 



((5)' P- 53-) ^"^^ I ^^^""'^ verified this in Adiantum cmargina- 



tuui and Polypodium falcatitm, that with the exception of the 



Fig. 186. — Scale from the stipe of 

 Cystoptcris fragilis, X^S- 



