ASPIDIUM SPOROPHYTE 305 



found in course of development in positions corresponding to the 

 rhizogenic cells. 



If transverse sections of the root be cut at a point not far 

 removed from the apex, it will be seen that the xylem is not yet 

 developed at the central part of the vascular cylinder while the 

 peripheral parts may be fully formed : thus the development of 

 the xylem is centripetal, the root is diarch, and the arrangement 

 of the vascular tissues is radial. 



XIV. Cut median longitudinal sections of the apex 

 of a root which has been hardened in alcohol : at most 

 only one absolutely median section can be obtained 

 from a single root: it will be found convenient to 

 embed the apex of the root in paraffin, or to hold 

 it between pieces of pith or carrot. Mount in 

 glycerine, and examine first with a low power : choose 

 out those sections in which there is a symmetrical 

 arrangement of tissues around a single, large, appa- 

 rently three-cornered apical cell, which lies at some 

 distance from the extreme apex. (Compare Fig. 14.) 

 Note 



1. That the orientation of the apical cell is constant, 

 i.e. one corner is directed towards the older part of the 

 root, while the side opposite that corner, i.e. the anterior 

 face or base of the cell, is perpendicular to the axis of 

 the root. 



2. That around the apical cell are regularly arranged 

 segmental cells, which have successively been cut off 

 from it by walls parallel to the sides of the apical cell. 

 Of these 



a. Those successively cut off from the base form the 

 root-cap, dividing up by regularly arranged walls into 

 a mass of regular cells. 



x 



