NEPHRODIUM-SPOROPHYTE 169 



flattened tangentially, and having the usual dot-like marking of 

 the radial walls : this may be difficult to observe, as the radial 

 walls are often pressed out of shape. Within this layer lies 



4. The pericycle, which usually consists of two layers of cells 

 with thin walls, and obvious protoplasmic contents. The 

 vascular tissues inclosed within these layers are arranged 

 according to the ordinary radial type : thus there will be 

 seen : 



5. Two groups of xylem abutting on the pericycle and 

 composed of tracheides of various size, the largest being near 

 the centre of the root ; as the root develops, the two originally 

 separate groups of xylem unite at the centre by formation of 

 fresh tracheides, and together form a flat band which traverses 

 the root longitudinally. Alternating with the groups of 

 primary xylem at the periphery of the vascular cylinder 

 are 



6. Two groups of phloem, consisting mainly of sieve-tubes 

 having the same characters as those of the stem. Scattered 

 among the vascular elements are cells of conjunctive paren- 

 chyma. 



XIII. 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, apparently three-cornered 

 apical cell, which lies at some distance from the extreme apex. 

 (Compare Fig. 13.) 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 seg- 

 mental 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- 



