ALLIUM CEPA 



189 



axially elongated so that the cap is wedge-shaped in outline. In 

 the central portion of the root cap, where there is the least variation 



Fig. 87. A and B, transections of mature portions of adventitious roots showing pentarch 

 and tetrarch types ; C, longisection of primary root grown in soil ; D, adventitious root 

 grown in water : co, cortex ; dgn, dermatogen ; en, endodermis ; ep, epidermis ; mx, metaxy- 

 lem ; pbm, periblem ; pel, pericycle ; ph, phloem ; pi, plerome ; px, protoxylem ; r c, root 

 cap. (After Hoffman.) 



in the size of the cells, a longisection shows two to four irregularly 

 vertical rows of cells, each of which may be traced back to a single 

 cell of the histogen. (Fig. 87, D.) 



