THE ANATOMY OF THE ROOT 



41 



may occur, and it is not uncommon for a plant to have a different 

 number of points in the primary root as compared with its laterals. 

 The most common types are the diarch (two protoxylem points), 

 represented by the primary roots of Beta, Solanum, and Raphanus 

 (Fig. II.); the triarch, found in Pisum and Medicago (Fig. 13); 

 and the tetrach, in Cucurbita, Gossypium, and Ipomoea. (Fig. 



Fig. IX. Transection of the primary root of Raphanus showing the diarch primary xylem 

 strand prior to secondary thickening of the axis. Outside the right-hand protoxylem point, 

 the pericycle has divided tangentially, initiating lateral root formation. 



3 10.) Pentarch, hexarch, septarch, and octarch types occur; and, in the 

 Gramineae, there may be a larger number of arcs. In the radial 

 siphonostele, the number of xylem arcs is usually considerably 

 greater than found in radial protosteles; and they are commonly 

 designated as being polyarch. (Fig. 14.) The number of arcs in 

 the primary root of corn may be xo or more; and, in secondary 

 roots, twice that number frequently occur. 



In some of the older literature dealing with the stelar anatomy 

 of the root, the term axile bundle has been used to designate the pro- 



