MORPHOLOGY, GENETICS AND BREEDING 



35 



The central cylinder of the primary root differentiates pericycle, phloem, 

 xylem, and pith in normal fashion. The phloem fibers occur in four 

 homogeneous bands. These mature about the same time or a little later 

 than the last primary xylem. Thus the primary vascular structure of the 

 root is tetrarch, external evidence of which can be seen in the 4-ranked 

 arrangement of the first lateral roots. 



Figure S.— Older primary root of peanut, cross section. Secondary wood has ap- 

 peared ; cortex and pith have begun to disintegrate, (after Yarbrough, 76.) 



