36 THE PEANUT— THE UNPREDICTABLE LEGUME 



Yarbrough (76) emphasizes the following characteristic features of 

 the root system in the young peanut seedling : 



1. The central stele shows a tetrarch or four-pointed pattern of four 

 xylem groups with alternating phloem groups. This is the usual radial 

 arrangement of conducting tissue found in roots of all higher plants. 



2. Although Compton (19) stated that the peanut root has no pith, 

 note the large pith in the center of the cross section shown in figure 4. 

 Old roots are quite woody and the pith breaks down, leaving the root 

 hollow. 



3. Old roots have their internal structure modified by two meri- 

 stematic zones : (a) the cork cambium which develops from the pericycle 

 and causes the death and loss of all tissues external to it, and (b) the 

 vascular cambium which develops within the phloem and forms consid- 

 erable amounts of wood (figure 5). 



4. Sparsely scattered hairs may be seen on peanut roots under con- 

 ditions of high temperature and high humidity with rosettes of hairs more 

 common at the bases of some lateral roots. 



5. No true epidermis exists on peanut roots. 



TRANSITION FROM ROOT TO STEM 



The hypocotyl, that portion of the peanut stem below the cotyledons 

 and above the root, never emerges from the soil and thus becomes part 

 of the anchoring root system of the plant. The transition from root to 

 hypocotyl, however, is marked and abrupt, both internally and externally. 

 The transition from radial to collateral bundles was described by Richter 

 (59), by Compton (19) and by Yarbrough (76). According to Yar- 

 brough, ". . . there are four exarch xylem groups in the upper primary 

 root; just below the collar there are eight endarch groups formed by 

 division of each original one ; in the collar or slightly above some fusions 

 may reduce the xylem groups to seven, six, or even four ; above this level 

 and throughout the hypocotyl further branching and anastomosing cause 

 the number of xylem groups to vary from nine to eighteen." The four 

 phloem groups of the primary root pass through the zone of xylem 

 transition without modification but finally divide and with the associ- 

 ated xylem give rise to the collateral bundles of the hypocotyl. Externally, 

 at the collar the smooth, intact epidermis of the hypocotyl gives way to 

 the sloughing outer surface of the root. Internally, special tannin cells are 

 associated with collateral strands throughout the stem and in leaves, 

 flowers, and fruits. These disappear abruptly at the collar. Thus the 

 transition zone from root to stem is defined by : 



