244 Mineral Nutrition of Plants 



may be accumulated throughout the length of unbranched roots of 

 succulent species. Concentrations of bromide at various levels were 

 represented in Figure 4. At the time of measurement, a gradient of 

 concentration was evident, decreasing from the meristematic distal end 



MM FROM ROOT TIP 



Figure 27A. Graphs here and in Figure 27B show the magni- 

 tude of nonmetabolic absorption of Rb 86 , P 32 , Sr 85 , and I 131 (ex- 

 pressed in counts per second per mm.) as a function of distance 

 from root apex. In 27A, apical segments of barley roots 1 cm. 

 long and approximately 0.4 mm. in diameter were immersed 

 in Rb*Cl or KH 2 P*0 4 ; in 27B, in Sr*Cl, or KI* solutions for 

 30 minutes at o° C, and subsequently sectioned and counted. 

 The dotted line labeled "outside solution" in each case corre- 

 sponds to the activity of a volume of the bathing solution equal 

 to that of 1 mm. of root segment. — Overstreet and Jacobson (56) 



toward the proximal or root-stem transition part of the root. A similar 

 distribution was shown spectrographically for potassium and rubidium 

 by Steward, Prevot, and Harrison (79). The similarity in shape be- 

 tween a distribution curve for metabolically accumulated rubidium 

 (Figure 24, from 79) and a respiration gradient curve (Figure 25, from 

 50) on the same types of roots is probably further evidence for the 



