202 Mineral Nutrition of Plants 



external or environmental conditions, or to internal controlling con- 

 ditions. Of the external or environmental conditions, aeration, tempera- 

 ture, and the supply of water and mineral salts are important. These 

 conditions may alter and determine the internal physico-chemical con- 

 ditions and the relative rates of processes, some interrelated with absorp- 

 tion, in roots. Studies with abscised roots of barley (jo) and oats (59), 

 and roots of decapitated wheat plants (42) have clearly shown the 

 necessity of an aerobic environment for the accumulation of salt in such 

 organs. Experimental results with abscised roots of tomato and rice, 

 as well as barley, are presented in Figures 8A and 8B, from experiments 

 of Vlamis and Davis (88). Here, the reduced accumulation of potassium 

 and bromide at oxygen values below 3-5 per cent is clear. A fairly 

 close correlation exists between these effects of oxygen supply on ac- 

 cumulation, and on total respiration by comparable tissues. This is 

 evident when the respiration curves shown in Figure 9 are compared 

 with the accumulation curves of Figures 8A and 8B. Similar concord- 

 ance between respiration and ion accumulation rates in potato roots has 

 been reported (77). The relationship between these factors will be dis- 

 cussed further in another section. 



Temperature 



The temperature of the environment is a second factor which may 

 control the rate of physico-chemical processes. As with aeration effects, 

 a similar correspondence between the effects of various temperatures 

 on the processes of accumulation and respiration have been obtained. 

 Several investigators have reported these phenomena with roots (30, 

 42,84,91). Such effects may be exemplified by data derived from ex- 

 periments of Ulrich (84) with abscised roots (Figure 10). In these 

 histograms, it may be observed that increases in both accumulation and 

 respiration rates are associated with rise in temperature of the medium 

 bathing the tissues. More than doubled increases in rates for a ten- 

 degree rise in temperature are indicative of these processes being of 

 physico-chemical nature. From the concordance of temperature effects 

 obtained between accumulation of inorganic solutes and respiration 

 (especially the enhanced respiration), investigators have generally in- 

 ferred some interrelation between them. This is quite possible, since 



