74 TRANSLOCATION IN PLANTS 



mobility and by the colloidal condition of the membrane 

 as altered by the action of other ions present in inter- 

 ferences or antagonisms," and goes on to say that the 

 " transpirational pull" might vary widely without influenc- 

 ing mineral nutrient absorption. He is partly correct in 

 stating that the rate of ionic movement is determined by 

 their own ionic mobility and by the condition of the mem- 

 brane, but he seems to overlook the fact that, if the solutes 

 move across membranes abutting on the water-conducting 

 vessels — and they must so move if these solutes are carried 

 through the water-conducting tissues as he assumes — then 

 their concentration on the vessel side of the membrane 

 must have an important effect on the rate at which they 

 move across. The rate of water movement through these 

 vessels, as determined by the rate of transpiration, should 

 therefore directly influence the rate of solute movement 

 away from the absorbing regions and thus, in turn, the 

 rate of absorption. The fact that the rate of absorption 

 is not determined by the rate of transpiration is therefore 

 clear evidence that the translocation is not dependent upon 

 water movement. 



Instead of finding that high transpiration decreases the 

 ash content of the roots and increases that of the tops, as 

 might be expected on the assumption that nutrients are 

 translocated in the transpiration stream, Muenscher (1922) 

 found that, when transpiration was altered by Ught and 

 shade, the plants with high transpiration had a much 

 higher ash content of the roots and less in the tops, while 

 those with low transpiration had less in the roots and more 

 in the tops. This difference was very marked whether 

 the ash contents were expressed as percentages of dry 

 weight or fresh weight or as ratios of total ash. Differ- 

 ences in the same direction but less marked were evident 

 when differences in transpiration were brought about by 

 differences in humidity. It is highly probable, as previ- 

 ously mentioned, that the differences in ash distribution are 

 not directly determined by transpiration influencing absorp- 

 tion or translocation, but indirectly through the influence of 



