Hoagland — 62 — Plant Nutrition 



will necessarily be absorbed at different rates. Steward 

 has in fact emphasized that the discs of potato tuber 

 usually absorb and accumulate potassium and bromide 

 ions for a considerable period of time, in practically 

 equivalent amounts. This equivalent absorption may 

 also occur over certain experimental periods in studies 

 on barley roots. This type of absorption has been 

 called by Steward "primary" absorption as distin- 

 guished from those absorptions which involve ion 

 exchanges, without net gain of salt by the cell, for 

 which the term "induced" absorption has been pro- 

 posed. 



Finally, as part of this general survey, I should 

 state that the absorption and accumulation of salt 

 by a root is not uniform at all points. The apex is 

 the most active region and the activity decreases at 

 points successively farther from the apex, as Prevot 

 and Steward (1936) noted. Microrespirometer meas- 

 urements in this laboratory and evidence by others 

 indicate that oxygen consumption follows the same 

 pattern. 



Accumulation of Salt and Permeability : — I have 

 considered some of the environmental factors and con- 

 ditions of internal metabolism which lead to salt ac- 

 cumulation. Under what circumstances will the ac- 

 cumulated salt be retained in the cells? Interesting 

 data bearing on this question have come from experi- 

 ments in which radioactive isotopes have been utilized. 

 Roots were allowed to accumulate a small amount of 

 radioactive potassium and then, after rinsing, the 

 roots were immersed in a large volume of distilled 

 water (Jenny and Overstreet, 1938). Almost no 

 potassium left the tissue as indicated by the extremely 

 delicate test for radioactivity. According to this test the 

 healthy roots were practically impermeable (as a net 

 effect) to the outward movement of salt. Nevertheless, 

 much more salt could have been caused to enter the cells 

 under the influence of active cell metabolism. 



