Hoagland —54— Plant Nutrition 



been grown for a long time in solutions of radioactive 

 phosphate of much higher activity than that regularly 

 employed, without any perceptible injury to the plant. 

 In the field of plant physiology we regarded the 

 radioactive isotopes to be of particular value as an 

 additional tool of unique kind for an investigation 

 already established. We have not devised any in- 

 vestigation to fit the tool. Often the isotopes are em- 

 ployed for convenience; the same things might some- 

 times be done in other ways, although perhaps with 

 far more difficulty. There are, however, certain facts 

 to be learned only by the isotope method. For ex- 

 ample, it may be shown with roots that it is possible 

 for ions to move out of a tissue while other ions of 

 the same species are moving in more rapidly, with 

 the result that there is a net gain of the ions in the 

 tissue. Further, the radioactive elements can make 

 their own pictures and thus demonstrate directly their 

 general distribution in the plant. They can indicate 

 their presence in some cases without any operation 

 on the plant, by application of a Geiger-Miiller counter 

 to the undisturbed tissue.* 



Metabolism and Salt Accumulation with Special Ref- 

 erence to Barley Roots : — Returning now to the main 

 discussion, following the earlier work on Nitella cells, 

 F. C. Steward (1932) with the cooperation of several 

 members of our laboratory, began a series of re- 

 searches on potato tuber tissue. This relatively uni- 

 form tissue was prepared in the form of thin discs 

 and the processes of ion absorption and accumulation 

 were followed under carefully controlled conditions in 

 the culture solution in which the discs were immersed. 

 The bromide ion was again found to be very useful 



*It is desired to acknowledge here the indispensable work 

 of Dr. P. R. Stout in the many studies with radioactive isotopes 

 made in the laboratory of Plant Nutrition, and also the coopera- 

 tion of Prof. E. 0. Lawrence and members of the Radiation 

 Laboratory. 



