Rothamsted Symposium on Trace Elements 38 



interpreted as permitting no final conclusion as to the role of 

 cadmium, lead and mercury. The possibility that these or other 

 elements studied in a similar manner may be required in amounts 

 smaller than the incidental impurities which could not be removed 

 from the culture solution by the present technique cannot be a 

 'priori excluded. 



If these views are accepted there can be no objection to re- 

 garding almost every element in the periodic table, and particu- 

 larly those most frequently encountered in plant tissues, as sus- 

 ceptible of being shown at some time to be essential for plants. 

 What can be asserted definitely is that if an element now regarded 

 as dispensable for a given plant should at some future time be 

 found essential, it will be shown to be required in exceedingly 

 small amounts— within the limits of contamination still encom- 

 passed by the refined methods now used for purifying the nutrient 

 medium. 



This quantitative approach to the problem of essentiality of 

 micronutrients is regarded not as a mere theoretical generalization 

 but as a point of view conducive to a search for more refined ana- 

 lytical methods and procedures for growing plants which would 

 make it possible to investigate the role of a number of new ele- 

 ments in plant nutrition. 



References 



1. HoAGLAND, D. R., 1944: Inorganic Plant Nutrition (Waltharn, 

 Mass.: Chronica Botanica Co.). 



2. Arnon, D. I. and Stout, P. R., 1939: Plant Physiol. 14:599. 



3. Arnon, D. I., 1937: Soil Science 44:91. 



4. Arnon, D. I., 1938: Amer. J. Bot. 25:322. 



5. Burk, D. and Horner, C. H., 1935: Third Intern. Soil Sci. 

 Congr. Trans. A. 1:152. 



6. BoRTELS, H., 1937: Arch. Mikrob. 8, 1:13. 



7. Arnon, D. I. and Stout, P. R., 1939: Plant Physiol. 14:371. 



8. Stout, P. R. and Arnon, D. I., 1939: Amer. J. Bot. 26:144. 



9. Piper, C. S., 1940: Austr. Inst. Agri. Sci. 6:162. 



10. Hoagland, D. R., 1940: Proc. Amer. Soc. Hort. Sci. 38:8. 



