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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



Even this small amount was sufficient to make the grain toxic to 

 white rats.^ 



When a crop like alfalfa, clover, or grass on an artificially 

 selenized soil was repeatedly cut and allowed to grow up again with 

 no further additions of selenium, the amount of selenium found in 

 successive cuttings of similar age became progressively less, suggest- 

 ing the gradual depletion of the selenium in the soil. Such de- 

 creasing concentrations in the tissues of successive crops from the 

 same roots are shown in table 3. 



Table 3. — Selenium in successive crops cut at intervals from the same roots, in 

 greenlwuse plots treated icith sodium selenate at a rate of 5 parts of selenium 

 per million of soil.* 



* Analyses by A. Van Kleeck, U. S. Bureau of Cbemistry and Soils, and by R. B. Deemer 

 and R. F. Gardiner under the direction of Drs. E. C. Sliorey and P. R. Dawson, U. S. 

 Bureau of Plant Industry. 



Not all elements in the soil are taken up as readily as selenium. 

 The accumulation of a given element depends on the nature of the 

 absorbing mechanism, which permits one substance to enter in greater 

 amount than another. This capacity to control to some extent the 

 materials entering the plant is known as " selective permeability ", 

 and it would seem that the mechanism involved should be such as 

 to keep out a poison, like selenium, which so far as we know now is 

 of no use to the plant. It seems quite possible that the easy entrance 

 of selenium may be due to its chemical similarity to sulphur. Sul- 

 phur is essential to plant growth, being a constituent of the proteins 

 and other compounds necessary to the plant's metabolism. Selenium 

 and sulphur are closely related with respect to their chemical prop- 

 erties. It does not seem unreasonable, therefore, to assume that 

 selenium gets in with sulphur, so to speak, and that they enter in 

 proportion to their relative availability in the substratum. 



The most striking evidence of such an interrelationship between 

 sulphur and selenium appeared in the fact that when excess sulphur 

 was made available to the plant the symptoms of selenium injury 

 that developed otherwise could invariably be prevented (pi. 3, fig. 1). 

 By means of sulphur treatments of both naturally seleniferous and 

 artificially selenized soils the entrance of selenium into the plant 



^ The analyses for selenium content were made by W. O. Robinson, U. S. Bureau of 

 Chemistry and Soils. The feeding tests on white rats were carried on in the laboratories 

 of Dr. Hazel Munsell, U. S. Bureau of Home Economics. 



