SELENIUM — HURD-KARRER 297 



each flask of a series and the sulphate variable. One of these series 

 is shown in plate 5. The fact that the selenium-sulphur ratios as- 

 sociated with the different degrees of injury to the plants were the 

 same here as in the preceding experiments, although the nutrient 

 solutions were of such different composition, increased the certainty 

 and significance of an exact quantitative relation between sulphur 

 availability and selenium toxicity. 



Of course, there is the possibility that with some entirely different 

 set of conditions the absolute values of these ratios might be changed 

 somewhat. However, the environment was changed purposely in 

 the different experiments over a period of several years by growing 

 the plants at different times of the year and at different tempera- 

 tures, by varying the composition and the acidity of the nutrient 

 solutions, and by renewing the solutions after each week with some 

 and after 5 Aveeks with others. The critical sulphur-selenium 

 ratio for the appearance of visible injury varied only between 9 

 and 11. No relationship of this nature has been established for 

 any other pairs of elements, the calcium-magnesium antagonism ap- 

 proaching it most closely, perhaps, but apparently not with this 

 high degree of reproducibility. 



The only way I have found so far to explain the relationship of 

 sulphur and selenium is to assume that the root cannot tell the 

 difference between them because of their chemical similarity. Assum- 

 ing that this is true, then the amount of selenium taken in with a 

 given amount of sulphur would depend on the proportionate amounts 

 of the two which are available, the total absorbed being limited. Thus 

 if there is a large excess of sulphur the root will get relatively little 

 selenium. After the selenium gets in, it can be assumed that the 

 plant proceeds to use it as if it were sulphur, but with serious results. 

 Every molecule that gets selenium instead of sulphur would be 

 disabled, as it were, and fail to function normally. When a large 

 enough proportion of the molecules are affected the plant shows ex- 

 ternal signs of injury. This theory of substitution adequately ac- 

 counts for the quantitative aspects of the dependence of selenium 

 toxicity on relative rather than absolute sulphur availability; and for 

 the fact that chemical analyses show that excess sulphur reduces the 

 amount of selenium taken up by the plant. 



The nature of the white chlorosis, which is typical of selenium in- 

 jury to such plants as wheat and Ijarley, suggests that some of the 

 molecules susceptible to this substitution are involved in the synthesis 

 of the green coloring matter, chlorophyll. Thus if the plastids were 

 disabled by such a substitution in their protein molecules, then they 

 might not function and the tissues would be white instead of green. 

 It is interesting that one of the earliest experimenters with the effect 



