SELENIUM — HUED-KARRER 295 



was markedly reduced, although in no instance was it entirely pre- 

 vented (Hurd-Karrer, 1935). The proportionate amount of sele- 

 nium entering apparently decreases as the proportionate amount of 

 available sulphur increases. Of course, to have this effect elemental 

 sulphur must become available to the plant, largely through the 

 action of the soil organisms that convert it to soluble sulphates. 



Typical demonstrations of the inhibition of selenium injury to the 

 plant by the addition of sulphur in the form of sulphate to quaitz 

 sand cultures are illustrated in plates 2 (fig. 2) and 3 (fig. 2). In the 

 experiment shown in plate 3 the plants were gi^own with 0.033 gram 

 of sodium selenate added to each pot of sand. The sand in the dif- 

 ferent pots was kept moist with nutrient solutions containing differ- 

 ent concentrations of sulphur in the form of magnesium and ammo- 

 nium sulphates. The plants with the nutrient solution containing no 

 sulphate died in the seedling stage (a), those with a small amount 

 were extremel}^ chlorotic and stunted (5), those with a moderate 

 amount were but slightly chlorotic (c), while those supplied with a 

 large amount showed no chlorosis {d and e) and were as good as the 

 corresponding controls without selenium in both height and develop- 

 ment. Chemical analyses of the plants of two such experimeiits 

 showed that selenium absorption was reduced by the excess sulphate 

 to about one-fourth that of plants with little or no sulphate. 



It was apparent from such experiments that the toxicity of sele- 

 nium for plants is determined not by the absolute amount of selenium 

 present but by the proportionate amount with reference to sulphur. 

 To determine the critical ratio of the two elements — that is, the rela- 

 tive amount of sulphur necessary to prevent injury from a given 

 amount of selenium — I grew wheat seedlings in flasks containing 

 nutrient solutions. These solutions were all of the same composition 

 with respect to the essential ions potassium, calcium, magnesium, 

 phosphorus, nitrogen, and iron; but the sulphur, in the form of 

 sulphate, was varied in the different series. In some none was 

 added,^ causing the plants to be a pale green but not greatly stunted 

 or otherwise injured by the sulphur deficiency over the 5 weeks' 

 period of the experiment. The highest sulphur concentration used 

 was 192 parts per million, which was slightly toxic as shown by a 

 perceptible reduction in height of the control plants. The selenium 

 (as sodium selenate) was varied in the different flasks of each series 

 in order to determine the amount necessary to injure the plants at 

 each sulphur level. Growth of the plants in 3 of the 6 series is 

 shown in plate 4. 



•There was probably about 1 part per million of sulphur in these cultures from extra- 

 neous sources, such as impure chemicals and fumigants used in the greenhouse. 



