Daniel I. Arnon 323 



successive vegetative and fruiting cycles. The indeterminate type of 

 growth is desirable when an experimentally produced deficiency is 

 later to be corrected by adding the missing element. 



In purification of the nutrient medium, stress was laid first on the 

 preparation of nutrient media of reproducible degree of purity and, 

 second, on determining and expressing the level of remaining contami- 

 nation in quantitative terms (49). It was possible, using this technique, 

 to obtain consistent and reproducible responses in plants by adding 

 minute amounts of metals to the nutrient medium (for example, 1 

 part of zinc in 200,000,000 parts of culture solution, which amounted to 

 0.001 mg. of zinc to a plant). 



The proof that molybdenum is an essential nutrient for higher plants 

 illustrates how this approach was followed in practice. When tomato 

 plants were grown from the seedling stage in nutrient media purified 

 in this manner and supplied with a complete nutrient solution includ- 

 ing the four micronutrients, boron, manganese, zinc, and copper, charac- 

 teristic deficiency symptoms became apparent in a few weeks (/o). The 

 lower leaves developed a distinct mottling, different from any other 

 deficiency symptom previously noted in the tomato. In later stages, 

 necrosis at the margins and a characteristic involution of the laminae 

 accompanied by abscission of blossoms were noted. Thus, it was found 

 that by the first criterion of essentiality, the completion of the life cycle, 

 the nutrient medium was deficient in some essential element. Com- 

 plete recovery was obtained upon adding the B7 solution. A breakdown 

 of this group of seven elements disclosed that molybdenum was the 

 needed micronutrient. The development of these deficiency symptoms 

 was prevented by adding 1 part of molybdenum as molybdic acid to 

 100,000,000 parts of nutrient solution. 



After experimentally producing a characteristic deficiency syndrome 

 and demonstrating that it could be prevented or cured by the addition 

 of molybdenum, the next step was to show that this effect was peculiar 

 to molybdenum and that other elements could not be substituted. This 

 was done by supplying the cultures with the six other metals in the B7 

 solution and with the thirteen elements in the C13 group. The defici- 

 ency symptoms persisted unless molybdenum was provided. Neither 

 was there any additional improvement in growth when the applica- 



