35 Aknon: Criteria of Essentiality 



rification procedures are, they cannot be regarded as having re- 

 moved the last atom of a contaminant originally present in the 

 water and nutrient salts, or one that is derived from the container 

 in which the plants are grown, or one gaining access to the nu- 

 trient medium in the course of an experiment. 



Experimentally the problem resolves itself into selecting a 

 species which has a high requirement for a given micronutrient 

 and using purification procedures capable of reducing to a mini- 

 mum the level of contamination in the nutrient medium. Dif- 

 ferent species vary greatly in their requirement for a given micro- 

 nutrient. Beans, for example, have a far greater requirement for 

 boron than barley. Alfalfa is capable of absorbing enough zinc 

 from a medium in which corn shows acute deficiency symptoms. 

 The extent to which it is necessary to purify the culture medium 

 in order to produce deficiency symptoms may be reduced through 

 selection of plants having a high requirement for an element. 



In our experiments the water culture technique was used and 

 tomato was selected as the principal test plant. This plant is 

 characterized by a relatively small seed in relation to the emerged 

 plant, thus rendering it likely that seed reserves would prove 

 inadequate for the requirements of the growing plant which is, 

 in addition, capable of successive vegetative and fruiting cycles. 

 The indeterminate type of growth is desirable when an experi- 

 mentally 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 

 contamination in quantitative terms (8). Pyrex glassware was 

 adopted for containers as the best material as far as freedom from 

 metal impurities was concerned (it is not suitable, however, for 

 boron work). Molar stock solutions of the various nutrient salts 

 were individually purified by precipitation and adsorption of im- 

 purities at high temperatures and slighdy alkaline reactions in 

 the presence of calcium and phosphate ions. Distilled water was 

 purified by redistillation, using a Pyrex glass condenser. The 

 purified water and chemical stock solutions were stored in Pyrex 

 containers. 



After purification the purity of water of the various molar 

 stock solutions of nutrient salts was determined by means of an 

 adaptation of the diphenylthiocarbazone (dithizone) test (8). 



