322 Mineral Nutrition of Plants 



to complete the vegetative or reproductive stage of its life cycle; (b) 

 such deficiency is specific to the element in question and can be pre- 

 vented or corrected only by supplying this element; and (c) the ele- 

 ment is directly involved in the nutrition of the plant quite apart from 

 its possible effects in correcting some unfavorable microbiological or 

 chemical condition of the soil or other culture medium. 



The criterion of the foremost physiological significance is the re- 

 quirement of an inorganic element for the successful completion of the 

 life cycle of a plant. This is, of course, different from merely demon- 

 strating a favorable effect on growth. The experimental procedure in- 

 volved in putting this criterion to the test must be based on removing 

 the element in question from the nutrient medium of the plant. This, 

 however, is beset with difficulties; first, it is impossible to remove 

 completely an element that may be contained in the seed. Second, the 

 same obstacle applies to the nutrient medium. Regardless of how effec- 

 tive purification procedures are, they cannot be regarded as having 

 removed 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 nutrient 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 minimum the level 

 of contamination in the nutrient medium. Different species vary greatly 

 in their requirement for a given micronutrient. 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 re- 

 duced through selection of plants having a high requirement for an 

 element. 



In our experiments the water-culture technique was used and the 

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

 terized 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 



