ESSENTIAL AND NON-ESSENTIAL ELEMENTS 417 



Similar antagonistic effects are evident in toxicity phenomena. For example 

 in one experiment it was found that the roots of lupines would elongate only 

 about 3.5 mm, per day in a solution of about 0.000015 IVI CuCU, but if 

 sufficient CaClo were added to make its concentration in the solution about 

 0.0078 M the roots elongated at a rate of 10.5 mm. per day. The antagonism 

 between the Cu + + and Ca++ ions was sufficient to reduce greatly the toxicity 

 of the Cu++ ions. 



8. Catalytic Effects. — Certain effects of mineral elements in plants have 

 generally been interpreted as due to their action as catalysts. Iron and man- 

 ganese, for example, are considered to act in a catalytic or regulatory role in 

 the synthesis of chlorophyll. The action of some mineral compounds as co- 

 enzymes (Chap. XXVII) may also be an example of such effects. 



Foods or Raw Materials ? — As with most of the other terms used in the 

 biological sciences a rigid definition of the word "food" is impossible. The 

 statement that a food is any substance which can be utilized directly by a 

 living organism as building material and also as a source of energ>^ (upon 

 oxidation) probably comes as close to a satisfactory definition of a food as 

 is possible. It is commonly considered that carbohydrates, fats, and proteins 

 are the most important foods of animals. Less generally is it recognized 

 that compounds of these same three classes are also the most important foods 

 used by green plants. The fundamental difference between the two t^pes of 

 organisms is that animals ingest carbohydrates, fats, and proteins from the 

 outside environment, while green plants synthesize them from a small num- 

 ber of relatively simple compounds which they obtain from their surround- 

 ings. Failure to appreciate this fundamental point regarding plant metabolism 

 has contributed to the persistence, especially in the popular mind, of the long 

 disproved idea that plants obtain their food from the soil. 



Those compounds which enter a green plant from its environment and 

 which cannot in any critical sense of the word be considered foods are most 

 conveniently termed raw ruaterials. The raw materials used by green plants 

 include, in addition to the necessary mineral salts, the carbon dioxide and 

 water used in photosynthesis. The compounds classified as raw materials are 

 not only utilized in the synthesis of foods but function in the plant in many 

 other ways. This is true not only of the mineral salts, as the immediately 

 preceding discussion has shown, but of water and carbon dioxide as well. 



Essential and Non-essential Elements. — Of the large number of ele- 

 ments which have been identified as occurring in plant tissues, only a limited 

 number have been found to be indispensable. Beginning about i860 a num- 

 ber of extensive investigations were undertaken to determine specifically which 

 elements are essential for plants, and which are not. Some of the earliest work- 



