BIOCHF.MISTRV OF PLANTS 



encountered as the effort is made to go further in the exckision of im- 

 purities from the nutrient medium. It should be noted, however, that 

 various chemical elements not indispensable for growth of the plant 

 may modify its biochemical reactions either beneficially or adversely, 

 under the conditions of a natural environment. 



Many of these facts have been established primarily through the 

 use of artificial culture methods, among which the so-called water- 

 culture method is especially useful for studying the effects of a deficiency 

 of an element needed by the plant in minute quantity. Sometimes, 

 however, special care in the selection and purification of a solid inert 

 medium, to which a purified nutrient solution is applied, pro\ides an 

 alternative technique, with certain advantages. 



Innumerable experiments, some of them with meticulous care, 

 have been made on many species of plants with these artificial culture 

 techniques. The control of the inorganic nutrient medium represents 

 only a partial control of the environment; and frequently there remains 

 the necessity or desirability of control of the atmospheric factors to 

 which the plant is subjected: light, temperature, humidity, carbon di- 

 oxide concentration, and air movement. The control of these factors 

 obviously demands costly equipment and usually is not attempted in 

 nutritional experiments with plants. But some laboratories have had 

 the opportunity to grow plants under conditions of controlled air tem- 

 perature and controlled artificial illumination. In recent years, 

 fluorescent lamps have proved especially valuable for this purpose. 



Most artificial culture experiments have not been designed for 

 the primary purpose of obtaining information about the metabolic 

 mechanisms of the plant and the specific enzyme systems concerned. 

 Observations on plants grown in the presence of selected combinations 

 of inorganic nutrients are likely to be confined to measurements of rate 

 of growth of the plants, total yields of the tissue produced on a fresh 

 or dry weight basis, or on yields of some part of the plant of special 

 interest from an agricultural point of view. Chemical studies are 

 often limited to the determination at the end of a selected growth 

 period of certain chemical elements absorbed by the plant, or of well- 

 known organic compounds formed as a net result of innumerable bio- 

 chemical processes that have proceeded perhaps for a considerable 

 period of time during which the plant has increased in size and difleren- 

 tiated its tissues. In other cases, the purpose may he to record the 



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