The Plant as a Metabolic Unit 221 



may be overcome in sufficient degree to permit the realization of 

 certain ideals o£ quantitative measurement, despite the com- 

 plexity of the biological system. As an illustration, experiments 

 conducted in this laboratory may be cited. Wheat plants w^ere 

 grown in chambers controlled with respect to light, temperature, 

 humidity, air flow, and nutrient solution. In an earlier form of 

 these chambers (Davis and Hoagland),'* control was available 

 only within a limited range of conditions, but a more elaborate 

 installationf makes feasible the accurate control of humidity and 

 temperature over a wide range, illumination of regulated in- 

 tensity and duration being supplied by Mazda lamps. 



By the use of such chambers, experiments can be replicated at 

 will, with a very small probable error. Therefore it becomes possi- 

 ble to compare with quantitative accuracy many known com- 

 binations of variables in successive experiments. Using a pure 

 line seed, "standard" sets of plants may be grown at any time 

 which are essentially identical, as judged not only by the super- 

 ficial criteria of growth, but also by organic and inorganic com- 

 position, as has been shown recendy by A. R. DavisJ and his 

 coworkers. 



It is, of course, recognized that there are formidable obstacles 

 to the wide extension of this kind of experimentation. Chief 

 among these obstacles is the practical difficulty of producing 

 artificial light of suitable intensity and quality. The growth of 

 many species of plants, under the kind of artificial illumination 

 usually employed, is very different from that occurring under 

 ordinary natural illumination. However, present difficulties are 

 not insurmountable and there exists a rich field for exploration. 



* Superior figures refer to items in the bibliography at the end of this essay. 

 tThis apparatus, erected by the Carriere Corporation, in consultation with 

 Professor A. R. Davis and others, will be described later. 

 % Unpublished data. 



