220 D. R. Hoagland 



plant itself. This latter view recognizes a system of reciprocal re- 

 lations of extraordinary complexity. At times it may appear that 

 this complexity renders hopeless the attainment of any real com- 

 prehension of the phenomena involved, but the development of 

 methods of investigating plant growth under conditions which 

 permit control of the principal factors in the environment of the 

 root and shoot has renewed faith in the possibility of resolving 

 some of the problems. Many past investigations lacking any 

 elements of controlled experimentation have led to vast increases 

 in observational data, and in general information, but not to 

 secure advances in the understanding of basic principles. 



The indispensability of the ecological or agricultural approach 

 to the study of soil-plant relationships is not open to question. 

 The whole natural environment of the plant cannot be imitated 

 in the laboratory, and important physiological problems may 

 first be suggested in the field. Yet the knowledge gained from ob- 

 servations made under natural conditions seldom suffices to 

 establish the physiological nature of the processes involved. Such 

 knowledge is indeed often self-limited, since new methods of 

 studying plants in the field, and new points of view in the inter- 

 pretation of data gained by ecological and agricultural investi- 

 gation, are generally developed in the course of experiments in 

 which some element of exact laboratory control enters. Field 

 research and laboratory research should be regarded as different 

 means of attacking the same basic problems, not as separate 

 branches of plant science. 



Methods of Controlled Experimentation 



The complete control of the environment of a plant in an experi- 

 mental procedure is obviously beset with manifold technical 

 difficulties, but it has been demonstrated that these difficulties 



