222 D. R. Hoagland 



Although the artificial environment necessarily departs in many 

 respects from any natural environment, certain general princi- 

 ples governing the response of the plant to its environment can 

 be developed by laboratory research and may find an application 

 in the study o£ plants in the field. 



The solution of certain physiological problems requires the 

 complete and accurate control of environment of the whole 

 plant, but other problems, including most of those to be discussed 

 in this paper, may be usefully studied for the present through 

 methods of partial control, accomplished in the greenhouse or 

 laboratory. Water and sand culture technique afford indispensa- 

 ble means of controlling the nutrient environment of the root. 

 It is true that this technique has been employed by plant physi- 

 ologists for many years, but only recendy have some of the 

 factors been properly recognized and controlled. The impor- 

 tance of root aeration and of certain chemical elements formerly 

 considered unessential, may be mentioned. Partial control of 

 the aerial environment serves effectively for certain purposes. 

 Duration of illumination can be increased through the use of 

 artificial light at night; the intensity and quality of the natural 

 light can be regulated to some degree by the use of special 

 light filters, or screens; partial control of humidity or of air 

 movement can be accomplished in suitable glass chambers. 

 Furthermore, accurately controlled experiments may be made 

 with isolated tissues, such as storage organs or root systems, 

 where the factor of illumination does not immediately enter. 



These methods of quantitative control are capable of leading 

 to an understanding of certain interrelations between the plant 

 and its environment which cannot be gained from observations 

 on plants growing under uncontrolled conditions. Quantita- 

 tively controlled experiments have an obviously broad signifi- 



