345] B. E. Livingston 147 



but much of it, as so far carried out, has resulted in little 

 more than giving us certain methods of study and certain 

 incomplete results. The problems are so complex that broad 

 generalizations cannot be looked for for a long time. 



It has so happened that two phases of dynamic physiology 

 have thus far occupied much of our attention, as far as 

 research is concerned. At the same time, these two phases are 

 among the most fundamental of all, as regards plant growth 

 in general, and the agricultural and forestal production of 

 plant material in particular, and they also appear to repre- 

 sent the very simplest problems in plant control. The first 

 of these phases, or groups of dynamic problems, deals with 

 the water relations of plants; the second deals with their 

 inorganic-salt relations. The connotation of these two groups 

 of problems may be roughly suggested to the reader by the 

 statement that the agricultural operations of drainage and 

 irrigation are related to plant water relations, while fertilizer 

 practice is related to inorganic-salt relations. A large number 

 of the contributions from this laboratory have dealt with one 

 or the other of these general phases. The measurement and 

 experimental control of the environmental conditions of mois- 

 ture and of inorganic salts, and the relation of these condi- 

 tions to plant growth, have thus received a large portion of 

 our attention. 



Along with the study of external conditions, the internal 

 conditions of our experimental plants must, of course, receive 

 consideration. While these conditions are generally much more 

 difficult of adequate measurement than are those of the en- 

 vironment, some progress has nevertheless been made in this 

 direction. For example, the work of this laboratory has 

 aided the advance of our knowledge of the manner in which 

 internal conditions control the rate of water-loss from plant 

 leaves, a very important subject, both to the science of plant 

 physiology and to the arts of agriculture and forestry. 



The relations of temperature and of oxygen supply have 

 recently begun to receive attention here, as well as the light 



