345] B. HE. Invingston 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 
