146 The Department of Plant Physiology [344 



ceases, and is replaced by one of carbon-exit, when light is 

 absent, but the same alteration may be induced by many other 

 changes in the surroundings ; for example, by sufficiently high 

 or low temperature, by sufficiently low water-content of the 

 leaves, by a sufficiently high concentration of a poisonous gas 

 in the air, etc. It soon becomes clear that no physiological 

 process is to be regarded as at all well understood without a 

 considerable knowledge of its quantitative control, and dyna- 

 mic physiology deals with the more elaborate and quantita- 

 tive statement of the physiological changes thus suggested. 

 It relates them to their determining conditions within and 

 without the organism. 



From the work of earlier students many plant processes are 

 now fairly well known in the simple, descriptive way, but none 

 of these is yet at all well understood in the dynamic or etio- 

 logical sense. This latter is the phase of physiology which is 

 now beginning to attract the most serious attention, and to it 

 will be devoted the energies of investigators for many genera- 

 tions to come. It is to this field of dynamic physiology that 

 the researches .of this laboratory are planned to apply. 



While descriptive physiology, as above defined, is a compar- 

 atively old science, dynamic physiology is a young one, and 

 the problems of the latter are complicated in the extreme 

 there are so many different kinds of conditions that may take 

 part in the control of plant processes, and each of these condi- 

 tions may be effective with so many different intensities. The 

 complexity and newness of these dynamic problems explain the 

 fact that the very methods needed for the sort of study here 

 suggested are, for the most part, still to be devised. It is ob- 

 vious that dynamic physiological investigations must rest upon 

 comparative measurements of the intensities of effective condi- 

 tions and of the concomitant or resulting rates of the processes 

 that are to be investigated, so that studies of the possible ways 

 by which such measurements and comparisons may be made 

 must constitute the beginnings in this field. All of our work 

 has aimed at this causal sort of explanation of process rates, 



