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, ete. 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, 
