Jan., 1922] PULLING BIOPHYSICS IN PLANT PHYSIOLOGY 
45 
ments in accord with the theorem regardless of whether or not both coil 
and magnet are parts of one structural entity, a dynamo; indeed, the 
magnet may be excited by a part of the current induced in the coil and thus 
coil and magnet have part in another process. It is the thesis of this paper 
that these internal interrelations of processes are denoted in biological terminol- 
ogy as correlations, which are thus seen to be not biological peculiarities to 
be considered merely as interesting phenomena but manifestations of the 
operation of an ordered universe to be investigated for the light they may 
shed on the plant as a reactant. 
So far the discussion has been vague with respect to the kind and number 
of variables that must be considered. Of course, since there are no rules 
to guide us, correlations and their mathematical relationships may be 
looked for among any characteristics of the plant. No one can say that 
random search would be barren of result. But since we have a somewhat 
extensive list of correlations already observed, which seem to have some 
physiological significance, it would seem to be the part of wisdom to begin 
with these. The Le Chatelier-Braun theorem plainly indicates a functional 
relation between the connected variables. A first-hand acquaintance with 
the general deductions of plant physiology should thus be a guide. Nat- 
urally, as the body of this sort of knowledge increases, relationships hitherto 
unsuspected will probably appear and, in their turn, lead to new ideas 
regarding the mechanisms of functional adjustment. 
One must be constantly on guard, however, not to infer that use of the 
same term in two cases indicates real correspondence of function. The 
word growth is such a term. It is common, for example, to use dry weight 
of plants as a measure of growth and to use increase in size as likewise 
indicative of growth. Dry weight has been an elusive quantity to say the 
least, perhaps because it is made up of varying proportions of differently 
usable substances, but at any rate it would seem on purely physiological 
grounds that both terms should not be directly and positively indicative of 
the same thing. Respiration is a fundamental activity of plants. Enlarge- 
ment is also. Put a plant in a situation that permits these two processes 
to proceed but that prevents additions of substances contributing to the 
dry weight, and the plant will enlarge and respire at the expense of com- 
pactness of solid substance. The more actively a plant or a tissue respires 
and enlarges the less compact is its solid matter. This is a matter of common 
observation. It would seem then that the plant's dry weight is but the 
material unused in respiration and enlargement and should not, by its 
mere magnitude, be taken as indicative of conditions favorable to the in- 
dividual plant itself, although the value of dry weight to its offspring or to 
mankind may be large. 
Regarding the number of variables to be considered, it would seem that 
these should be taken, not as two, as is usually done in correlation studies, 
but as three, for this seems to be a necessary consequence of the Le Chatelier- 
