Jan., 1922] PULLING — BIOPHYSICS IN PLANT PHYSIOLOGY 
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physiological bases, may not morphological correlation be associated with 
physiological correlation? 
In this we come to the root of the matter. The attention of plant 
physiologists has ostensibly been directed chiefly to the relations between 
plant and environment. I say ostensibly because, although the environ- 
ment is measured as such, the plant is not. The plant is always measured 
in terms of the results of the interaction between environment and plant. 
We have no word for the plant itself, its inherited potentialities, the features 
that make it an individual. We know that plants of the same species, 
variety, and crop are physiologically different and remain so all their lives. 
These quantitative differences we vaguely ascribe to "individual vari- 
ability," and on it we blame many, if not most, of our experimental diffi- 
culties. 
To present the matter from another standpoint and thus, perhaps, 
make it clearer: suppose we could measure this internal, physiological 
constitution of the plant, that we knew all there was to know about a 
plant's ability to operate. Suppose we allowed these plants to grow in 
hermetically sealed boxes, of various sizes, shapes, and contents, the contents 
being wholly unknown. Suppose that after an interval of time we should 
analyze the contents of the boxes by every means at our command. How 
much knowledge would be acquired regarding the plant's relations with its 
environment? How much of the content of the box was put there by the 
plant and how much was residue? What compounds were formed by the 
plant and what by interaction of plant excretion and box content? How 
many repetitions of the experiment would be necessary before sure knowl- 
edge would be gained? True, we might select the boxes as to uniformity 
of size, shape, color, etc., but what would that avail if there were no rela- 
tion between externals and internals? Now, restate this, replacing the 
words box and box-content by plant, and vice versa, and we have a picture 
of the present situation. 
It must be evident that the saving feature in the case of the measurable 
environment and the unknown plant, the situation in which we actually 
find ourselves, is that there is a relation between externals and internals, 
and that a plant's activities do not have an indefinitely large range but are 
limited. Why then do we have so much difficulty in ascertaining the 
relation between environment and plant? In the absence of a problem's 
solution no one, of course, can state all reasons for failure to find one, but 
there are considerations that offer probable answers to the question. In 
the first place, the environment has not been completely stated in any 
instance, nor consciously duplicated in any two instances. Accordingly, 
the situation is more complicated than the term "measurable environment" 
might seem to indicate. As has so many times been pointed out, it should 
be measured and controlled — and if I may interject a remark, a first-hand 
acquaintance with the first story of the physics building is required for 
this: a precise workable equipment of laboratory knowledge. 
