GENERAL RELATIONS 103, 
in them neither stimulus nor adaptation has received anything approaching 
adequate treatment. 
148. The measurement of response. The amount of response to a 
stimulus is proportional to the intensity of the factor concerned. This does 
not mean that the same stimulus produces the same response in two distinct 
species, or necessarily in two plants of one species. In these cases the rule 
holds oniy when the plants or species are equally plastic. For each in- 
dividual, however, this quantitative correspondence of stimulus and response 
is fundamental. It is uncertain whether an exact or constant ratio can be 
established between factor and function; the answer to this must await the 
general use of quantitative methods. There can be no doubt, however, 
that within certain limits the adjustment is proportional to the amount of 
stimulus, whereas reaction is well known to be abnormal or inhibited beyond 
certain extremes. It is quite erroneous to think that reaction is independent 
of quantity of stimulus, or to liken the stimulating factor to “the smallest 
spark (which) by igniting a mass of powder, produces an enormous 
mechanical effect.” Such a statement is only apparently true of the action 
of mechanical stimuli upon the few plants that may properly be said to 
possess irritability, such as sensitive plants and certain insectivorous ones. 
Of the normal relation of response to direct factors, water, light, etc., it is 
entirely untrue. Axiomatically, there is ordinarily an essential correspond- 
ence, also, between the amount of adjustment and of adaptation. This 
correspondence is profoundly affected, however, by the structural stability 
of the plant. 
From the preceding it follows that the measurement of response and the 
relating it to definite amounts of direct factors as stimuli are two of the 
most fundamental tasks of ecology. The exact determination of physical 
jactors has no value apart from its use for this purpose. -It is perfectiy 
clear that precise methods of measuring stimuli call for.similar methods in 
determining the amount of adjustment and of adaptation. The problem is 
a difficult one, and it is possible at present only to indicate the direction 
which its development should take, and to describe a few methods which 
will at least serve as a beginning. To cover the ground adequately it is 
necessary to measure response by adjustment and by adaptation separately, 
and in the latter to find a measure for the individual and one for the 
species. The one is furnished by the methods of morphology and the other 
by biometry. 
JPFEFFER-EwarT. Physiology of Plants, 1:13. 1900. 
