GENERAL RELATIONS IO3 



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 only 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 

 factors has no value apart from its use for this purpose. It is perfectly 

 clear that precise methods of measuring stim.uli 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. 



*Pfeffer-Ewart. Physiology of Plants, 1:13. lOCO. 



