THE PHYSIOLOGICAL BASIS. 43 



ents and Long (Clements, 1918:29; 1919; cf. Long, 1919) in the habitats at 

 the Alpine Laboratory, and the chemical procedure has been refined to furnish 

 a basic method of universal application. The use of plants as instruments for 

 habitat analysis is further discussed on a later page. 



THE PHYSIOLOGICAL BASIS. 



Kinds of response. — With rare exceptions a physical factor produces a func- 

 tional response. Such responses are the most direct and the most accurate 

 measures of the habitat, and hence would serve as nearly perfect indicators 

 were it not for their being invisible. Fortunately, functional responses when 

 marked regularly bring about structural changes which are visible. This is 

 especially true of growth which, as the middleman between function and form, 

 has the advantage of being direct as well as visible. Growth, like structure, 

 has the further merit of showing qualitative as well as quantitative differences 

 and thus serves as an obvious record of abnormal response. From the stand- 

 point of indicators, it is desirable to take all three kinds of response — function, 

 growth, and structure — into account and to assign to each its proper value. The 

 relative value is indicated by the sequence of the three as successive effects of 

 controlling factors as causes. The rapidity and accuracy of the response 

 decreases with the distance from the impinging factors, while the readiness of 

 its recognition correspondingly increases. As a consequence, indicator values 

 have so far been based largely upon species and form. The importance of 

 growth has later been recognized and it is but recently that function has been 

 taken into account. In the further investigation of plants as habitat measures 

 and indicators, it is essential to determine the functional responses first, as the 

 most direct and quantitative. These should then be correlated with growth 

 measures and the latter with structural adaptations. When this has once 

 been done, either structure or growth can be used as ready and accurate meas- 

 ures, without resorting each time to the experimental analysis involved in 

 functional measurements. As a matter of practical application, however, it is 

 probable that growth and reproduction will serve as the best indicators of 

 conditions for crop plants since the habitat is more or less controlled. In the 

 case of forest and grassland, where the factors are essentially natural, a further 

 analysis by means of functional determinations seems desirable if not imperative. 



Effect of habit — There are three reasons for the superiority of function over 

 form for indicator correlations. The first is that considerable adjustments to 

 factors can occur without affecting structure at all, the demands being fully 

 met by functional responses. Another is that there is almost always a lag 

 between function and structure, by which the effects of a factor appear in the 

 latter only after a time or in diminished degree. These reasons are relatively 

 unimportant compared with the role of habit, however, and the second is 

 perhaps only a consequence of the latter. While there has been little experi- 

 mental study of habit as such, there are many suggestions of its importance in 

 modifying or reducing response, especially in structure. This influence of 

 habit is well known to foresters and agriculturists in connection with the 

 germination of seeds from different regions and the behavior of their seedlings. 

 It has also been shown in the case of alpine species transplanted to lower levels 

 in that some retain the dwarf habit and others do not (Bonnier, 1890), and 



