FACTOR INDICATORS. 81 



of indications. The widespread occurrence of certain herbaceous societies 

 throughout one or more forest associations, or even formations, affords a 

 striking opportunity for correlating the light relations for dominants asso- 

 ciated under varying conditions as to other factors. The perennial herbs are 

 of especial importance in this connection, as the effects of differing light 

 intensities are clearly reflected in a variety of ways, in density, form, height, 

 flowering, etc. 



In definitizing the use of light indicators, it will be necessary to resort more 

 and more to quantitative measurements of responses and factors. The most 

 important responses in this connection are photosynthesis and growth. Both 

 of these have certain values, and they will be more and more employed in 

 combination, as complete and accurate results become necessary. At present, 

 however, the determination of photosynthesis and its correlation with light 

 is a much simpler and more exact process. As a consequence, the best deter- 

 mination of indicator values for light will continue to be initiated by close 

 observation of general correspondences, which are first tested by means of 

 measurements of intensity and then by studies of photosynthate production. 

 It is probable, indeed, that this will give the real light indication without 

 recourse to growth responses, but the latter will prove necessary to obtain the 

 full indicator value for practical purposes. 



Temperature indicators. — Temperature produces no clear-cut response in 

 structure or grouping, and hence its indicators are not readily recognized by 

 observation alone, as in the case of water and light. The most obvious response 

 to it is growth, but this is affected so profoundly by other factors in nature 

 that a primary correlation with temperature is always difficult and usually 

 impossible. As a consequence of their striking distributional correlation with 

 latitude and altitude, a number of endeavors have been made to classify 

 plants with reference to temperature. The most suggestive are the classi- 

 fications of A. de Candolle (1874) and Drude (1913 : 154). Both of these are 

 based upon general climatic features, and take some account of water as well 

 as temperature. While they have more or less interest, their ecological value 

 is slight, owing to the almost complete lack of experimental and quantitative 

 bases. Moreover, the usefulness of the groups is further reduced by such 

 terms as "Etesial-Poikilotherme-Psychrochimenen." 



The most notable attempt to correlate flora and fauna with temperature is 

 that of Merriam (1890, 1894, 1898). The laws of temperature control of the 

 geographic distribution of plants and animals are stated by him as follows : 



"The northward distribution of terrestrial animals and plants is governed 

 by the sum of the positive temperatures for the entire season of growth and 

 reproduction, and the southward distribution is governed by the mean tem- 

 perature of a brief period during the hottest part of the year." 



His well-known system of life-zones was established upon the basis afforded 

 by these hypotheses. As indicated by his discussion of the Arctic, Hudsonian, 

 and Canadian zones (1898 : 54), the life-zones appear to be actually based 

 upon the outstanding vegetation zones of the continent, with temperature 

 control as a more or less correlated principle. While Merriam's system has 

 been of undoubted service in studies of floristics, its ecological value rests upon 

 the extent to which it has followed the natural vegetation zones and climaxes, 

 and upon the correlation of these with crops. It can not be regarded as fur- 



