70 CARNEGIE INSTITUTION OF WASHINGTON. 



Auxo-thermal Integration of Climatic Complexes, by D. T. MacDougal. 



It is obvious that if we are to make any rational interpretation of the 

 partial and totalized effect of temperature upon the organism in any 

 phase of its activity, or during all of its ontogeny, a method must be 

 formulated by which the duration and intensity of the temperature 

 exposure of the organism may be calculated. 



The author's work led him in 1900 to a realization of the necessity 

 for such a method, and the first attempts at anything definite were 

 presented at the Denver meeting of the American Association for the 

 Advancement of Science in 1901 . The method then proposed consisted 

 simply in estimating the number of hours at which the temperature 

 stood above the freezing-point from the beginning of a season until a 

 plant had attained a certain stage of its development. This method 

 was superior to all previous methods of summation of temperature 

 effects, in that it gave full value to the time factor of exposure, which 

 the older methods of cumulating or totalizing maxima or averages of 

 daily temperatures did not. It implied an empirical procedure, as 

 growth at all temperatures above the freezing was taken as uniform. 



The first step in the selection of a standard by which the constructive 

 processes of organisms might be measured consisted in fixing upon 

 some form of activity which was delicately affected by temperature 

 and was readily measurable. Growth extension or expansion seemed 

 to meet these requirements most fully, and this selection had the 

 additional advantage that a large number of measurements of the 

 actual rate in several species is already available. 



It was therefore determined that evaluation of climatic factors of 

 any kind must necessarily be made in terms of some single plant, or as 

 may fall out later in groups of plants with similar procedure. Next 

 it was seen that the data which might be obtained by the measurement 

 of the elongation of internodes in a monocotyledonous stem would be 

 more easily secured and corrected and could hence be made more exact. 

 A plant of this kind with a range from the tropics to the arctics was 

 represented by wheat and the growth-data of this plant was already 

 available, enabling us to measure the temperature factor of a climate 

 in terms of the growth velocity of wheat, as it might be done with any 

 other plant the growth of which has been determined. 



Assuming now that it is desired to evaluate the variable temperature 

 of any place or of any experimental setting, it is first of all necessary 

 to secure a reliable thermograph record for the period under investi- 

 gation, which might include the entire frostless season or the time in 

 which a certain stage of development of the selected organism had been 

 accomplished. Next this record is ruled by lines which will divide it 

 into figures, the area of which (measured by a planimeter) represent the 

 length of time and intensity of temperature. The obvious procedure 

 is simply to construct regular figures which shall include the area of the 



