72 EXPERIMENT STATION. [Jan. 



duration." This minimum of temperature, which rimst l>e 

 reached before any development takes place, has been called 

 the critical temperature. Do Candolle considered 43° as the 

 critical temperature for all plants. Previously it had been 

 placed at the freezing point. Now it is known that this point 

 varies for diil'erent species and varieties, and for different func- 

 tions. 



The thcorv as above stated assumes as the constant the sum 

 total of temperatures above a certain minimum point for the 

 elapsed time. Such a constant is of use in places having simi- 

 lar climates, but obviously is not suitable for comparisons be- 

 tween places having different lengths of growing seasons ; for 

 plants of the same species come to maturity in northern latitudes 

 with a very much less sum of heat than in more southern loca- 

 tions. In order to correct this inaccuracy, Linsser 1 proposed 

 the aliquot idea. To determine the aliquot for any physiologi- 

 cal function, the sum temperature for the given phase is di- 

 vided by the sum temperature for the entire year, as observed 

 at the same station. Thus, instead of depending upon the 

 production of a certain constant sum of heat, certain stages are 

 considered as due to be completed when the sum temperatures 

 above the critical temperature equal a definite fraction, of the 

 sum temperature of the year. Linsser called this fraction the 

 physiological constant. 



Another question is presented by this study of the aliquot, 

 namely : Is the critical temperature constant for a given function 

 and species in different latitudes? No investigations are known 

 which have sought to determine this point, but theoretically it 

 must be answered in the negative, as a little thought will show. 

 If we consider this constant to be the same in all latitudes, how 

 can we conceive of certain trees and shrubs having any dormant 

 periods in locations where the temperature rarely falls as low 

 as that at which they bloom in our northern climate? That is, 

 the temperature is continually above the critical temperature, 

 and no chance is offered for the plants to rest. 



Heretofore all investigations of this subject have depended 

 upon thermometer readings for their measurements of the sum 

 temperatures. These readings were taken two or three times a 



1 Carl Linsser, " Die Periodisohe Erscheinungen des Pflanzenlebcns in ihrem Verhaeltniss 

 zuden Waermeerscheinungen." Mem. Acad. Sci., St. Petersb., sev.VII., U (1867), No.7,p. 





