72 EXPERIMEXT STATION. [Jan. 



duration." This minimum of temperature, which must be 

 reached before any development takes place, has been called 

 the critical temperature. De 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 different species and varieties, and for different func- 

 tions. 



The theory as above stated assumes as the constant the sum 

 total of temperatures above a certain minimum point for tlie 

 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 diff'erent 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^ proposed 

 the aliquot idea. To determine the aliquot for any ph3^siologi- 

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

 vided by the sum temperature for the entire year, as obsei-ved 

 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 temperatm-e 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 diiferent latitudes ? No investigations are known 

 which have sought to determine this point, but theoretically it 

 nmst 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 shrul)s 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 



u})on thermometer readings for their measurements of the sum 



, tein])eratures. These readings were taken two or three times a 



1 Carl LlnBser, " Die Perlodische Erscheinungen dcs Pflanzenlebens lu Ihrem Verhaeltniss 

 zuden WiienncerBclieinungeu." Mem. Acad.Sci., St. Petersb., sev. VII., 11 (1S67), No. 7, p. 

 35. 



