XVII.] THE PHYSIOLOGICAL CONSTANT. 291 



Frederick Brendel, of Illinois,* who, in comparing the 

 late spring of 1857 with the very early spring of 1859, 

 "found in some species a striking coincidence of the 

 sums of the mean temperature, and of the number of 

 days on which the temperature rose above freezing 

 point," when "comparing the periods of flowering with 

 the accumulation of heat in both years, from January 

 to that period." More recently, the same writer has 

 reaffirmed! that the physiological constant is the "sum 

 of daily mean temperature, commencing with January 

 and excluding all temperatures below freezing point," 

 up to the time of the event in question. It is believed 

 by most phenologists that a certain life -event takes 

 place in any species whenever the species has been ex- 

 posed to a certain sum -total of heat. This sum -total 

 is reckoned from the lowest assumed temperature at 

 which vegetation takes place. This lowest temperature 

 was first put at freezing-point, but in later years it is 

 placed at six degrees Centigrade, or about forty -three 

 degrees Fahrenheit. Below this point all temperatures 

 are disregarded, and the sum -temperature for the life- 

 event is obtained by adding together all the positive 

 daily thermometric readings, — all those above this 

 awakening point. This sum -temperature is itself 

 taken as the physiological constant by Hoifmann, and 

 perhaps most phenologists, but Linsser considers it 

 erroneous, and establishes his constant by ascertaining 

 the ratio of this sum -temperature of the event to the 

 sum -temperature for the year. "The sums of temper- 

 ature above zero which are necessary to the same vege- 

 tation-phase in two places are proportional to the sums 



♦Trans. 111. Agric. Soc. ili. 674, 

 t Flora Peoriana, 19 (1887). 



