FACTORS AFFECTING SEASONAL ACTIVITIES. 7 



tions more widely separated from each other, and in the seed 

 are again able to endure any cold which they may encounter. 



THERMAL REQUIREMENTS OF A PLANT. 



This brings us at once to the consideration of the practi- 

 cability of some estimation of the thermal constant of any form, 

 or the amount of heat necessary for its seasonal or cyclical de- 

 velopment. The first effort toward fixing any such standard 

 appears to have been made by Reaumur, the inventor of the ther- 

 mometric scale which bears his name. He adopted the sum of the 

 mean daily temperatures, as recorded by his thermometer in the 

 shade, as an index of the amount of heat required to bring a 

 plant to any given stage of development, using averages of the 

 daily maximum and minimum to obtain his mean daily temper- 

 atures. According to Abbe, Reaumur calculated the sum 

 of the averages constituting" the heat exposure of a plant at his 

 locality in France during the 91 days of April, May and June, 

 1734, to be equivalent to 1160° C, but in the following year it 

 amounted to only 1015° C. 



Adanson disregarded all temperatures below freezing, tak- 

 ing only the sum of the positive temperatures on the centigrade 

 scale, and began the summation of heat exposures thus derived 

 with the first day of the calendar year for any given season. 



Boussingault attempted to derive the thermal constant of a 

 vegetative period, or any part of it, by multiplying the mean tem- 

 perature of the air by the duration in days. 



Gasparin calculated thermal constants from temperatures 

 obtained from insolation thermometers exposed to direct sun- 

 light while lying on the sod, which would record 20° to 30° C. 

 higher than shade temperatures, and showing a difference 

 equivalent to three to six degrees latitude. By this method, the 

 thermal constant required for the germination and maturity of 

 the seed of wheat amounted to 2450° C. 



Variations in the method of calculation of the thermal con- 

 stant have been made by various investigators, in which this 

 standard has been obtained by multiplying the mean tempera- 

 ture by the square of the number of days involved, others multi- 

 plying of days by the square of the mean daily temperatures. 



