169 



months — Api-il, May, and Juno, lT3-t — the sum of the daily tempera- 

 tures for ninety-one days was equivalent to 1,1()0° C, but for 1735 

 it was 1.015° C, whence he concluded that the ripening of the vege- 

 tation Avould be retarded in 1785 as compared with the preceding 

 year. 



This idea had been familiar to Reaunuu- for some time j)i'evi<)usly, 

 and in 1735, as cited by Gasparin, Met. Agric, Vol. II, 1st ed., Paris, 

 1844, he says : 



It would be interesting to continue such comparisons between the 

 temperature and the epoch of ripening and to push the study even 

 further, comparing the sum of the degrees of heat for one year with 

 the similar sums of temperatures for many other years; it would be 

 interesting to make comparisons of the sums that are eft'ective during 

 any given year in warm countries with the effective sums in cold and 

 temperate climates, or to compare among themselves the sums for the 

 same months in different countries. 



Again, Reaumur says : 



The same grain is harvested in very different climates. It would be 

 interesting to make a comparison of the sum of the temperatures for 

 the months during which the cereals accomplish the greater part of 

 their growth and arrive at a perfect maturity both in warm coun- 

 tries like Spain and Africa, in temperate countries like France, and 

 in cold countries like those of the extreme north. 



This passage, says Gasparin, is the germ of all the works which 

 have been executed since that time in order to determine the total 

 quantity of heat necessary to the ripening of the different plants that 

 have been cultivated by man. 



Adanson (1750) disregarded all temperatures below 0° C, and took 

 only the sums of the positive temperatures. He expressed the law as 

 follows: The development of the bud is determined by the sum of 

 the daily mean temperatures since the beginning of the year. 



Humboldt early insisted upon the necessity of taking the sunlight 

 itself as such into consideration in studying the laws of plant life. 



Boussingault (1837), in his Rural Economy, introduces the idea 

 of time by adopting the principle that the duration of any vegetating 

 period nndtiplied by the mean temperature of the air during that 

 period gives a constant product. He takes the sum of the tempera- 

 tures from the time when vegetation begins and finds the length of 

 the period of vegetation from germination up to any phase, to vary 

 from year to year, inversely as the total smns of the daily temper- 

 atures. 



Thus, for winter wheat to ripen, he found that there was necessary 

 a sum total of from 1,900° to 2,000° C. of mean daily air tempera- 

 tures in the shade, which constant sum is equivalent to saying that the 

 average temperature of the growing period is found by dividing this 

 number by the number of days. This method of computation takes 



