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months—April, May, and June, 1734—the sum of the daily tempera- 
tures for ninety-one days was equivalent to 1,160° C., but for 1735 
it was 1,015° C., whence he concluded that the ripening of the vege- 
tation would be retarded in 1735 as compared with the preceding 
year. 
This idea had been familiar to Reaumur for some time previously, 
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 effective 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 ike 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 ab 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 multiplied 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 fhe length of 
the period of vegetation from germination up to any phase, to vary 
from year to year, inversely as the total sums 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 
