Climate of Middle Illinois. 19 



56.7. The mean of the first ten years was 52.1, of the second, 51.4, of the 

 last, 52.7. 



TEMPEEATUEE ACTING UPON PLANTS. 



One plant is more sensible than others; cultivated plants introduced 

 from a warmer climate more than indigenous ones, and often in fall the 

 leaves of the tomatoes and dahlias may be killed by a temperature of the 

 atmosphere, which is above freezing point. On the other hand I observed 

 in April 1857, when the gooseberry bushes were green for a week, that the 

 mercury descended to 19° without injury to the leaves. 



In Transact, of 111. Agric. Soc, Vol. Ill, is published a paper read 

 before the Illinois Natural History Society in June, 1859, " On Meteorol- 

 ogy in connection with botanical observations," in which I did show that 

 each plant require a certain sum of heat in a certain space of time to per- 

 form its physiological functions, and that the degrees below the freezing 

 point, if not destructive, be not reactive, but inactive, and, therefore, all 

 the degrees below freezing point be of no account and excluded from the 

 computation. 



In the summer of 1857 I made some observations on the growth of 

 Indian corn. On the 16th of May, two days after- a. heavy rain, I planted 

 some corn in the yard of my residence, it sprouted on the 25th of May and 

 was ripe on the 30th of September. During these 138 days the sum of 

 daily mean temperature, 5 feet above the ground in the shade, was 3064 c. 



The sum of daily mean temperature of the soil four inches below the 

 surface, at 3 p. m., was 3,443; the quantity of rain, 13.2 inches; the mean 

 humidity of the atmosphere, 68 p. c. of saturation. The result of this ob- 

 servation is about the same as that reported by Bousingault upon an ob- 

 servation made at Alais in South France (44° N. L.) 



In the above observation, during the period of which the minimum 

 of the temperature was not falling below freezing point, the meteorologi- 

 cal observations as made for the Smithsonian Institution, and now for the 

 Signal office, could be used, and the means were computed from the three 

 daily observations at 7 A. M., 2 p. m. and 9 p. m., although this mode of 

 calculation does exclude for the whole summer the minimum (before sun- 

 rise) and the maximum (about 3 p. m.); and so the above sum, which 

 would be necessary to ripen Indian corn, was obtained, provided that the 

 temperature of each degree above freezing point have any effect upon its ■ 

 growth. 



That, in winter, the temperatures below freezing point are not reac- 

 tive, can be proved by observations of the periods of blooming of woody 

 plants. 



In 1857 spring was tardy, in 1859, very early. Comparing the time 

 of flowering of certain species with the sum of daily mean temperature, 

 commencing with January and excluding all temperatures below freezing 

 point, it is surprising to see the coincidence of figures, and the great dif- 

 ference when the negatives are not excluded. A table published in the 



