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no account of any temperature at which the growth of wheat ceases. 
A lower limit for such temperature has been adopted by several 
investigators, such as the 0° C., already mentioned as adopted by 
Adanson. An upper limit has not yet been ascertained. Edwards 
and Colin put it at 22° C.; but in Venezuela Codazzi found wheat to 
mature under a constant*temperature of 23° or 24° C. throughout the 
whole period of vegetation, and, as we shall see hereafter, the upper 
limit undoubtedly depends upon the humidity of the air, the moisture 
of the soil, and the total radiation from the sun quite as much as upon 
temperature. Similarly Marié-Davy calls attention to the fact that 
maize grows poorly at Paris, where it is cloudy and warm, but well in 
Alsace, where it is dry and clear, the temperature of the air averaging 
about the same in both, the difference being in the quantity of sunshine 
and rain. 
Gasparin (1844) adopted the mean temperature of the day as de- 
rived from observations made at any convenient hours and took the 
sum of such temperatures from and after the date at which the plants, 
especially the cereals, begin to actively develop, or to vegetate, or 
when the sap flows readily throughout the day. For this “ effective 
temperature ” he adopts 5° C. 
Subsequently Gasparin adopted a thermometer placed in full sun- 
shine on the sod as giving a temperature more appropriate to plant 
studies, but still retaining the lower limit of 5° C. for the mean daily 
temperature of the initial date. Thus he obtained for wheat a sum 
total of 2,450° C. as the sum of the effective daily temperatures from 
sowing to maturity. 
Gasparin also observed the temperature of a blackened metallic 
disk in the sunshine and the temperature of the sunny side of a ver- 
tical wall, and again the temperature of a thermometer at the surface 
of a sandy, horizontal soil, all in full sunshine. He recognized that 
the loss of heat by evaporation must keep the temperature of the soil 
slightly lower than that of the surface of the wall; but, in default of 
better methods, he kept a record of the temperature of the wall for 
many years. From his average results I give the following abstract: 
Observations by Gasparin at 2 p.m. daily. 
| January. August. 
Locality. | Year. ms ; = = 
| Air. | Wall. | Air. | Wall. 
Oranges: s2s 286 2-2 ae Pee eee ee 1836-1850 | 6.7 15.4 30. 2 44.1 
Parist.ce2 2220. po sic ol ee ee Se eee ee 1838-1850 | 4.0 6.3 23.6 30.2 
Peissenberc: (Mbanich)) oases eee a eee 1786 | —1.3 11.0 14.6 22.0 
The warmth in the sunshine is to the warmth of the air in the shade 
as though one had been transported in latitude from 3 to 6 degrees 
farther south. 
oe 
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