170 



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, alrea'dy mentioned as adopt'ed 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 Marie-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 dirt'erence 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 wdiich 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 : 



The warmth in the sunshine is to the warmth of the air in the shade 

 as though one had been transported in latij^ude from 3 to 6 degrees 

 farther south. 



