15i INUTILITY OF MEAN AIK TEMPERATURES. 



means of temperature, as furnishing the elements required to 

 determine whether a given plant could be adTantageously 

 cultivated in a given country. But these means were aU more 

 or less fallacious, and might have led to serious mistakes. 

 Mean temperatures are useless to cultivators unless they repre- 

 sent what takes place during the period of vegetation. We do 

 not want to know what the temperature is of seasons when, or 

 of places where, plants do not grow, unless for the purpose of 

 determining the amount of winter protection which they may 

 require; and all indications of climate in which the dormant 

 season is mixed with the growing season only mislead. Suppose, 

 for example, it was to be said that the mean annual tempera- 

 tures of Longville and Bretville are the same (say 35°), this 

 would be no proof of similarity of climate, for Longville might 

 have the winter mean 30°, the summer mean 50°; while 

 Bretville might have the winter mean 30°, and the summer 

 mean 40° — cold winters and temperate summers characterisiag 

 one place, mild winters and bad summers characterising the 

 other. Nor are daily means much more useful. Let us suppose 

 that Longville has in June a daily mean of 45°, while that of 

 BretviUe is 50° ; it might be that these means represented hot 

 days and cold nights in the one case, and cool days and 

 mUd nights in the other — conditions which for the purposes of 

 cultivation are whoUy different. That M. Boussiugault's 

 method of explaining the relation between plants and climate 

 was an important improvement upon the usual indications is 

 not to be denied. But it was not wholly satisfactory. Pushed 

 to its limits the theory was manifestly untenable, for it amounted 

 to this — that if a plant requires 30 days with 10° of heat in each 

 day, or 300° to do a certain thing, and if it can do the same 

 thing in 10 days, with 80° of heat in each day, then it ought to 

 accomplish the same end in one day by the aid of 800° of heat, 

 which is absurd. 



In all considerations relating to ground temperature, the 

 gardener should inform himself more especially upon three 

 points : — 1, the temperature of the soil when plants are at rest; 

 8, that when they are in vigorous growth ; and 3, that when 

 they are ripening their fruit. The first points out the bottom 



