222 THE PLANT WORLD. 



Boussingault attempted to derive the thermal constant of a 

 vegetative period, or any part of it, by multiplying the mean tem- 

 perature of the air by the duration in days. 



Gasparin calculated thermal constants from temperatures 

 obtained from insolation thermometers exposed to direct sun- 

 light while lying on the sod, which would record 20 rk-grees C. 

 to 30 degrees C. higher than shade temperatures, aii<l showing 

 a diiference equivalent to three to six degrees latitude'. By this 

 method, the thermal constant required for the germination and 

 maturity of the seed of wheat amounted to 2450 degrees C. 



Variations in the method of calculation of the thermal con- 

 stant have been made l^y various investigators, in vdiich this 

 standard has been obtained by multiplying the mean tempera- 

 ture by the square of the number of days involved, others miilti- 

 plying of days by the square of the mean daily temperatures. 

 Some begin this calculation of the heat exposure with the appear- 

 ance of the earliest species to show sign of awakening activity. 



As an application of the principle of the thermal constant 

 many bio-geographers have attempted to explain distribution by 

 the mean annual temperature to the regions concerned. Among 

 the most notable of such works is to be mentioned that of Hoff- 

 man of Giessen, South Germany, who used the sum of tlie inso- 

 lation temperatures from the 1st of January in calculating the 

 thermal constants, and it is his data which are quoted so freely 

 in all general treatises on plant geography and the thermal rela- 

 tions of vegetation. Drude uses the mean annual temperatures 

 in his treatment of the subject, in Avhich he is followed by Pound 

 and Clement in their Phytogeography of l^ebraska. 



It need scarce be said that the data accumulated by the 

 various methods described durino- the last centurv and a half is 

 confusing on account of the highly empirical charactei' of the 

 principles upon which each separate investigation has been 

 based, the diiferent standards of thermometry, and the utter lack 

 of uniformity of technique. The last defect alone is sufficient 

 to invalidate most of the results, which are nearly all valueless 



