8 HORTICULTURAL SOCIETY OF NEW YORK. 



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 the inso- 

 lation temperatures from the ist 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 which he is followed by Pound 

 and Clement in their Phytogeography of Nebraska. 



It need scarce be said that the mass of data accumulated by 

 the various methods described during the last century and a half 

 is confusing on account of the highly empirical character of the 

 principles upon which each separate investigation has been 

 based, the different 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 

 so far as any application is concerned, in this connection. Con- 

 cerning the futility of research upon this subject it is most sig- 

 nificant that Warming and Schimper refuse to recognize the 

 thermal constant as a definable factor in the relations of plants 

 to climate. 



In the eft"ort to outline some method for the calibration of 

 heat exposure of plants growing in the open, the work of Plerve 

 Mangon seems to offer the most valuable suggestions. Mangon 

 computes all shade temperatures from the time of germination 

 of seeds until maturity of the plant was reached, disregarding all 

 measurements in which the mean daily temperatures is less than 

 6° C. (42° F.). By this method he found the sum of mean 

 daily temperatures necessary for the ripening of wheat in Nor- 

 mandy in 1 870- 1 879 to vary between 2219 and 2517, and with 

 the data of several seasons at hand it was possible to predict 

 the date of ripening of wheat within three or four days. 



The great variation shown by a plant with regard to the 

 heat exposure calculated by Mangon's method is in all probabil- 



