SEASONAL ACTIVITIES OF PLANTS. 223 



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 effort to outline some method for the calibration of 

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

 Mangon seems to offer the most valuable suggestions. Mangon 

 computes all shade temperatures from the time of germination 

 of seeds of the plants produced were mature, disregarding all 

 measurements in which the mean daily temperatures is less than 

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

 mean daily temperatures necessary for the ripening of wheat in 

 l^ormandy in 1870-1879 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- 

 ity due to the faulty method of calculating such exposures. The 

 performances of an engine are not to be calculated by the total 

 averages of the steam pressure during its working days, but may 

 be quite exactly determined by multiplying the pressure by the 

 number of hours during which pressure was kept up and used. 



A similar relation holds with regard to the use and effect 

 of radiant energy in the plant, and although any method of es- 

 timation must be more or less arbitrary, yet it seems possible to 

 select one which will be capable of wide application and corre- 

 sponding value. In the evolution of such a method for plants 

 in the temperate zone it seems less artificial to begin the calcu- 

 lation of the heat exposure w^ith the winter solstice instead of 

 January 1st, and as has been done by several writers, or if eco- 

 nomic plants are under consideration, take the date of planting 

 as a starting point. It also seems most convenient to use the 

 temperature of the freezing point of water as a base line for the 

 thermometry of the plant. 



