18 



the second of these threee methods has been employefc 3.n the 

 present paper. 



To bhow the reason for using the physiological tempe- 

 rature sumnaticn index in thip study, rather than the suiri- 

 mation of the remainder or exponatial indices, it will he 

 necessary to anticipate somev7hat the discussion tha"^ is to 

 follow, The three climatic conditions ( temperature, evapo- 

 ration and light) shov/ a definite seasonal mar?h for each of 

 the stations employed in the investigation. The temperature 

 r.i ses from low values in thp Rin-ing to a midsummer maximum 

 which is follov/ed hy a subsequent fall to low autumnal values. 

 Cn the other hand, the values representing both light and evap- 

 oration decrease, in general, throughout the season. Tf, now, 

 a generalized curve of plant growth be drawn, plotted against 

 the time of year, and employing average values to represent 

 all the stations together, sxich a curve follows the seasonal 

 march of the tempera +"ure and shows only secondary variations 

 due to the effect of the other climatic conditions. The growth 

 of the plants is thus determined mainly by temperature. 

 Obviously, also, the seasonal march of the temperature values 

 must show the same general form of curve no matter wha"^ scheme 

 is used in expressing teraperf-ture efficiency. In view of 

 these facts, and in considera-^ion of the general comparative 

 purpose of the present study a method should be used for ex- 

 pressing temperature efficiency, that gives a seasonal march of 

 the efficiency values in accord with the corresponding march 

 of plant growth. Of the three methods mentioned, the physiol- 

 ogical efficiency index fulfill'^ -^his requirement best, and 

 this has accordingly been selected for use throughout the entire 



