17i NINETEENTH REPORT. 



measuring the elongation per unit time, using that at tO'^ F. as unity. 

 Livingston has this to say in regard to this method : 



"While it is quite apparent th;it the system of physiological indices here 

 described is far superior, in several resjiects, to the other systems heretofore sug- 

 gested, it is equally clear that these indices are to be regarded as only a first 

 approximation and that much more physiological study will be required before 

 thev may be taken as generally applicable. In the first place, they are based ujjon 

 tests of only a tihn/Ie plant species, maize, and there are probably other plants, 

 (perhaps even other varieties of the same species), for which they are not even 

 approximately true. Second, these indices are derived from the growth of 

 se-ecllhujx, and no doubt other phases of growth in the same plant may exhibit 

 other relations between temperature and shoot elongation. Third, these indice's 

 refer to rates of xhoot elongation, and there are many other i)rocesses involved in 

 plant growth, \vhich may require other indices for their jiroper interpretation in 

 terms of temperature efficiency. Eourth, they apply strictly only under the 

 moisture, I'ujht and chemical conditions that jirevailed in Lehenbauer's experi- 

 ments; with more ligiit or with different moisture or chemical surroundings about 

 the routs, these same i)lants, in tlie same seedling stage may exhibit very different 

 values of the temperature efficiency indices. Eifth, and finally, plants in nature 

 are never subject to any temperature maintained for any considerable time, and 

 these indices are derived from lx?-hour exposures to maintained temperatures. As 

 McDougaT" has well emphasized, the indices really needed for the ecological and 

 physiological interpretation of temperature must take account of the \arying 

 temperatures that are almost always encountered in nature. In the face of these 

 considerations it would be rash indeed to suppose that the index series here 

 brought forward may be found to ap])ly in a wholly satisfactory way to all ]ilants 

 or even plants in general." 



In order to test out Livingston's index system, to determine wliat im- 

 provement, if any, is gained over the summation process, I have applied 

 his indices of temperature to the daily mean and maximum temperature 

 readings at Wauseon, Ohio, during the periods in the growth of maize, 

 from the time of the appearance above ground to the time of blossoming, 

 in the years 1893 and 1897. These were the years in wliicli the greatest 

 and least values, respectively, were found by Smith by the summation 

 process. Livingston's indices were also applied to the daily mean and 

 maximum temperature readings during the periods from blossoming to 

 ripening in the years 1906 and 1907, when the highest and lowest values, 

 respectively, were found to have obtained by the summation method. It 

 was believed that if the system of indices brought these extreme thermal 

 values nearer together then it would be an indication that the method 

 was an improvement over the summation system. The results obtained 

 were as follows : 



