208 ENVIRONMENTAL CONDITIONS. 



indicated as a temperature scale on the thermometer-tube, and these 

 volumetric graduations may be of any convenient magnitude. Thus, 

 centigrade, Fahrenheit, etc., degrees are the respective increments in 

 the volume of the thermometer liquid corresponding to equal incre- 

 ments in temperature rise. 



The term ''temperature" often means temperature reading (on some 

 thermometer scale), and it is also used in a self-evident adjectival 

 sense, meaning pertaining to temperature. In order to apply construc- 

 tive reasoning to temperature conditions it is necessary to be somewhat 

 more explicit than is usually the case, so that the terms we shall use 

 require definition. 



Temperature readings or measurements are simply numbers that 

 represent comparative temperatures, and they may therefore be con- 

 sidered as conventional indices of temperature. The numbers of the 

 Fahrenheit and centigrade scales are merely these indices expressed in 

 two different kinds of units. Thus, the normal daily means of Bulletin 

 R of the United States Weather Bureau are to be regarded as averages 

 of a number of means for the given date, these means themselves being 

 averages of a series of temperature indices representing the different 

 temperatures encountered in the air throughout the day. According 

 to this terminology, all climatological studies of temperature conditions 

 thus start with temperature indices. These indices tell nothing about 

 the possible effects of the temperatures represented, as these may 

 accelerate or retard any process; they have reference only to the state 

 of molecular motion of the particular body whose temperature is 

 considered. 



For the problems before us, as has been mentioned, the various 

 degrees of temperature effectiveness upon plant activities must be 

 measured and compared, rather than temperatures themselves, and 

 it thus follows that we require indices of temperature efficiency. These 

 are to be derived from the indices of temperature, with due regard to 

 the nature of the process to be studied, and various attempts have been 

 made to obtain from temperature indices these other indices that are 

 to be measures of the plant-producing power, etc., of given atmos- 

 pheric temperatures. We shall deal below with the various methods 

 that have been tried. The relative applicability of these methods is 

 to be determined only empirically, but certain a priori considerations 

 need to enter into the critical discussion of this relatively new and ver^' 

 important subject. According to the way in which indices of tempera- 

 ture efficiency are derived from temperature indices, we may have four 

 distinct classes of the former, which will now be taken up separately. 



(l) Direct Indices of Temperature Efficiency for Plant Growth. 



Direct efficiency indices are obtained directly from the corresponding 

 temperature indices, the numerical values being the same in both cases 



