70 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



B. COMPARATIVE INSTRUMENTATION AND METHODS OF SUMMARIZING EXPRES- 



SIVE CLIMATIC FACTORS. 



1. Temperature summations on a physiological basis according to 

 the Lehenbauer plan have shown much promise in correlating air 

 temperature with physiological plant activities. The summation of 

 the effective temperature, namely, the temperature above 40° F., as 

 proposed by Merriam, also appears to have much promise. This 

 method in general compares favorably with temperature summations 

 made on the physiological (Lehenbauer) basis. 



2. Summations of average daily mean and seasonal mean tempera- 

 tures appear to have little value in showing correlations between 

 the factor in question and physiological activities in plants. 



3. The evaporation for short periods, such as a part of a day or a 

 fractional part of a week, for example, when compared with relative 

 humidity, temperature, and wind velocity, can be obtained more 

 accurately by means of the porous cup atmometer than by the free 

 water surface evaporimeter of the Weather Bureau pattern. For 

 periods of a week or longer either instrument will serve. 



4. In recording sunshine as related to plant activities, both dura- 

 tion and intensity should be considered. Such records can be ob- 

 tained approximately by noting the difference in evaporation between 

 similarly exposed black and white porous cup atmometers. These 

 instruments appear to have some advantages over the Marvin sun- 

 shine recorder, which furnishes a record only of sunshine duration. 



C. CORRELATIONS BETWEEN ENVIRONMENTAL FACTORS AND PLANT GROWTH. 



1. Lack of uniformity in the fertility and, of course, in the tex- 

 ture of the soil in which the plants are grown may cause consider- 

 able variation in their water requirements and in the total dry mat- 

 ter produced. Soil collected within a restricted habitat often varies 

 considerably in productivity, and unless thoroughly mixed may 

 become an important source of error in experimentation. 



2. The total effective heat units and length of growing season in 

 the three types studied are such that only in the lowest association 

 do crops like wheat and peas reach full maturity. Hence, farmers 

 should not attempt locally to grow the ordinary agricultural crops, 

 such as cereals, above an elevation of about 8,000 feet. The eleva- 

 tion at which there are normally sufficient heat units to develop and 

 mature cereal crops in general varies, of course, with the latitude 

 and longitude. 



3. The rate of maturity of the plants decreases directly as the 

 effective heat units decrease, as is the case in passing from the lowest 

 to the highest type. This decrease in the rate of maturity of the 

 plant in the type stations may be shown, up to a certain point, at 



