706 STATE BOARD OF AGRICULTURE. 



been called, for various crops, in terms of air temperature during the 

 life phase of any crop, under the supi)Osition that a uniform quantity 

 of heat is necessary to produce a given increase in growth. But widely 

 different results are found from year to year when dail}^ air temperatures 

 are added together over a period during which the plant is passing 

 through a certain growtli stage. Souiewhat closer results are obtained 

 by subtracting a constant amount, usually 42, from each air temperature 

 considered, because 42° F. may be talcen as zero for plant growth for 

 ordinary farm crops. This process of adding together the remainders 

 after subtracting 42 from each daily temperature reading, has been 

 called the "summation method" of studying the relation between air 

 temperature and plant groAvth. As stated above it does not give con- 

 sistent results. 



Livingston,- Lerenbauer," and others have introduced Van't Hoff's law 

 into the problem, to the effect that chemical action, and hence plant 

 growth, which is largely chemical in nature, is accelerated and doubled 

 with each increase of 18° F. in temperature. This system seems to hold 

 good for medium temperatures, and gives closer results than the sum- 

 mation process, but it fails, as does the latter also, when higher tem- 

 peratures are concerned, because it takes no account of the fact that 

 the rate of growth decreases after a certain temperature is passed. 



To overcome this defect and to further perfect the system, Livingston* 

 has worked out a series of values or indices of growth corresponding 

 to each degree of temperature. These indices were determined from 

 Lerenbaurer's^ curve of growth in maize seedlings as controlled by tem- 

 perature. He took the rate of growth at 40° F. as unity and found 

 value for each succeeding temperature on this basis. The highest value 

 was 122 at 89° F., after Avhich the indices rapidly decreased to unity 

 again at 116° F. When this system is applied to air temperatures it 

 gives slightly better results for the earlier growth phases, but seems to 

 be no improvement over earlier methods for later periods of growth. 

 Seeley*' has pointed out that the temperature of the plant itself is widely 

 different from that of the air which surrounds it, especially when the sun 

 is shining, and suggests that plant temperatures be used instead of air 

 temperatures in studies of plant growth. He found that plant leaves 

 were, on the average, 15° F. warmer at midday, in clear weather, than 

 the air temperature, 10° F. warmer when the sun was partially obscured 

 in partly cloudy weather, and practically the same temperature on 

 cloudy days. These averages were obtained from over 300 observations 

 made during the growing season of 1915 and 191G. He proposes a 

 formula for evaluating air temperatures in terms of the true plant tem- 

 perature as follows : T = t —42 N + 15 C + 10 P, where ''t," is the 

 sum of the daily maximum temperatures during a given period, "W 

 the number of days in the period, "C" the number of clear days and 

 "P" the number of partly cloudy days. Applying this formula to the 

 average temperatures at a number of stations in Michigan, using the 

 normal number of clear, partly cloudy and cloudy days during the 

 growing season, in each section, gives a more accurate idea of the true 

 thermal conditions under which crops grow in this state. Chart XIV 

 shows the results obtained in this wa3^ It will be noted that the largest 

 value, 7183 is about 40 per cent greater than the least value, 5048, the 

 stations having these values being, respectively'', Adrian, in the extreme 



