MICHIGAN ACADKiVn- OK SCIENCK. 17S 



plant growtli, is broui;liL out in l''i<iiirc S, wliicli shows the rate of growth 

 in plants in connection with both the maximum and mean temperatures 

 from May 23 to June 30, 1916, at East Lansing, Mich., which will be 

 discussed further later on. 



If the summation methods are to be continued, therefore, I would 

 recommend that maximum temperature readings be considered rather than 

 tlie means. But neither gives satisfactory results. 



As a modification of the summation method of studying the heat 

 requirements of plants, Lehenbauer", Livingston' and others have used 

 van't Hofi's law in regard to the acceleration of chemical action with 

 increase of lieat in evaluating temperature readings. They reasoned that 

 as plant growth is largely chemical in nature, this activity should in- 

 crease and double with each rise of about 18° F. in temperature, as it 

 does in purely chemical reactions. The formula used is simply 

 u=2 *^g-, where ii is the value to be derived, and t is the temperature 

 on tlie Fahrenheit scale. The value derived is therefore the exponential 

 function of the temperature itself, and the method has been termed the 

 exjDonential method. A temperature of 60° F. would have a value of 2, 

 78° F., 1, etc. 



While tliis method is an improvement over the summation method, in 

 that it gives increasingly greater weight to liigher temperature readings, 

 and therefore probably represents more truly their effectiveness in pro- 

 moting plant growth, up to a certain limit, yet it does not stop at this 

 optimum for tlie plant and gives too great values, by far, for very high 

 temperatures. Tehenbauer found, in his measurements of the rate of 

 growth of corn seedlings as influenced by temperature, that van't Hoff's 

 rule applied only to medium temperatures. 



This method also fails then, for the same reason, as Zon*^ has pointed 

 out in the case of the summation process, because it does not take account 

 of the fact that there is an optimum temperature for growth in each 

 plant, beyond which the growth rate decreases with further increase in 

 temperature. 



Realizing this fact, Livingston" has worked out a series of indices of 

 temperature efficiency for plant growth, based on Lehenbauer's measure- 

 ments of growth of maize seedlings as influenced by temperature. These 

 he terms physiological indices because they are actually based on 

 physiological processes. He has determined a value for each tempera- 

 ture reading on the Fahrenheit and Centigrade scales. In the former 

 he starts with unity at 1'0° F., increasing to a maximum value of 122.3 

 at 89° F., and then rapidly decreasing to unity again at 116° F. These 

 values were determined directlv from the Lelienbauer jj-rowth curve bv 



