MICHIGAN ACADEMY OF SCIENCE. 



175 



Table III. 

 Comparison of Indices and Summation System. 



While the Livingston system brought the thermal values for the tvi'o 

 years nearer together in the ease of the earlier phase of growth, from 

 time of appearance above ground to blossoming, there is even a greater 

 difference in the results obtained for the two years for the period 

 blossoming to ripening when mean temperatures are used, but a slight 

 improvement when maximum temperatures are used. It would seem, 

 therefore, that the indices derived by Livingston from the growth of 

 Lehenbauer's maize seedlings, apply more consistently during the time 

 the plant is passing through the earlier growth phases, but do not hold 

 good for the later blossoming to ripening stage. It may be, as Livingston 

 suggests, that a separate set of indices will be needed for each life phase. 

 The closer results obtained by considering the maximum temperature 

 readings rather than the means, is marked, amounting to eight per cent 

 in the early phase of growth and nine per cent in the later one. This 

 would seem to support my contention that maximum temperatures should 

 be used in these researches, rather than mean temperatures. 



It may safely be stated, then, that no way lias yet been suggested 

 for interpreting air temperature readings in terms of their efficiency 

 in promoting plant growth, which really satisfies the requirements when 

 put to the test. May it not be that sufficient attention has not been 

 given to the temperature of the plant itself, as it is freely exposed in 

 the air, under more or less sunshine, wind and moisture. The various 

 parts of the living plant have far different powers of absorbing insola- 

 tion than the air which surrounds them. The color and texture of the 

 surfaces of leaves and stems are such as to make them good absorbers 

 and radiators of heat. Many investigations have shown that leaf 

 temperatures are higher when the sun is shining on them than the air 

 which surrounds them. Ehler^^ has found that the temperature of pine 



