8oo Journal of Agricultural Research voi.xx.No.xi 



influence on the rate of ripening. But these ordinary temperature 

 readings do not furnish a basis for a quantitative comparison of the tem- 

 perature efficiency in reference to these processes. 



Various methods have been proposed for interpreting the observed 

 climatic temperatures in different localities and for different seasons in 

 the same locality, with reference to plant growth. Three of these 

 methods were applied to the fairly definite set of physico-chemical 

 processes involved in the ripening of sweetcorn. The first method 

 employed was one of direct temperature summation, similar to that 

 described by MacDougal. 1 



The integration was performed, with a planimeter, upon thermograph 

 records. The area between the 40 F. line and the pen tracing for each 

 day of the two ripening periods was first measured. Then the mean 

 temperature for each hour of a chosen day was computed from a thermo- 

 graph record, and 40 was subtracted from each hourly temperature. 2 

 The sum of these results divided by the planimeter reading for the same 

 day gave a factor by which the planimeter reading for any 24-hour 

 period could be converted into hour-degree units of effective temperature. 

 The total number of hour-degree units was computed for the 6- and 15-day 

 ripening periods of the early and late crops, respectively. These units 

 express both the intensity and duration aspects of the temperature 

 factor. The adoption of the 40 as the starting point for the temperature 

 summations was based upon the facts that carbohydrate changes are 

 chiefly involved in ripening and that carbohydrate transformations in 

 green corn during storage are extremely slow below this temperature. 



The results of the direct temperature summations given in Table III 

 show a slightly greater total number of hour-degree units of effective 

 temperature in favor of the late crop. Stevens and Higgins 3 have 

 shown that the temperature of green corn on the stalk in the shade is 

 nearly that of the air, while in the sun it is often above that of the air. 

 The period of ripening for the early crop here considered was character- 

 ized by high temperature and clear days, while the ripening period of 

 the late crop contained 2.5 times as many days, many of which were 

 cloudy. Since the temperature records from which the units of effective 

 temperature were computed were taken in an instrument shelter, the 

 sum of the hour-degree units for the early crop is probably a little less 

 than actually required. 



Livingston and Livingston, 4 realizing the need of some fundamental 

 principle of physiology upon which to base the value of temperature 



1 MacDougal, D. T. the temperature of the soil. In Jour. N. Y. Bot. Garden, v. 3, no. 31, p. 125- 

 131, fig. 19-21. 1902. 



2 The thermograph records were furnished by Dr. Earl S. Johnston of the Laboratory of Plant Pathology, 

 Maryland Agricultural Experiment Station. 



3 Stevens, Neil E., and Higgins, C H. temperature in relation to quality of sweetcorn. In 

 Jour. Agr. Research, v. 17, no. 6, p. 275-284, 1 fig. 1919. Literature cited, p. 283-284. 



4 Livingston, Burton Edward, and Livingston, Grace Johnson, temperature coefficients in 

 plant geography and climatology. In Bot. Gaz., v. 56, no. s, p. 349-375, 3 fig. 1913. 



