product of the number of hours the plant stood above the temperature at which growth began and of the averaged intensities during this period. The method was obviously empirical, as it assumed that the rate of growth was the same at all temperatures above its zero point, which might be freezing or above it.
Fig. 6. Graph Showing Rate of Growth of Seedlings of Wheat at Temperatures Between 40° and 108° F., plotted from data given in text-books of plant physiology.
Next, Professor B. E. Livingston, using the exponential law of chemical velocity in the interpretation of temperature effects, found that survival and distribution of some types of vegetation were explainable upon the temperature integrations arrived at in this manner. This method, however, still depends upon averages or summations of temperature and does not evaluate the higher temperatures correctly as the
