The quantity of heat required to complete some stage, or the whole, of a plant's growth. It was first investigated by Réaumur about 1735, using the sum of the mean daily temperatures. It may also be measured by the sum of air temperatures above some standard such as 42°F (see degree-day). In modern agricultural climatology this concept has been largely abandoned in favor of complex influence factors such as evapotranspiration. It has also been established that soil temperatures at or somewhat below 10 cm are more important for plant growth than air temperature.