The'heat unit approach," in use for over two centuries, is a scheme for studying plant-temperature re-lationships by the accumulation of daily mean temp) era-tures above a certain threshold temperature during the growing season. If a plant has a base temperature of 40F1 anl the mean temperature oil a given day is 551?, the difference in degrees for that day is termed" degreedays" or'heart units."'Inl this example, wve would have 15 degree-days. If 120 () degree-days are reciuired for maturity, the plant should reach maturity by the time 1200 degree-(lays have been accumulated. this sum re-quire (l for a particular crop variety has been assumed by heat unit workers to be a constant value and is termed the varietal constant. By knowing the varietal constant, users llredict the date on which their crops should be harvested.'T'his linear relationship, known as the''remaiinder index method," gave rise to a number of expressions, sLuch as" degree-days,"" heat units," groovingg de-gre-davs,"'growthwh Units," and others relating plant rcs) oiises to season thermal levels. l'Uiv w or L ITERATUr uE Silence 1mutich of the valuable vork done ini the past 230 years has beeni neglected by many modern users, a worldwide survey of major contributions to the heat Unit ap-proach is in order.

Around 1730), Rena6 AF (le Reaumnur invented a temllperature scale in which () marks the freezing point and 80'the boiling point of water. Although his temperature scale, known as the Rlaumtir scale, is now almost ob.)-solete, except amiionig SOIIme cheese-makers, his ideas onl the quantitative study of planit-temiperature relationships are worthy of mention. Lie summe (I up the mean (laily air teml) eratures for 91 (lays (luring the months of April, II ay and June in hlis locality and found the sum to be a n lcarly constant value for the (leveloilment of anyv plant from v ear to year.'i'his summation of temperatures, plll) iis. ed (l in 1715, was later known as Rvatumiir's thermal conlstanlt (if phenology. Hte assumed that his tbcrmio-metric constant exl) resse (l thle amount of heat required for a llaialt to reach a given stage of maturity.'T'llis i (lea gave rise to tlhe hreat unit system of today. otherss have adopted Reaumtir's idea as a I) lasis for their own n (lices..\danson i17550) borrowed it but dlisregar (llel all temperatures b) elo\v () C and took only thle skills of positive telmlperatures onl thle Centigrade scale. This res' Llted ill the estal) lishluent of a" remainder index netllol.'Ill collsi (lering sunlight as an imliortalnt physio-