HEAT 



111 



perature zero represents only one of its 

 many components (see below). In studies 

 related to life histories and behavior, the 

 first temperature just given is often referred 

 to as the "threshold of development" and 

 the second approaches the "developmental 

 zero." Actually, some development fre- 

 quently takes place at temperatures that 

 will not allow the successful completion of 

 a given stage or process; hence the ecologi- 

 cal zero represents a somewhat higher 

 temperature than the developmental zero 

 (Parker, 1930; Belehrddek, 1935; Powsner, 

 1935). The biological zero of Belehradek 

 denotes "the temperature at which a given 

 protoplasmic action is arrested by cold 

 without formation of ice." Its precise re- 

 lation to the ecological zero has not been 

 determined; presumably it approximates the 

 developmental zero. Belehradek (pp. 139- 

 145) bsts the biological zero for diverse 

 processes in different organisms. These 

 "zeros" and "thresholds" may be affected by 

 time of exposure as well as by temperature 

 and such physiological factors as age, pre- 

 vious conditions, and temperature adapta 

 tion. 



The location of the ecological tempera- 

 ture zero for a given process or stage in 

 development can be determined only by 

 experimentation. Its position can be approx- 

 imated rather closely at times by plotting 

 the point at which the straight-line recipro- 

 cal of the temperature hyperbola crosses 

 the temperature axis (see Fig. 17). This is 

 the so-called alpha point for the curve; it 

 has theoretical rather than ecological signifi- 

 cance. Usually the true ecological zero lies 

 at some lower temperature than is indicated 

 by this intercept, a fact that has aheady 

 been considered in connection with the 

 correction of the simple equation for the 

 hyperbolic temperature curve (see p. 107). 



The effects of exposing organisms to 

 temperatures between the ecological zero 

 and the complete stoppage indicated by 

 the developmental zero and then returning 

 them to higher temperatures are not uni- 

 form. In some instances, acceleration up to 

 80 per cent occurs (Parker, 1930; for 

 grasshopper eggs). In other cases there is 

 retardation (cf. Ludwig, 1928; Powsner, 

 1935). This discrepancy may be interpreted 

 as follows: Some processes always continue 

 at temperatures at which the organism is 

 not killed by cold. The effect of low tem- 

 perature is probably differential, and some 



processes are slowed down more than 

 others. If not pronounced, this may weL! 

 have an accelerating influence when the 

 organisms are placed in medial tempera- 

 tures. The "disorganization" that results 

 from chilUng increases with time; if the 

 temperatures again become favorable, a lag 

 period follows before metabolic processes 

 become sufficiently well correlated to pro- 

 ceed normally. If the "disorganization" has 

 gone too far (and this depends on the 

 length of exposure as well as on low tem- 

 perature), the process cannot be completed 

 or completion will follow only at a retarded 

 rate. 



SUMMATION OF HEAT 



The useful ecological practice of sum- 

 mation of temperature represents in reaUty 

 an attempt to find an index for a summa- 

 tion of the heat energy required to complete 

 a given stage in the life history of an ani- 

 mal or plant. As such it has a theoretical 

 basis in the "law of constant heat summa- 

 tion" of thermochemistry. This generahza- 

 tion states that the quantity of heat 

 involved in a chemical process is the same 

 whether it takes place in one or in several 

 steps (Getman and Daniels, 1931). Actu- 

 ally, summation of the capacity aspect of 

 heat (see p. 91) has not been practiced 

 by ecologists in connection with life his- 

 tories; they have used temperatures instead. 



Modern ecological summation of tem- 

 perature developed from the extended 

 experience of the phenologists that the ac- 

 cumulation of a given daily excess of tem- 

 perature above some convenient base will 

 approximately coincide with the completion 

 of a given stage in development. This 

 amount is usually found by summing so- 

 called day-degrees. In present usage, a day- 

 degree represents 1 degree of mean temper- 

 ature above the ecological zero lasting for 

 one day. The needed accumulation to a 

 selected end point is called the thermal 

 constant for that set of processes. In more 

 refined usage, especially for shorter fife his- 

 tories, hour-degrees are used for summa- 

 tion, and attention may be turned from 

 such environmental units to reciprocal 

 values called developmental units. A de- 

 velopmental unit is defined as the amount 

 of organismic development produced in a 

 given amount of time, frequently one hour, 

 by an increase of 1 degree of medial tem- 

 perature. Developmental units are obtained 



