166 Temperature 



are indicated by heat sums. In the first place the heat sum for a 

 process is constant only for that range within which there is direct 

 proportionality between growth rate and temperature. Lack of such 

 proportionality is usually found near both the upper and the lower 

 limits of tolerance. Indeed, as mentioned earlier, above certain 

 "optimal" temperatures the rates of many metabolic processes ac- 

 tually decrease. Furthermore, since heat sums are based on average 

 daily temperatures, they do not take into account the possible special 

 effects of the maximum or minimum daily values, night temperatures, 

 or differences between diurnal and nocturnal temperatures. 



The critical importance of these more detailed aspects of tempera- 

 ture in controlling life processes is discussed in greater detail by 

 Daubenmire (1947) particularly in relation to land plants. Some 

 plants exhibit normal responses only when grown in temperatures 

 that fluctuate with a regular diurnal rhythm— a phenomenon known as 

 thermoperiodism. And to make matters more complicated, the pre- 

 cise action of temperature may be further modified by the intensity 

 of light and the length of day. Went (1950) points out that no heat 

 sum can account for the ripening of the tomato. In addition to 

 proper light intensity, length of day, and day temperature, the tomato 

 plant sets seed only if exposed to a series of five or more nights dur- 

 ing which the temperature remains above 15°C. These findings are 

 compatible with the general principle of minimum heat requirements, 

 but they show that in certain land plants at least the matter is far 

 more complicated than first supposed. 



Control of the geographical distribution of many animals can sim- 

 ilarly be related to the need for a minimum amount of heat. The 

 American oyster as an adult can survive at temperatures from 32°C 

 down to the freezing point of salt water. A temperature above 15°C, 

 or higher in some races, is necessary for spawning to take place in this 

 species, and good development of the larvae requires water above 

 18 or 20°C. The adult lobster can live at temperatures ranging from 

 about 17°C down to 0°C, but breeding will take place only in water 

 warmer than 11°C. The lower maximum temperature tolerated 

 means that the lobster cannot range to the south as far as the oyster 

 and the lower heat requirement for lobster reproduction produces a 

 marked difference in the breeding range of the two animals. For a 

 complete study of the effects of temperature on distribution the ther- 

 mal conditions in each bay and inlet should be investigated, but the 

 range of temperature along the coast is sufficient to illustrate the gen- 

 eral influence of this factor. 



A glance at Fig. 5.15 will show that north of Cape Cod coastal 



