154 Temperature 



and in most other biological processes is not linear, and sometimes 

 varies in a complicated way. For the cod egg a rise of 4° from 

 — 1°C to + 3°C produces a doubling in the rate of development, but 

 a similar increase of 4° from 10°C to 14°C results in only a slight 

 acceleration. A temperature of 10°C, which is the minimum at which 

 the mackerel egg will develop, is near the maximum temperature for 

 the cod egg. As the temperature is raised from 10°C to 21°C the rate 

 of development of the mackerel egg is increased until at the upper 

 value it will hatch in just over 2 days. 



An increase in temperature beyond a critical point may cause a 

 reduction in the rate of some life processes in certain species. The 

 growth of the hypocotyl of the pea seedling shown in Table 11 illus- 



TABLE 11 



Relation between Temperature and the Growth of the Pea Seedling 

 As Indicated by the Elongation of the Hypocotyl 



Temperature Growth 

 °C Mm per Day 



14.1 5 



18.0 8 



23.5 30 



"16 . 6 54 



28 . 5 40 



33 . 5 23 



36.5 9 



trates this point. As temperature increases from 14.1°C to 26.6°C 

 the amount of growth recorded each day steadily increases, but at 

 higher values the daily elongation of the hypocotyl becomes less. In 

 contrast to the growth in fish eggs, the pea seedling continues to 

 elongate after the temperature has been reached at which growth is 

 the fastest. This fact does not mean that increase in temperature has 

 a negative effect on a reaction. Growth, like most other biological 

 processes, is the result of a number of biochemical reactions going on 

 simultaneously. In the present instance the rate of some inhibiting 

 reaction may have been increased more by a rise in temperature than 

 the reaction for elongation with the result that after a certain point 

 the rate of growth becomes less. 



In other instances temperature may have little or no effect on the 

 rate of biological processes. Between certain limits the incubation 

 period for many reptiles is only slightly influenced by thermal changes 

 in the surroundings. The diamond-backed terrapin, for example, 

 undergoes normal hatching between 18°C and 33°C, but within that 



