370 GROWTH 



on the rate of growth. The little Jap increases in size year by 

 year at the same rate as the tall Norwegian. The rate of grozvth 

 is a specific phenomenon governed by factors deep rooted in the 

 composition of the organism. 



2. External Factors. 



Quite apart from these more or less normal variations due 

 mainly to hereditary influences, there are various external factors 

 which have a modifying effect on the rate and amount of growth. 



(d) Temperature. As we have seen previously, all chemical 

 and -physical reactions respond to alterations in temperature by 

 an alteration in velocity. In the terminology of van't Hoff, it 

 may be said that if x is the temperature coefficient of a reaction, 

 and the temperature of the reacting mass is raised n degrees, the 

 consequent alteration in velocity will be as x n . Usually the 

 interval taken (i.e. n) is 10 C. and x is written as Q 10 . For most 

 chemical reactions Q 10 is -2. This may be taken to mean that 

 for an increase of temperature of 10 C. the velocity of the re- 

 action will be doubled. Van't Hoff noticed that, at low tempera- 

 tures, very high temperature coefficients were obtained in 

 some cases Q 10 reached the value of 5 or 6. Most physical re- 

 actions, as we have seen, differ from most chemical reactions in 

 having a negative temperature coefficient, i.e. their rate is decreased 

 by an increase of temperature. The various reactions which are 

 manifested as growth are some chemical and some physical, and 

 it is, therefore, somewhat difficult to apply the van't Hoff law to 

 this phenomenon. Moreover, as pointed out in the earlier pages, 

 Errera extended the principle of Le Chatelier by stating that- 

 every physiological process causing change, by its very action, set 

 in being other reactions to inhibit change. It is, therefore, a difficult 

 matter to interpret the figures obtained for the influence of 

 temperature on the velocity of growth in animals. 



Hertwig's classical work on the rate of growth of the tadpole 

 illustrates the type of result got in this line of research. He found, 

 for instance, that at 10 C. the tadpole took 6-5 days to reach 

 the same stage of development that at 20 would have taken two 

 days, i.e. the two rates are as 6-5/2 = 3-25. Using the equation 

 given above and putting n = 10, 



x 10 =3-25, 

 i.e. 10 log x = log 3-25 = 0-5119. 



Therefore log x = 0-05119, 



and x = 1-125. 



