224 



THE RATE OF GROWTH 



[CH. 



In Fig. 64 are shewn the mean rates of growth (based on about a hundred 

 experiments) at some thirty-four different temperatures between 0-8° and 

 29-3°, each experiment lasting rather less than twenty-four hours. Working 

 out the mean temperature coefficient for a great many combinations of these 

 values, I obtain a value of 1-092 per C.°, or 2-41 for an interval of 10°, and 

 a mean value for the whole series shewing a rate of growth of just about 

 1 mm. per hour at a temperature of 20°. My curve in Fig. 64 is drawn from 

 these determinations; and it will be seen that, while it is by no means exact 

 at the lower temperatures, and will fail us altogether at very high tem- 

 peratures, yet it serves as a satisfactory guide to the relations between rate 

 and temperature within the ordinary limits of healthy growth. Miss Leitch 



18 20 22 24 26 28 30 32 34°C 

 Fig. 63. Relation of rate of growth to temperature in maize. Observed 

 values (after Koppen), and calculated curve. 



holds that the curve is not a Van't Hoff curve ; and this, in strict accuracy, 

 we need not dispute. But the phenomenon seems to me to be one into which 

 the Van't Hoff ratio enters largely, though doubtless combine'd with other 

 factors which we cannot determine or eliminate. 



While the above results conform fairly well to the law of the 

 temperature-coefficient, it is evident that the imbibition of water 

 plays so large a part in the process of elongation of the root or 

 stem that the phenomenon is as much or more a physical than a 

 chemical one: and on this account, as Blackman has remarked, the 

 data commonly given for the rate of growth in plants are apt to 

 be irregular, and sometimes misleading*. We have abundant 



* F. F. Blackman, Presidential Address in Botany, Brit. Assoc. Dublin, 1908. 



