CHAPTER XXXIII 



ON THE RELATION BETWEEN TEMPERATURE AND GROWTH, 

 AND THE ACCURATE DETERMINATION OF OPTIMUM 

 AND MAXIMUM POINTS 



General consideration of difficulties of accurate determination of effects of 

 temperature on growth — Four accurate methods: (i) Method of discon- 

 tinuous observations — Accurate regulation of temperature by electrolytic 

 rheostat— (2) Method of continuous observations — Thermo-crescent curve — 

 Determination of the optimum point— (3) Method of balance — (4) Method of 

 excitatory response — Translocation of the optimum point. 



Determinations of the effect of temperature on growth are 

 usually carried out either by observing a single plant for 

 several hours in succession, or by obtaining the average 

 growth of various groups of plants, kept under different 

 temperatures. In the latter case the individual peculiarities 

 of different specimens cause them to give values which are 

 more or less discrepant, but this fact is to some extent 

 neutralised by taking the average of a large number. These 

 methods, however, are all very laborious, and the results not 

 highly consistent. Whichever method be adopted, we have 

 to remember that in all cases in which observations stretching 

 over periods of several hours are required, the growth of the 

 plant will be liable to spontaneous variations, resulting from 

 the periodicities impressed upon it by the changing conditions 

 of its environment. It is, perhaps, owing to this fact that the 

 results obtained by various authorities have been so widely 

 divergent. For example, the optimum temperature for 

 growth of Zea inais was found by Sachs to be 34° C. and by 

 Koppen to be 302° C. 



Clearly, the perfect method would be one in which we 

 should be able to measure the effects of temperature alone on 



