IRRITABILITY. 377 



The first point is this : that in estimating the effect of 

 a rise of temperature upon the rate of growth, it must be 

 borne in mind that the accelerating effect is to be calculated, 

 not with reference to the zero-point of the thermometer, but 

 from the ascertained zero-point or minimum temperature for 

 growth of the plant. Secondly, the acceleration of growth 

 due to a rise of temperature between the minimum and the 

 optimum is not proportional to the number of degrees, but is 

 greater for each degree as the temperature approaches the 

 optimum. This is illustrated by the figures given in a previous 

 lecture (p. 293). 



Temperature influences growth in this way, that for any 

 given degree of temperature, between the minimum and the 

 maximum, there is a corresponding rate of growth. But in 

 producing this effect, temperature does not act as a stimulus : 

 temperature, as such, exercises not a stimulating influence, 

 but a tonic effect, which is due to the fact that the manifesta- 

 tion of irritability is dependent upon temperature. Growth, 

 for instance, is more active at the optimum than at either the 

 minimum or the maximum temperature because at that 

 temperature the necessary evolution of energy is taking place 

 with sufficient activity (p. 295), and the protoplasm is pro- 

 bably in its most motile state. The arrest of growth at a 

 temperature below the minimum is probably to be ascribed 

 to an insufficient evolution of energy, whereas the arrest of 

 growth at a temperature above the maximum cannot be 

 ascribed to this cause, for at such a temperature the evolution 

 of energy, as estimated by the activity of destructive meta- 

 bolism (p. 295), is very considerable. The arrest of growth in 

 the latter case can only be accounted for by ascribing it to 

 an arrest of the motility of the protoplasm. The relation 

 between temperature and the manifestation of irritability 

 (thermotonus] will be frequently illustrated hereafter. 



Variations of temperature have, however, in some cases, a 

 stimulating effect. From his researches on the growth of 

 seedlings, Koppen came to the conclusion that frequent and 

 considerable variations of temperature cause a retardation of 

 growth, that is, that the growth in length of an organ in a given 



