TONIC INFLUENCE OP LIGHT 491 



i?idered. Plants are affected by variations in temperature in 

 ways very similar to those depending on changes in light. 

 In this case again we may discriminate between a tonic and 

 a stimulating influence, though both these afford evidence 

 of the ability of the plant to receive impressions from its 

 environment, and to respond to them in various ways. It is not, 

 however, always easy to ascertain the effects due to changes in 

 temperature alone, as usually other conditions, such as light and 

 moisture, vary at the same time as the temperature changes. 



As we have seen, the environment of the plant is partly the 

 soil and partly the atmosphere, and the temperature of both may 

 or may not vary simultaneously. The tonic effects of differ- 

 ences in the aerial temperature have already been alluded 

 to more than once in discussing the metabolic processes. 

 There is for each of these a particular temperature at which 

 it progresses to the greatest advantage. At lower and at 

 higher points the protoplasm is less active, and in each case 

 there is a point below which activity ceases, and one above 

 which also it does not go on. The same thing is true of the 

 processes of growth. We call these points respectively the 

 optimv/m, the mvnimum, and the maximum temperatures for 

 each function. They vary for different functions, and they 

 vary also for the same function in different plants. We have 

 said that this may be called a tonic influence ; the mani- 

 festation of it is seen constantly, and it modifies not only the 

 functions under consideration, but also the general power of the 

 plant to appreciate all the special stimulations that it from 

 time to time receives. We cannot say definitely how it affects 

 the protoplasm, but the constant round of activity, chemical and 

 physical, which constitutes its life, goes on for some reason 

 best at the optimum temperature, and is altogether suspended 

 at the minimum one. At the maximum temperature the death 

 of the protoplasm generally ensues. The average points of 

 minimum, optimum, and maximum temperature in the case of 

 metabohsm are about 10°, 30°, and 50° C, though a good deal of 

 difference can be seen in different plants. The points observable 

 for growth are slightly lower than those noticed for metabolism. 



Variation in the temperature of the soil has a considerable 

 influence on the absorption of water by the root-hairs, and in- 

 directly on the process of transpiration. It has been found 

 experimentally that warming the soil is attended by an increase 

 in the activity of both these functions. As the absorption of 

 water by the root -hairs is regulated by the protoplasm they 



