372 VEGETABLE PHYSIOLOGY 



retards growth, and the tonic influence is associated with 

 this retardation. This is, however, a somewhat incomplete 

 presentation of the case. Eetardation of the growth is not 

 the only effect produced by the access of a proper degree 

 of illumination. It is rather to be regarded as regulatory 

 than retarding, and as it affects many other functions 

 than growth, it seems more appropriate to consider the 

 influence of the light as directed to the maintenance of this 

 " tone, which is really one of the conditions of health. How 

 the actual effect upon the protoplasm is produced we cannot 

 say ; it may be that the motility which is characteristic of 

 healthy protoplasm and its control of its own permeability 

 are adjusted to a particular relationship with the environ- 

 ment, of which phototonus is one condition. 



The rays of the spectrum which exert this influence on 

 the living substance appear to be those of high refrangibility, 

 the blue and the violet. To these rays the protoplasm seems 

 to be excessively sensitive. We do not explain their action 

 when we say that they bring about a variation in the turgidity 

 of the cells, or that they set up a change in the manner of the 

 nutrition. The facts which we have called attention to can 

 only be referred to the power of the protoplasm to respond 

 to their influence. 



The question of the influence of temperature upon the 

 tone of the plant need not here be considered so fully, as 

 in a preceding chapter we have discussed the phenomena 

 of the general relations of temperature to the plant at 

 some length. We may, however, again point out that 

 plants are affected by variations in temperature in ways 

 very similar to those depending on changes in light. It is 

 not, however, always easy to ascertain the effects due to 

 changes in temperature alone, as other conditions, such as 

 light and moisture, usually 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. We have 



