364 VEGETABLE PHYSIOLOGY 



seen at their best. This is the condition of tone or photo- 

 tonus, and its maintenance may be spoken of as due to the 

 tonic influence of light. It is frequently said that light 

 retards growth, and the tonic influence is associated with 

 this retardation. This is, however, a somewhat incomplete 

 presentation of the case. Retardation 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 charac- 

 teristic of healthy protoplasm and its control of its own 

 permeability are adjusted to a particular relationship with 

 the environment, 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 refrangi- 

 bility, the blue and the violet. To these rays the proto- 

 plasm 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. 



