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, if 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, 
