372 VEGETABLE PHYSIOLOGY 
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 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 
