STIMULATION AND ITS RESULTS 377 
tions of the environment may be looked upon as the effect 
of any modification of the conditions which have induced 
tone. We have seen, for instance, that a particular degree 
or range of illumination sets up in a plant the condition 
of phototonus, which is one constituent of the healthy tone 
of the organism. Any modification of that illumination is 
followed by certain effects, the extremes of which we have 
already discussed. This alteration of the optimum illu- 
mination becomes at once a stimulating action, and we can 
speak of a stimulating influence of light, which is really 
any change in what we have called its tonic action. It can 
be in the direction of increase or decrease of the latter, but 
as it induces changes it must be regarded as stimulating. 
What is true of light is also true of the other factors 
which combine to produce the healthy tone of the plant. 
Changes of temperature bring the organism nearer to or 
farther from that optimum point at which it is in the most 
complete state of thermotonus, and are responded to in 
various ways accordingly. Any alteration in the fluid 
contents of a cell brings about a change in what we may 
call the tonic tension of that cell, in which condition the 
permeability of the protoplasm exists at its best, and again 
an appropriate response is made. 
In considering broadly the result of stimulation we 
must notice at the outset that it provokes a purposeful 
response. The living substance appears to have a definite 
aim; it may be to remove the stimulating cause if the 
latter affects it prejudicially; it may be to readjust its 
manifold forces to the new conditions to which the environ- 
ment is suddenly or gradually subjecting it. 
The means which the plant avails itself of are seldom 
abrupt and violent, like the manifestation of muscular con- 
tractility, but more frequently take the form of the modi- 
fication of some rhythm which is characteristic of its 
behaviour. A few cases of sudden and sharp change are 
met with, as when the leaf of Mimosa droops on being 
touched, or when that of Dionca rapidly closes over its 
