STIMULATION AND ITS RESULTS 369 
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 con- 
dition 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 
_illumination 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 
further 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 modifi- 
cation 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 Dionza rapidly closes over its 
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