428 PHYSIOLOGY 
Definition. A stimulus is any change in the intensity or direction of 
application of energy which produces an appreciable effect upon living 
protoplasts. Of course when no appreciable effect is produced, the 
energy may differ neither in amount nor form from that which does 
arouse a reaction; and effects may be produced which are not perceived 
because improper tests are applied. A stimulus, thus, has no absolute 
value; it implies not a definite amount of energy measured in physical 
units, but merely enough applied suddenly enough to call forth a reaction 
as revealed by some arbitrary test. Therefore, what is a stimulus under 
certain conditions, is not a stimulus under others. 1 Nor need the stimu- 
lus arise or act outside the plant as a whole. It may originate in one part 
and act upon an adjacent part, even in one protoplast and act upon 
another. These stimuli, in one sense external and in another internal, 
are most difficult to study. They are in part, and perhaps wholly, the 
occasion for the reactions that are called autonomic, or less properly 
" spontaneous." 
Kinds. Stimuli may be classified for convenience as mechanical, 
chemical, and ethereal. Under mechanical stimuli are grouped those 
which depend upon mass movements, resulting in contact, impact, 
friction, pressure, etc., upon the plant. For lack of definite knowledge 
of the nature of gravitation, the stimulus of gravity may be conveniently 
included here, since it depends upon mass attraction and induces mass 
movements. Under chemical stimuli are included those whose action 
depends on their chemical quality their composition and molecular 
structure rather than on their mass. Ethereal stimuli comprise 
those propagated as vibrations in the ether and distinguished according to 
the length of the waves as light, heat, and electricity. 
Modes of reaction. The action of a stimulus results in stimulation 
or excitation, and this may or may not lead to an observable reaction, 
depending upon the state of the protoplasm and the means used to detect 
a change in its behavior. Thus, immediately upon excitation a change 
in the electrical condition of the protoplast occurs, but this does not mani- 
fest itself to our senses, unless the stimulated region and an unstimulated 
one are put into electrical connection with the poles of a sensitive gal- 
vanometer (fig. 670). At the same moment a contraction of the proto- 
1 No sharp distinction can be drawn between the stimuli which are followed by a 
prompt and easily observable response and those external agents whose very gradual 
change has no early apparent effect, but produces ultimately some deviation from the 
usual course of development. In the broad sense both are stimuli, but the term is 
usually applied only to the former, in which sense it is here denned. 
