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 eSect 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.' 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- 



* No sharp distinction can be drawn between the stimuU 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 defined. 



