12 ZOOLOGY. 



lation, growth, etc. Protoplasm varies in the degree of irri- 

 tability. In general it responds to stimuli most normally under 

 those conditions which are most favorable to the ordinary vital 

 processes. 



20. Stimuli. All the disturbing forces or conditions, ex- 

 ternal or internal, which tend to cause response in living proto- 

 plasm, are called stimuli. The principal stimuli are, chem- 

 ically active substances, moisture, contacts, heat, light, elec- 

 tricity, and gravity. Inasmuch as irritability lies at the 

 foundation of the various protoplasmic activities mentioned 

 below, all the natural causes which modify irritability, also 

 modify, through it, the vital processes, such as motion, growth, 

 etc. 



Light affects protoplasm profoundly. The direction of motion in pro- 

 toplasm is largely determined by light. Light may either attract or repel 

 protoplasm. Excess of light retards growth. Heat strongly modifies the 

 rate of all the vital processes. There is an optimum temperature at which 

 the protoplasm best performs its work. An excessive increase or decrease 

 of this temperature produces a cessation of activity, a condition of rigor, 

 and death. The fatal maximum temperature for ordinary animal proto- 

 plasm may be said to be about 45 or 50 C. ; the minimum, o, or below. 

 Chemical agents may stimulate protoplasm in such a way as to attract or 

 repel organisms. Paramecia, which are single-celled animals, may be seen 

 to gather about an air-bubble, or at the margin of the cover-glass. They 

 will retreat before an encroaching solution of certain salts. 



It is a most significant fact in this connection that protoplasm may 

 become, so to speak, accustomed to a stimulus which has been long con- 

 tinued, so that it ceases to respond in the customary way. Protoplasm 

 may gradually be brought, for example, to endure and thrive at a tem- 

 perature which would have produced death if suddenly applied. It is 

 almost impossible to overstate the importance of this faculty in enabling 

 organisms to survive changing conditions. Stimuli, then, may be said to 

 be powerful in proportion to their suddenness and intensity. 



21. Assimilation. The process of changing food sub- 

 stances into protoplasm is called assimilation. It can be 

 effected only by protoplasm. Such foods may be relatively 

 simple substances or may be the complex protoplasm of other 

 organisms. The protoplasm of the green leaves of plants has 

 the power of utilizing the simple inorganic compounds, as 



