PROTOPLASM 13 



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. 



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

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

 toplasm, are called stimuli. The principal stimuli are, 

 chemically active substances, moisture, contacts, heat, light, 

 electricity, and gravity. Inasmuch as irritability lies at 

 the foundation of the various protoplasmic activities men- 

 tioned 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 protoplasm 

 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 protoplasm 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 continued, so that it ceases 

 to respond in the way it did when the stimulus was novel. Protoplasm may 

 gradually be brought, for example, to endure and thrive at a temperature which 

 would have produced death if suddenly applied. It is almost impossible to over- 

 state 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. 



22. Assimilation. The process of changing food substances 

 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 

 oxygen, water, and carbon dioxid, in a larger measure than 

 that of animals. Plants may build these up into foods, whereas 

 animals must have the complex organised foods to start with. 



