24 A. \V. GREELEY. 



sion accelerates these expressions of the general activity of the 

 protoplasm. These conditions are those which bring about a 

 liquefaction of the protoplasm, so that the protoplasmic activity 

 is seen to vary directly with the amount of water the protoplasm 

 contains. Conversely the assumption of a quiescent spherical 

 resting stage in various protozoa is the result of an increase in 

 surface tension, and is formed by those conditions which cause 

 a coagulation of the protoplasm and a loss of water. Thus a 

 slight increase in the temperature is seen to have the same effect 

 on these simple protoplasmic properties of Paramcscium as 

 anions ; a lowering of the temperature acts like cathions ; since 

 each set of conditions produces the same structural effects. 



II. The Tropisms. 



We have at the present time an enormous amount of informa- 

 tion concerning the reactions of organisms to external stimuli, but 

 we know almost nothing of the physical or chemical effects of these 

 attractive or repellent agents on the protoplasm of the organism, 

 and consequently we are not able to offer any satisfactory expla- 

 nation of the mechanism of the tropic response. In the follow- 

 ing experiments I have studied the reactions of paramoecia to 

 thermal, electrical and chemical stimuli, and have attempted to 

 show that the reaction of Paramcecium to each stimulus depends 

 on certain structural changes in the protoplasm, which are a result 

 of the stimulating action. 



I. Thermotaxis. The reactions of paramoecia to variations in 

 temperature are exceedingly definite. It has been stated by many 

 observers that they are positive to temperatures between approxi- 

 mately 23 and 27 C., and are negative to all others. This reac- 

 tion is beautifully demonstrated by placing the paramoecia in a 

 long, narrow dish which is heated at one end and cooled at the 

 other. It has been already shown that those temperatures to 

 which Paramcecium is positive constitute exactly those thermal 

 conditions which bring about a liquefaction of the protoplasm and 

 a reduction in the surface tension. Those temperatures to which 

 Paramcecium are negative, coagulate the protoplasm. We thus 

 see that, in the case of thermotaxis, attraction is accompanied by 

 a liquefaction of the protoplasm, repulsion by coagulation. The 



