262 HEAT AND PROTOPLASM [Ch. VIII 



a difference in the temperature of the two ends is sufficient 

 to determine direction of locomotion. In the case of Para- 

 mecium, it is improbable that radiant heat acted, since this 

 passes with difficulty through water. Finally, in the case of 

 the insects enclosed in a box there is evidence of no axis ori- 

 entation, and in the case of Verwokn's experiments with the 

 Amoeba the action of direction is clearly shut out. We must 

 therefore conclude that direction of locomotion in thermotaxis 

 is not usually, if ever, determined by direction of heat rays. 



Since it is not direction of heat rays, it is probably difference 

 of intensity of the agent at the two poles of the organism 

 which is the determining factor. This is clearly so in the case 

 of the Myxomycete and Amoeba. In the case of insects also, it 

 is clear that one part of the body being appreciably nearer the 

 source of heat would be appreciably warmer than the other, 

 and this difference in temperature might serve as an indication 

 to the organism of the direction of the source of heat. But 

 when we come to consider Mendelssohn's experiments on 

 Paramecium, we pause to think of the organism being so 

 sensitive as to be affected differently at the two poles by so 

 slight a difference, of intensity as these poles must experience. 

 Mendelssohn has found that the least difference of intensity 

 at the ends of his 10 cm. long trough which will call forth a 

 thermotactic response is 3° C. The length of a Paramecium 

 is about 0.2 to 0.25 mm., which corresponds to a difference 

 of 0.01° G. of temperature at its two poles. This is the minimal 

 temperature-difference which acts as a stimulus to Paramecium 

 and calls forth a thermotactic response. Although this differ- 

 ence is small, we must, with Mendelssohn, in the absence of 

 opposing data, consider it the determining factor in thermo- 

 taxis, and conclude that, in general, the thermotactic response 

 is a response to differences in the intensity of heat to which 

 the two poles of the body are subjected. 



Let us now sum up the results of our study of the effect of 

 heat on protoplasm. The rates of the metabolic processes and 

 of protoplasmic movements are controlled by temperature, since 

 they diminish from the optimum slowly towards the minimum 

 and rapidly towards the maximum. At these points movement 

 and irritability cease as a result of excessive stimulation, and 



