EXTERNAL INFLUENCES AS CAUSES OF VARIATION 255 



inactive (heat rigor); its minimum, below which all activity 

 ceases ; and its optimum, that point at which growth is most 

 rapid. Certain facts in this connection are noteworthy : 



1. The maxima, minima, and optima are not the same for 

 different species. 



2. Protoplasm is killed if carried much above the maximum, 

 the organism decomposes and is destroyed. 



3. Temperatures below the minimum are not fatal except in 

 the presence of moisture, which, on conversion into ice, destroys 

 the structure of protoplasm by the act of expansion. 



4. The optimum at which growth is most rapid is nearer the 

 upper than the lower limit. 



5. Both the optimum and the maximum may be raised by 

 careful methods involving gradual acclimatization. 



Specific effect of heat upon protoplasm. 1 Beginning at the 

 optimum and decreasing both ways to the limits, it may be said 

 in general that protoplasmic activity is in proportion to the tem- 

 perature. This is true of the amount of oxygen absorbed, of 

 carbon dioxid evolved, of chlorophyll formed, and of carbon 

 fixed, in other words, of metabolism. The same is true as 

 to movements of protoplasm and its irritability to light, contact, 

 or other stimuli. The following table exhibits the number of 

 electric shocks per second required at different temperatures to 

 produce tetanus in the neck muscles of a tortoise. 2 



EFFECT OF TEMPERATURES UPON ANIMAL ACTIVITIES 



Effect of heat upon the rate of growth in plants. 3 The relation 

 of temperature to plant growth is well shown in the tables on 

 the following page. 4 



1 C. B. Davenport, Experimental Morphology, Part I, pp. 222-231. 



2 Ibid. p. 230. 3 Ibid. Part II, pp. 450-460. * Ibid. p. 451. 



