244 GENERAL BIOLOGY 



cartridge. The most usual environmental changes 

 producing organic responses are those of tempera- 

 ture, chemical conditions, light, electricity, and 

 mechanical contact. This fundamental ability of 

 the living matter to respond to stimuli is known as 

 Irritability. 



THE USUAL CONDITIONS OF ENVIRONMENT 



Temperature. For all living things a certain 

 degree of warmth is requisite, but organisms vary 

 greatly in this regard. For every organism there is 

 an optimum temperature at which it grows and 

 thrives best, and this is apt to be the usual tempera- 

 ture in which the organism naturally occurs. Plants 

 and animals of the tropics require a higher degree of 

 heat than do those of temperate zones, and when 

 we transplant them they need " hothouses " in 

 order that they may thrive. There is also for each 

 sort of organism a minimum and a maximum tem- 

 perature. Since protoplasm is so largely made up 

 of water, its temperature cannot fall below 32 

 Fahrenheit, if life is to be maintained. 1 As we shall 

 see further on, the protoplasm of some kinds of 

 animals and plants may be protected against such 

 conditions and hence may be unaffected by freezing 

 temperature. It is equally obvious that the tem- 

 perature of boiling water will destroy living matter by 



1 The death of the living matter is due, in all probability, not so 

 much to the alteration of the temperature per se as to the fact that the 

 freezing of the water into needles of ice rends and tears the fundamental 

 structures of the cell beyond repair. 



