THE MANIFESTATIONS OF LIFE 39 



division occurs every four or five hours; at 25 C. every 

 fifteen or twenty minutes. Between this temperature 

 and 40 C. there is no essential change, but beyond it t 

 multiplication ceases or growth becomes modified so 

 that in certain species spores are formed as the bacilli 

 cease to develop, in others spore formation ceases. When 

 the temperature ascends beyond 60 C., no more spore- 

 free organisms can be found alive. The spores, however, 

 may endure ascending temperatures including exposure 

 to 100 C. for an hour, and 120 C. for a few minutes. 



This variation shows that there are several tempera- 

 tures deserving special mention; the lowest at which the 

 activity of the organism becomes manifested, known as 

 the minimum, that at which the vital manifestations 

 progress with greatest rapidity, the optimum, and the 

 highest at which they can be continued, the maximum. 



The temperature endurance of organisms differs in 

 many cases because of special adaptations. In the case 

 of the bacteria it is ability to enter upon a latent or spore 

 stage; in certain lowly animal forms, it is ability to enter 

 upon an encysted stage in which the delicate protoplasm 

 becomes protected by a dense capsule; in higher plants 

 protection against moderate cold is secured, in some 

 species, through the development of a hairy covering by 

 which the cold atmosphere is kept away, in others where 

 no such provision is made and the plant is killed by the 

 frosts, it prepared for the future generations either by 

 the formation of seeds, some of which can endure any 

 known degree of cold, or by a latent existence in the 

 form of rhizomes or bulbs. 



In the so-called "cold-blooded" animals, whose 

 temperatures differ little from those of the surrounding 

 atmosphere, cold retards the metabolic functions, and 

 heat accelerates them. If such animals can be kept from 

 actual freezing, they endure cold without much harm, 

 and heat only injures them when the accelerated metab- 

 olism becomes a source of excessive waste to the cells. 



The higher, "warm-blooded," animals and birds 



