10 



stance. A clear and atlc(|ualc clcliiiilion could llieii be 

 established. 



Much evidence, however, has been accumulated for an 

 entiri'Iy different notion of death temperatures. In some 

 l)hints, for example — and the same mii-ht be true of single 

 cells — death seems to result from the une(inal slowing 

 down by cold of the various metabolic activities. If, for 

 instance (in the case of a higher plant), less moisture is 

 absorbed by the roots than is liberated by the leaves, des- 

 iccation and death will follow. An exposui-e to a tem- 

 perature as high as 5° might become lethal if enough time 

 is allowed for the material to dry; a temperature a few 

 degrees lower will dehydrate the plant to death in a 

 shorter time. It is evident, then, that the time factor 

 must be introduced into the definition of the death tem- 

 peratures. On the other hand, the degree of moisture of 

 the atmosphere, the amount of light present, the water- 

 content of the soil, etc., will also affect the duration of 

 death and the temperature at which it occurs. The lethal 

 temperatures, therefore, represent simply conditions un- 

 favorable for life, in the same sense as the scarcity of light 

 or of oxygen or of water. In cases of this kind, the defi- 

 nition of death temperatures is rather involved; instead 

 of a lethal point one should use the notion of a lethal re- 

 gion ; the time necessary for death should be mentioned, 

 as should also the other conditions which hasten death or 

 delay it. The temperature at which the plant dies, by 

 desiccation, in an arbitrarily chosen very long time, can 

 be called the maximal lethal temperature. The lethal 

 zone extends from that maximum downward, but it has no 

 minimum since there is no temperature, however low, 

 which will kill the plant instantaneously by putting its 

 metabolic processes out of balance. 



If such a plant is cooled rapidly to the freezing point, 

 the injury produced by the congelation of its juices may 

 suffice to cause it to die. But death results here from an 

 enti]-ely different cause, and the plant has, besides the 

 death zone mentioned in the preceding paragraph, a death 



