THE METABOLISM OF PLANTS. 269 



the activity of the metabolism of living organisms. This is 

 especially evident in the case of animals. The metabolic 

 activity of cold-blooded animals is directly dependent upon 

 the temperature to which they are exposed, and Pfliiger has 

 shewn that even in warm-blooded animals the activity of me- 

 tabolism varies with variations in the temperature of the sur- 

 rounding medium. A remarkable illustration is afforded by 

 the phenomenon known as " hibernation ", exhibited even by 

 certain warm-blooded animals, that is, that when the animal 

 is exposed to a persistently low temperature its metabolism 

 is reduced to a minimum. Plants behave in relation to tem- 

 perature like the cold-blooded animals. When they are 

 maintained at a low temperature they cease to exhibit any 

 signs of life. The meaning of this is that, at a low tempera- 

 ture, the metabolic processes are arrested ; but this arrest 

 is not death, nor does it necessarily involve it, but, if the 

 exposure to the low temperature be long continued, it may 

 eventuate in death. 



We may cite the following examples in illustration of 

 the depressing effect of a low temperature upon the metabolic 

 processes. De Candolle found that of the seeds of ten species 

 of plants, those of one species only (Sinapis alba) germinated 

 at OC. ; and even in this case germination was much re- 

 tarded, for whereas these seeds germinated at o C. after 

 seventeen days, similar seeds germinated in sixteen days at 

 a temperature of I'QoC., and in four days at 5*70 C. Similar 

 observations have been made by Sachs. 



The minimum temperature for germination has been ascertained in 

 certain cases. Sachs has made the following determinations, the mini- 

 mum temperature being that at which a small proportion only of the total 

 number of seeds germinated : 



Minimum 



Zea Mais 9*4 C. 



Phaseolus multiflorus ... 9-4 



Cucurbita Pepo 14*0 



Wheat 5-0 



Barley. S'o 



