ON TEMPERATURE. 87 



they first begin to vegetate, of enduring extreme 

 cold.* 



113. The effect of cold is, as has been seen, to di- 

 minish excitability ; of heat, to stimulate it : but, if the 

 latter stimulus were constantly equal, it may be con- 

 ceived that the excitability would soon become im- 

 paired or expended. Nature has, however, provided 

 against this result, not only by the fluctuations of tem- 

 perature that occur at different periods of the day, but 

 more particularly by the periodical fall of temperature 

 at night and its rise during the day ; an arrangement 

 intimately connected with all the vital actions of vege- 

 tation. In the day, when light is strongest, and its 

 evaporating and decomposing powers most energetic, 

 temperature rises and stimulates the vitality of plants, 



* M. De Candolle gives the following as the laws of temperature 

 with respect to its influence upon vegetation: — 



1. All other things being equal, the power of each plant, and 

 of each part of a plant, to resist extremes of temperature, is in the 

 inverse ratio of the quantity of water they contain. 



2. The power of plants to resist extremes of temperature is direct- 

 ly in proportion to the viscidity of their fluids. 



5. The power of plants to resist cold is in the inverse ratio of the 

 rapidity with which their fluids circulate. 



4. The liability to freeze, of the fluids contained in. plants, is 

 greater in proportion to the size of the cells. 



6. The power of plants to resist extremes of temperature is in 

 a direct proportion to the quantity of confined air which the 

 structure of their organs gives them the means of retaining in the more 

 delicate parts. 



6. The power of plants to resist extremes of temperature is in 

 direct proportion to the capability which the roots possess of ab- 

 sorbing sap less exposed to the external influence of the atmosphere 

 and the sun. 



