EXTERNAL CONDITIONS OF ABSORPTION, ETC. 



513 



The power possessed by the soil of becoming heated by the sun exerts 

 as much influence on the health of the plant as its power of retaining 

 gases and vapour. The warmth of the soil acts upon plants entirely 

 independently of the temperature of the air, and frequently requires to 

 be much higher, in order that plants may flourish. Unfortunately, on 

 this subject we have but few observations, and these principally relate 

 to our hothouses and tropical districts. 



The following Table gives an approximation to the temperature of the 

 soil borne by plants without injury : 



The following interesting Table I have taken from Schiibler. The 

 columns A. relate to observations made by Schiibler in his own garden 

 at Tubingen, 1010 Paris feet above the level of the sea, with a southern 

 aspect, and at mid-day, between the hours of twelve and one o'clock. 

 The columns B. give the average of daily observations made in the 

 Botanic Garden at Gent in 1796, 1252 Paris feet above the level of 

 the sea. 



On the 16th of June, 1828, a thermometer in the soil during a west 

 wind rose to the temperature 54 R., whilst that of the air was 20-5. 



This condition of the earth must influence plants according to their 

 specific nature, and produce many peculiarities in the distribution of 

 plants upon the surface of the earth. The limitation of plants to a larger 



* The temperatures here are apparently marked according to Reaumur's scale. TRANS. 



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