166 SELECTIVE POWER OF PLANTS. 



where one of the same species had been previously 

 growing for any length of time, and that substances 

 existed in that soil which were not to be found in 

 other parts of the field. But this is by no means a 

 sufficient proof, because we should expect that if 

 the existing combinations were separated, others 

 would be formed. 



That the roots possess a general power of selec- 

 tion, we are led to believe from the fact that the 

 ashes of various species differ so widely even when 

 the plants are grown upon the same soil. It is 

 probably confined to the combinations which the 

 plant meets with in its indigenous soil, and does 

 not apply to those artificial substances which are 

 sometimes applied, nor to the tree when placed in 

 an unnatural position. Thus, the application of a 

 large quantity of iron to the roots of a plum tree, 

 resulted in a dark-colored exudation from its bark ; 

 and the colors of flowers are often changed by the 

 substances placed in the soil. 



It will be well to examine more particularly the 

 structure of those organs of the plant which perform 

 the function of digestion. The upper surface of the 

 leaves, and all green parts of other portions of the 

 tree, exhale the oxygen contained in their sap, while 

 the under surface of the leaves inhales carbonic acid 

 and some other fertilizing gases from the atmos- 

 phere. The cells, on the upper surface, are there- 

 fore very close, to prevent excessive evaporation, 



