54 THE PLANT. 



substances through the living vegetable membrane must, 

 therefore, be governed by very complex laws. 



Land plants act m the same manner with respect to the 

 soil in which they grow, as marine plants to sea-water. 

 One and the same field presents to the plants growing in 

 it, the alkahes, alkaline earths, phosphoric acid, and ammo- 

 nia, in absolutely the same form and condition ; but the 

 ash of no one species of plant ever shows the same 

 relative proportions of component elements as the ash of 

 another species. Even the parasitical plants, which draw 

 their mineral constituents in a certain state of prepara- 

 tion, from other plants on which they hve, as the mistletoe 

 (Viscum album), do not comport themselves to the latter 

 as a graftling to a tree, but absorb from the sap very 

 different proportions of mineral constituents ( 'Anual d. 

 Chem. und Pharm.' liv. 363). ISTow, as the soil is perfectly 

 passive in respect to the supply of these materials, there 

 must be some agency at work in the plant itself, which 

 regulates the absorption according to the requirements of 

 each plant. 



The observations made by Hales (see Appendix C.) 

 show that the exhalation from the surface of the leaves 

 and branches exercises a powerful influence upon the 

 motion of the fluids, and upon the absorption of water 

 from the soil. If the plant drew its mineral food fi'om 

 a solution moving about in the soil and passing imme- 

 diately into the roots, then two plants of different species 

 or kind, placed in the same conditions, would receive the 

 same mineral substances m the same relative proportions ; 

 but, as we have seen, two plants belonging each to a 

 different species contain these substances in the most dis- 

 similar proportions. 



That a selection takes place in the absorption of food 

 by the roots is a fact beyond dispute. 



