66 THE PLANT. 



water. One and the same field presents to the plants 

 growing in it, the alkalis, alkaline earths, phosphoric 

 acid, and ammonia, in absolutely the same form and 

 conctition ; but the ash of no one species of plant ever 

 shows the same relative proportions of component ele- 

 ments as the ash of another species. Even the parasit- 

 ical plants, which draw their mineral constituents in a 

 certain state of preparation, from other plants on which 

 they live, as the mistletoe ( Viscuin album), do not com- 

 port themselves to the latter as a graftling to a tree, but 

 absorb from the sap very different proportions of min- 

 eral constituents (' Annal d. Chem. und Pharm/ liv. 

 363). Now, 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 from 

 a solution moving about in the soil and passing imme- 

 diately into the roots, then two plants of different spe- 

 cies or kind, placed in the same conditions, would re- 

 ceive the same mineral substances in the same relative 

 proportions ; but, as we have seen, two plants belonging 

 each to a different species contain these substances in 

 the most dissimilar proportions. 



That a selection takes place in the absorption of food 

 by the roots is a fact beyond dispute. 



In the case of aquatic plants, which grow under 

 water, exhalation is altogether excluded as a possible 

 operating cause of the passing of the food into the body 

 of the plant. In these plants the absorbent surface 

 must exercise very unequal powers of attraction upon 

 the different materials, which are presented by the solu- 

 tion in the same form and in a state of equal mobility ; 

 or, what comes to the same thing, the resistance offered 

 to their passage through the outermost cellular layers 

 must be very dissimilar. The case cannot be different 



