POWER OF SELECTION BY ROOTS NOT ABSOLUTE. 55 



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 siuface must exer- 

 cise very unequal powers of attraction upon the different 

 materials, which are presented by the solution in the same 

 form and in a state of equal mobihty ; 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 with the roots 

 of land-plants, to judge from the unequal proportions of 

 the substances severally absorbed by them. 



The power of the roots to preclude the passing of 

 certain substances from the soil into the plant is not 

 absolute. Forchhammer (Poggend. 'Annal.' xcv. 90) 

 detected exceedingly minute traces of lead, zinc and 

 copper in the wood of the beech, birch, and fir ; and tui, 

 lead, zmc, and cobalt in that of the oak ; but the fact 

 that the outer rind or bark, in particular, is found to con- 

 tain metals of this Idnd in perceptibly larger quantities 

 than the wood, clearly points to tlie accidental natm-e of 

 their presence, and to their taking no essential part in the 

 vital processes of the plant. 



How small the quantities of these metals must be 

 which the roots of these trees absorb may be judged from 

 the fact that hitherto chemical analysis has not been able 

 to detect traces of any other metal than manganese and 

 iron, in the water of wells, brooks, or springs ; and their 

 appearance in these wood-plants, which dm-ing the growth 

 of half a centuiy or more have absorbed and evaporated 

 an immense quantity of water, is the only proof we 

 possess, that tliis water must actually have contained 

 these metals in some form or other. 



The observations of De Saussure, Sciilossberger, and 



