POWER OF SELECTION BY ROOTS NOT ABSOLUTE. 67 



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 

 tin, lead, zinc, and cobalt in that of the oak ; but the 

 fact that the outer rind or bark, in particular, is found 

 to contain metals of this kind in perceptibly larger 

 quantities than the wood, clearly points to the acci- 

 dental nature 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 man- 

 ganese and iron, in the water of wells, brooks, or 

 springs ; and their appearance in these wood-plants, 

 which during the growth of half a century or more have 

 absorbed and evaporated an immense quantity of water, 

 is the only proof we possess, that this water must actu- 

 ally have contained these metals in some form or other. 



The observations of DE SAUSSTJKE, SCHLOSSBERGER, 

 and HERTH, show that the roots of land and water 

 plants absorb from very dilute saline solutions water 

 and salt in proportions entirely different from those in 

 the fluid ; in all cases a greater proportion of water, and 

 a less quantity of salt. In plants watered with very 

 dilute solutions of salts of baryta, Daubeny found no 

 baryta, whereas Knop in similar experiments detected 

 this substance. The general result of all these experi- 

 ments is that, of themselves, the plants have not the 

 power of offering a permanent resistance to the chem- 

 ical action of salts and other inorganic compounds upon 

 the exceedingly fine membrane of the root. 



Most land-plants in their natural state in the soil 

 can bear no salt solutions, as concentrated as in these 

 experiments, without sickening and dying ; and even 



