24 THE VITAL FUNCTIONS. 



small proportion of the salts, being taken up, and the re- 

 maining part of the fluid being found to be more sti'ongly 

 impregnated with the salts than before this absorption liad 

 taken place. It would appear, however, that all this is 

 merely the result of a mechanical operation, and that it fur- 

 nishes no evidence of any discriminating faculty in the 

 .spongiole: for it is found that, provided the material pre- 

 sented be in a state of perfect sohition and limpidity, it is 

 sucked in with equal avidity, whether its qualities be dele- 

 terious or salubrious. Solutions of sulphate of copper, which 

 is a deadly poison, are absorbed in large quantities by the 

 roots of plants, which are immersed in them: and water 

 which drains from a bed of manure, and is consequently 

 loaded with carbonaceous particles, proves exceedingly in- 

 jurious when admitted into the system of the plant, from 

 the excess of nutriment it contains. But in the ordinary 

 course of vegetation, no danger can arise from this general 

 power of absorption, since the fluids which nature supplies 

 are always such as are suitable to the organs that are to re- 

 ceive them. 



The fluid, which is taken up by the roots, and which, as 

 we have seen, consists chiefly of water, holding in solution 

 atmospheric air, together with various saline and earthy in- 

 gredients necessary for the nourishment of the plant, is in 

 a perfectly crude state. It rises in the stem of the plant, un- 

 dergoing scarcely any perceptible cliange in its ascent; and 

 is in this state conducted to the leaves, where it is to expe- 

 rience various important modifications. By causing the 

 roots to imbibe coloured liquids, the general course of the 

 sap has been traced with tolerable accuracy, and it is found 

 to traverse principally tlie ligneous substance of the stem: 

 in trees, its passage is chiefly through the alburnum, or more 

 recently formed wood, and not through the bark, as was at 

 one time believed. 



The course of the sap, however, varies under different 

 circumstances, and at different epochs of vegetation. At 

 the period when the young buds are preparing for their de- 



