20 THK VITAL FUNCTIONS. 



vided the material presented be in a state of perfect 

 solution and limpidity, it is sucked in with equal 

 avidity, whether its qualities be deleterious or salu- 

 brious. Solutions of sulphate of copper, which is a J 

 deadly poison, are absorbed in large quantities by " 

 the roots of plants, which are immersed in them ; 

 and water which drains from abed of manure, and 

 is consequently loaded with carbonaceous particles, 

 proves exceedingly injurious when admitted into 

 the system of the plant, from the excess of nutri- 

 ment it contains. But in the ordinary course of 

 vegetation, no danger can arise from this general 

 power of absorption, since the fluids supplied by 

 nature are always such as are suitable to the organs 

 which are to receive 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 ingredients necessary for 

 the nourishment of the plant, is in a perfectly crude 

 state. It rises in the stem of the plant, undergoing 

 scarcely any perceptible change in its ascent ; and 

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

 to experience 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 prin- 

 cipally the ligneous substance of the stem : in trees, 

 its passage is chiefly through the alburnum, or more 



* The experiments of Dr. Daubeny, however, lead to the con- 

 clusion that plants possess, to a certain extent at least, a power of 

 selection, with respect, more particularly, to the earthy constituents 

 which form the basis of their solid parts. (Trans. Linn. Soc. xvii. 

 253, 266.) 



