FUNCTION.] POWERS OF SELECTION. 283 



ten gallons of distilled water, although the whole of that 

 quantity was expended upon them, a minute examination 

 demonstrated that the stems contained no trace whatever of 

 strontian, although a small portion appeared to be present in, 

 or at least adherent to, the roots. By other experiments it 

 was ascertained, that the strontian was not in these cases 

 first received into the system, and afterwards rejected through 

 the roots ; for when the roots of a Pelargonium were divided 

 into two nearly equal bundles, one of which had its extremity 

 immersed in a glass containing a weak solution of nitrate of 

 strontian, the other in one containing pure distilled water, 

 after the lapse of a week the water in the second glass was 

 tested, but no strontian could be discovered in it, although 

 a single grain in one pint would have been readily detected. 

 Hence it appears, " that plants do possess, to a certain extent 

 at least, a power of selection by their roots, and that the 

 earthy constituents which form the basis of their solid parts 

 are determined as to quality by some primary law of nature, 

 although their amount may depend upon the more or less 

 abundant supply of the principles presented to them from 

 without." (Linn. Trans, xvii. 266.) 



Bouchardat, however, maintains, that plants have no power 

 of selection. He thinks that the experiments of Theodore de 

 Saussure, who maintained the contrary, are not sufficiently 

 free from all chances of error to be conclusive. The way in 

 which the experiments of Theodore de Saussure were made 

 may be stated in a few words. He dissolved in 793 cubic 

 centimetres of water two or three different salts, each weigh- 

 ing 637 milligrammes; he analysed the residue of the solu- 

 tion when it was reduced one-half by absorption by the 

 roots of the plants. The quantity of salts contained in the 

 residue, minus that which the liquid contained before the 

 introduction of the plants, indicated the quantity of salts 

 absorbed. Theodore de Saussure saw that with several salts 

 this quantity was very unequal ; thus, to cite only one ex- 

 ample, in a mixed solution of nitrate of lime and muriate of 

 ammonia, a Polygonum absorbed two of nitrate of lime, and 

 fifteen of muriate of ammonia. 



The differences were particularly great with the soluble 



