to the Earth)/ Constituents presented to their Absorbing Surfaces. 265 



the ith of a grain, that is, 2 per cent, of the whole quantity of earthy matter 

 present, my analysis indicating 



Gr. 

 Of lime 7'30 



— strontian 0']8 



Total quantity of earth . . . 7'48 



The conclusion to which I have been led by the foregoing experiments may 

 appear at first sight inconsistent with those deduced by M. de Saussure in his 

 elaborate work on vegetation before referred to, in which he has shown that 

 some poisonous substances, such, for example, as salts of copper, are freely 

 absorbed by the roots of vegetables, and retained in considerable quantities in 

 their tissue. 



But it M'ill be recollected, that this philosopher himself accounts for the cir- 

 cumstance by the disorganization which such bodies, by their presence, occa- 

 sion in the fibres of the roots. 



I have myself found that when a Pelargonium had a portion of its roots 

 immersed in a solution of bichromate of potass, a trace of this salt was con- 

 veyed into a second glass containing distilled water, which had no con- 

 nexion with the former, except through the medium of a parcel of the roots 

 which dipped into it. Nor was this owing to capillary attraction, for the 

 same effect did not take place in another experiment, in which the roots were 

 detached from the body of the plant, and therefore acted as dead matter ; and, 

 moreover, the salt was detected by appropriate tests applied to the stems and 

 leaves. 



In this instance, then, the substance was seen to circulate through the 

 whole texture of the vegetable, and ultimately to be excreted by its roots ; 

 and a similar result was obtained in the case of another plant, in which a 

 solution of proto-sulphate of iron had been dissolved in the water in contact 

 with its extremities*. 



* That is to say, the salt was detected by ferro-cyanate of potass in many parts of the stem and 

 branches ; but it did not reach above a certain point, nor was it excreted by the roots, this difference 

 arising from the absorption of oxygen by the salt, which, being thereby converted into a persulphate, 

 became insoluble in the juices of the plant, and consequently clogged up the canals by which the sap 

 is conveyed. 



