264 Dr. Daubeny on the Degree of Selection exercised hy Plants, with regard 



admission. The latter supposition seems tlie more probable one, since, if we 

 adopt the former, we ought to be able always to find traces of the earth dif- 

 fused throughout the vegetable tissue ; and I may relate an experiment of 

 my own, which seems to confirm it, undertaken after the plan of those by 

 means of which the ingenious M. Macaire of Geneva established his important 

 doctrine with respect to the excretory function discharged by the roots of 

 plants. 



A small Pelargonium was taken out of its pot, and its roots divided into 

 two nearly equal bundles, one of which had its extremities immersed in a glass 

 containing a weak solution of nitrate of strontian ; the other, in one contain- 

 ing pure distilled water. 



After a week had elapsed, the water contained in the second glass was tested ; 

 but no strontian could be discovered in it, though a single grain in one pint of 

 water would have been readily detected by my method. Hence it would 

 seem that the strontian is not excreted by the roots. 



Yet this power of rejecting the earth in question, if possessed by the plant, 

 must be held compatible with that of absorbing the water containing it, with 

 which its roots are in contact. I took out of the ground a sinall Lilac {Syringa 

 vulgaris), and introduced its roots into a glass globe containing seven pints of 

 a weak solution of nitrate of strontian. In about a fortnight the quantity was 

 reduced to three pints, the remainder having for the most part been absorbed 

 by the roots ; for evaporation was prevented by covering the surface of the 

 water with a stratum of olive oil, and the mouth of the vessel with a cork. 

 Unluckily, the original quantity of salt had not been estimated ; but it was 

 found that what remained in the water at the close of the experiment yielded 

 69'4 grains of sulphate of strontian, equivalent to 39"2 of the earth. The four 

 pints of water therefore consumed, if they had passed through the organs of 

 the vegetable charged with their original quantity of nitrate of strontian, 

 would have carried into its circulation 22*4 grains of this earth ; and as the 

 water was absorbed at the average rate of about 4^ ounces per diem, it fol- 

 lows that more than a grain and a half would have been carried daily through 

 the substance of the plant, supposing the salt to have been taken up in the 

 same ratio as the water. Now on burning the plant, and examining its ashes, 

 a trace of strontian certainly was detected, but its whole amount did not reach 



