174 On the Selecting Power of' Plants 



birds had been entirely debarred from every kind of earthy 

 matter. It may be asked, whether the strontian is taken first 

 into the system, and afterwards excreted from it, or whether the 

 spongioles of the roots refuse it admission. The latter supposi- 

 tion seems the more probable one, since, if we adopt the former, 

 we ought to be able always to find traces of the earth diffused 

 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 Pelargo- 

 nium 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 containing 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 me- 

 thod. 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 small lilac {Syringa vul- 

 garis), 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 remain- 

 der 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 es- 

 timated ; 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 89.2 of the earth. The four pints of 

 water therefore consumed, if they had passed through the or- 

 gans of the vegetable charged with their original quantity of ni- 

 trate of strontian, would have carried into its circulation 22.4 

 grains of this earth ; and as the water was absorbed at the ave- 

 rage rate of about 4^ ounces per diem, it follows that more than 

 a grain and a half would have been carried daily through the 



