308 PHYSIOLOGY. BOOK II. 



proof of its contributing, in some way or other, to the nutri- 

 tion of the vegetable system." 



Fixed as plants are to the soil, deprived of volition, and 

 incapable of removing their highly absorbent roots from what 

 is hurtful to them, except with extreme sloA\Tiess, it appears 

 scarcely probable that they should have any power of select- 

 ing their food ; on the contrary, the facility with which they 

 are poisoned would seem to confirm the correctness of the 

 usual supposition. But, if roots are made to grow in 

 coloured infusions, it is said that they take up only the co- 

 lourless parts, leaving the coloured behind ; and we know that 

 if an apple tree is planted in a piece of ground in which an- 

 other apple tree has been grov/ing many years, the new plant 

 will languish and become unhealthy, whatever quantity of 

 manure, that is of new food, may be offered to its roots. This 

 last fact is accounted for upon the supposition that the soil 

 contains some peculiar principles which are necessary to the 

 health of an apple tree, and that the old tree, having selected 

 for its own consumption all that the soil contained, has left 

 none behind it for the new comer ; but the probability is that 

 this hypothesis is untenable, and that the fact is to be ex- 

 plained upon very different principles (see Chap. X.). It has 

 been, however, demonstrated by Daubeny that plants have, to 

 a certain extent, a power of selection by their roots. He 

 found that when barley was watered with distilled water, con- 

 taining in every two gallons two ounces of nitrate of strontian, 

 not a trace of that earth could be detected in the ashes of 

 the plants; and when Lotus tetragonolobus was treated in a 

 similar manner, excepting that only two ounces of nitrate of 

 strontian were dissolved in ten gallons of distilled water, 

 although the whole of that quantity was expended upon them, 

 a minute examination demonstrated that the stems contained 

 no ti'ace whatever of strontian, although a small portion ap- 

 peared 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 solu- 



