732 PHYSIOLOGY. 



growth and development, it may be shown by chemical analysis 

 afterwards, that the wheat stalk will contain a much larger pro- 

 portion of silica, (which it must have obtained from the soil,) than 

 that of the bean. 



The experiments of Bouchardat, Vogel, and others, appear, 

 on the contrary, to indicate that roots absorb all substances 

 presented to them indifferently, and in equal proportions. The 

 simple fact, however, which is easily proved by chemical analysis — 

 that the ashes of different plants contain different substances or 

 in different proportions, seems to me to prove incontestably, that 

 roots have a power of selecting their food. In using the term 

 selecting, however, we do not intend to imply that roots have 

 any inherent vital power of selection resembling animal volition, 

 but only to express the result produced by virtue of the mutual 

 actions of the root and the substances which surround it in the soil. 

 This power or property of selection is without doubt due to some 

 obscure, and at present but little understood molecular relation 

 which exists between the membranes of the cells of different 

 plants and the substances which are taken up or rejected by 

 them, by which different roots acquire different endosmotic 

 action for the same substances, while those of the same plant, 

 in like manner, possess a varying endosmotic action for different 

 substances. It follows also, from the recognition of endosmotic 

 action as the cause of the absorption of fluid matters by the 

 plant, that poisonous substances may be also taken up when 

 in solution by the roots, provided their tissues are not injured 

 by them in their passage ; and we find accordingly, that when 

 injurious substances are found in the soil a corresponding effect 

 is produced upon plants by their absorption. 



Excretion of Roots. — The roots of plants have been considered 

 by Urugmans, Plenk, De Candolle, Macaire-Prinsep, and others, 

 to possess the power of excreting into the soil certain of their 

 peculiar secretions. Thus Prinsep found, that the Lettuces 

 and Poppies excreted a matter analogous to opium ; the Legu- 

 minoscB gummy matters ; the Euphorbiacea acrid matters, &c. ; 

 and it was therefore concluded by De Candolle and others, that 

 such substances were thrown off by plants because they were in- 

 jurious to them. It was also believed that while such substances 

 were injurious to the plants producing them, they were bene- 

 ficial to others; and as plants could not therefore be grown 

 for any length of time in soil impregnated with their own 

 injurious excretions, rotation of crops was necessary. (Seep. 777). 



These experiments, when repeated by Braconnot, Walser, 

 Meyen, Boussingault, &c., with every precaution, did not lead 

 to the same results, but, on the contrary, to the conclusion — 

 that the effects observed by Macaire-Prinsep were due to his 

 experiments having been made without sufficient care, and 

 that no excretions of the peculiar substances of the plant took 



