68 AHATOMY ANB PHYSIOLOGY OF 



lie near together, some healthy, others diseased, the latter are very imioh 

 more quickly penetrated by the tincture of iodine. 



An important question in absorption is this : are the different 

 substances absorbed by different plants in equal relative quan- 

 tity, or does one plant take up one substance, another a second 

 in greater abundance? Saussure, who thought the latter condi- 

 tion not improbable, could not find any confirmation of it in 

 his experiments, for the variations which'^he found in the absorp- 

 tion of different substances by different plants, were not more con- 

 siderable than the variations which occurred in different experi- 

 ments with the same plant. Trinchinetti (Sulla facolta assorhente 

 delle radici) made experiments on this question, by ])lacing differ- 

 ent species of plants in mixtures of two salts which do not decom- 

 pose one another, whereby he shewed that certainly one plant ab- 

 sorbed one, another plant the otlier salt, in preference, from a mix- 

 ture of nitrate of potass and common salt. Thus Mercurialis annua 

 and Qhenopodium mride absorbed much nitre and little salt, 

 while Satureia hortensis and Solanum Lycosijersicum took up 

 much salt and little nitre, and from a mixture of sal-ammoniac 

 and salt Mereurialis absorbed more sal-ammoniac, while Vicia 

 Fa a took more salt. 



If, however, as there is every appearance, the result obtained by 

 Trinchinetti be correct, we can by no means deduce from it the con- 

 clusion that the plant possesses the power of absorbing substances 

 useful to it and excluding those Avhich are injurious, for experi- 

 ence has amply demonstrated that it does not possess this latter 

 power, that it can even, m Saussure's experiments with sulpliate 

 of copper shew, absorb injurious substances more easily than those 

 which it applies to its nutrition, and we must assume that the 

 cause of the differences in question is to be sought in the physical 

 and chemical peculiarities of the particular substances, and their 

 relation to the cell-membrane and the cell-contents. 



^ Observ, It is a known fact that different species of plants which grow 

 side by side in the same soil, to the roots of which the same nutriment is 

 conveyed, shew by analysis of their ashes a very different composition of 

 fixed constituents derived from the soil This circumstance may be ex- 

 plained in two ways; either through the assumption tliat different species 

 of plants take up different constitvients in unequal quantity from the 

 same solution, for which the experiments of Trinchinetti above-mentioned 

 furnish positive evidence, or through the hypothesis defended by Liebig, 

 that different plants take up equally, like a sponge, all that is dissolved in 

 water, hut again reject all superfluous or injurious substances. The first 

 must be regarded as hj far the more probable in the present state of our 

 knowledge, since the second hypothesis, which is based upon Macaire- 

 Princep's experiments, presently to he mentioned, that substances imfit 

 for the plant can be again excreted by the roots, has not been confirmed 

 by later researches. It is certainly not to be denied that plants possess 

 in the fall of the leaves a means of removing a part of the substances 



