174 DR ALISON'S OBSERVATIONS ON 



which is merely the selection, by a portion of a living structure, of some one suh- 

 stance existing in a fluid, and the consequent attraction of this to a particular 

 part of the structure, while other materials, equally presented to that living part, 

 are excluded. 



We need not here enter into the question, on which chemists and agricul- 

 turists are not yet agreed, whether the nourishment of plants, in the present con- 

 dition of the earth's surface, does or does not recjuire the pre-existence, in the soil, 

 of organic compounds, resulting from previous living beings, which are absorbed 

 from it. But we may justly give the name of vital attraction or affinity to 

 that power by which certain saline matters, dissolved in the compoimd fluid 

 which is absorbed, are retained in the substance of the plant, while others are 

 returned to the soil. " The experiments of Macairi: Prixcep," saj's Leibig, 

 " have shewn that plants, made to vegetate with their roots, first in a solution of 

 acetate of lead, and then in rain-water, give back to the latter all the salt of lead 

 which they had previously absorbed. Again, when a plant, freely exposed to the 

 air, rain, and light, is sprinkled with a solution of nitrate of strontian, the salt is 

 absorbed, but is again separated by the roots, and removed farther from them by 

 every shower of rain, so that at last not a trace of it is to be found in the 

 plant. A fir-tree, the ashes of which were analysed by a most accurate chemist, 

 grew in Norway, on a soil to which common salt was conveyed in great quantity 

 by rain-water. How did it happen that its ashes contained no appreciable quan- 

 tity of salt, although we are certain that its roots must have absorbed it after 

 every shower '! We can explain this only by the observations above referred to, 

 which have shewn that plants return to the soil all substances unnecessary to 

 their own existence; and we are thus led to the conclusion that the alkaline bases, 

 existing in the ashes of plants, must be necessary to their growth, since, if this 

 were not the case, they would not be retained." (lb. p. 103,4.) Another in- 

 ference is at least equally obvious, that plants have the power of fixing and 

 retaining within them, those matters which are suited or essential to their com- 

 position ; and this power we regard as the simplest form of vital affinity. It 

 may be said, that the alkaline bases are thus fixed in plants, because they enter 

 into combination with organic acids, and that, therefore, it is the formation of 

 these acids, not the retention of the bases which combine with them, that is truly 

 the vital change. But this does not apply to other saline matters contained in 

 vegetables, which must have been taken up from the soil in the same state in 

 which they are found in the plants, e. g., the phosphate of magnesia, which is 

 " an invariable ingredient in the seeds of grasses ;'' or the silica which is found 

 in certain parts of various plants. 



Were it not for this selecting and appropriating power, indicating a simple 

 attraction of some parts of the vegetable for certain earthy or saline matters only, 

 we should find some salts of alumina, as well as of lime or magnesia, in the ashes 



