174 DR ALISON’S OBSERVATIONS ON 
which is merely the selection, by a portion of a living structure, of some one sub- 
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 require 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 compound fluid 
which is absorbed, are retained in the substance of the plant, while others are 
returned to the soil. ‘‘ The experiments of Macarre PRINcEP,” says 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.” (Ib. 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 sclecting 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 

