CHAP. XI. 
FOOD OF PLANTS. 
369 
thing to do with the nutrition of plants ; and that, in those 
cases where it was met with, it was merely in a state of sepa- 
ration from the atmospheric air which had been inhaled and 
deprived of oxygen and carbonic acid. But its constant 
presence in combination with the tissue of Mushrooms and of 
Cruciferous plants, in gluten, and what chemists call vegetable 
albumen, and also in vegetable alkalies, seems a sufficiently 
strong proof of its contributing, in some way or other, to the 
nutrition of the vegetable system.” And M. Boussingault 
has shown that it is in fact a constant element of vegetation, 
most concentrated in seeds, to the maturation of which it is 
essential, and dispersed through the other parts of the tissue. 
(Comptes rendus^ vi. 105.) 
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 slowness, 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 growing 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 Daubeiiy, 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 
B B 
