524 
On the Food of Fhuits. 
associated in the two ti'ees with equal proportional quantities of 
organic acids. The same thing was observed in two fn-trees, one 
of Avhich grew in France, the other in Norway. The conclusion 
arrived at is the following : — every vegetable requires for the ful- 
filment of its vital functions the presence of a certain quantity of 
some particular organic acid or acids, of the use of which, how- 
ever, in the economy of the plant we are yet ignorant ; and, 
further, it is required that these acids be in union with a base. 
Now, although it appears that such substances as potash and soda, 
and lime and magnesia, can to a certain extent replace each other 
in this office, if it should so happen that the supply from the 
ground is insufficient for the purpose, the plant cannot thrive 
unless it possess the power, a power which appears very rare, of 
secrctinc/ an organic alkali for its own use. 
Again, it is known that wheat-straw, in common with the stalks 
of all the grasses, contains a very large quantity of siliceous matter, 
while the grain itself contains phosphoric acid, potash, and mag- 
nesia. These substances are always present ; so far as we know, 
they are indispensable. A land, the richest on the face of the 
earth in other respects, must be absolutely barren for corn-crops 
if it be destitute of these substances, although other plants not 
requiring them, if such could be found, might flourish in the great- 
est luxuriance. Why are clay lands in general the most favour- 
able to corn ? Because, among other reasons, from the nature of 
their origin, soluble siliceous salt must be present. Such crops 
exhaust a soil of this kind by robbing it not so much of its humus 
as of its salts, and cannot again be advantageously grown upon 
the same spot until this loss is supplied either by a further disin- 
tegration of the soil itself, which happens in a fallow, according to 
Liebig, or by the addition of suitable manure. 
The settlers in the American woods, those who for the first 
time put the plough into new land, which year after year has for 
centuries had its surface manured by the decomposing leaves of 
the forests which before grew upon the spot, and whose deep 
penetrating roots sought out these saline substances in every 
direction, raise crop after crop of the heaviest grain without addi- 
tion of any kind, to the astonishment of the inhabitants of long- 
settled districts; but even this comes to an end, and the ground 
refuses to give its increase without an artificial supply of the ex- 
hausted elements. 
In some of the older provinces of the United States great tracts 
of once fertile land are described to be lying barren from having 
been injudiciously forced to grow tobacco, a plant rich in nitrate 
of potash ; and no suitable manure being at hand to recover them, 
they remain wild and waste. 
Such is the state of the case. We have reason to believe that 
