On the Food tf Plants. 
531 
Grain from preceding; straw : — 
500 grains yielded of ash . 12 grains. 
500 . . . . 13 „ 
Or, 2*5 per cent. 
10 grains gives, on analysis : — 
In 100 lurts. 
Phosphate of potash, with trace of chloride 3 "9 . 39 
Phosphates of lime and magnesia . . 2"3 . 23 
Siliceous matter . . • . 3*8 . 38 
10-0 100 
The ash of this grain thus tlocs not differ sensibly from those 
of the preceding, except in the ])roportion of silica in an insoluble 
state, derived from the inseparable husk. The composition of 
the straw-ash diflcrs widely from that of all the others, inasmuch 
as it contains a large proportion of silicate of potash, and also of 
soluble phosphate ; which are both absent, the first nearly, the 
second completely, in all the rest. Setting aside this apparent 
exception (which perhaps we may be able in part, at least, to 
account for), the most important and invariable mineral con- 
stituents of the cereals appear, from the experiments, to be silica 
and sulphate of potash in the straw, and the phosphates of potash, 
magnesia, and lime in the seed. 
So far as it is fair to judge from so few cases, although the 
absolute quantity of ash produced from corn-plants grown under 
different circumstances may vary very much, yet the composition 
of the ash itself, both of the straw and of the grain, undergoes very 
little change. 
How shall we account for the introduction of so large a quan- 
tity of silica into the plant? The foregoing experiments prove 
that this is not silicate of potash, but nearly pure silica, and that 
substance is well known not to be sensibly soluble in water. 
Possibly the anomalous case of the overgrown oat-straw cut down 
before complete ripeness, may afford a key to this difficulty. We 
may conjecture that it is introduced in the state of soluble silicate 
of potash, which salt is afterwards decomposed in the plant itself 
by a vegetable acid there produced, the new potash salt being ex- 
creted by the roots, or otherwise disposed of. It would be easy 
to obtain evidence on this point by careful analyses o!f the ashes of 
corn-plants from the same field in their green and in their mature 
state. 
It is possible, also, that the absorption by these and other 
plants of phosphates from the soil, bone-earth for example, may 
be connected with an acid excretion from their roots, by the aid 
of which it is rendered soluble ; this is, however, merely a con- 
jecture. 
