ule
holds true with regard to all of the constituents required; Nature
seeming to have made it a law that if one of the important ingredients
of the plant is absent, the others, though they may be present in
sufficient quantities, cannot be used. Thus, if the soil is deficient in
potash, and still has sufficient quantities of all of the other
ingredients, the plant cannot take up these ingredients, because potash
is necessary to its life.

If a farmer wishes to make a cart he prepares his wood and iron, gets
them all in the proper condition, and then can very readily put them
together. But if he has all of the _wood_ necessary and no _iron_, he
cannot make his cart, because bolts, nails and screws are required, and
their place cannot be supplied by boards. This serves to illustrate the
fact that in raising plants we must give them every thing that they
require, or they will not grow at all.

In the case of our young plant the following operations are going on at
about the same time.

The leaves are absorbing carbonic acid from the atmosphere, and the
roots are drinking in water from the soil.

[What becomes of the carbonic acid?

How is the sap disposed of?

What does it contain?

How does the plant obtain its carbon?

Its oxygen and hydrogen?

Its nitrogen?

Its inorganic matter?]

Under the influence of daylight, the carbonic acid is decomposed; its
oxygen returned to the atmosphere, and its carbon retained in the plant.

The water taken in by the roots circulates through the sap vessels of
the plant, and, from various causes, is drawn up towards the leaves
where it is evaporated. This water contains the _nitrogen_ and the
_inorganic matter_ required by the plant and some carbonic acid, while
the water itself consists of _hydrogen_ and _oxygen_.

Thus we see that the plant obtains its food in the following manner:--

CARBON.--In the form of _carbonic acid_ from the atmosphere, and from
that contained in the sap, the oxygen being returned to the
air.

OXYGEN } From the elements of the water constituting the sap.
& }
HYDROGEN.}

NITROGEN.--From the soil (chiefly in form of ammonia). It is carried
into the plant through the roots in solution in water.

INORGANIC} From the soil, and only _in solution_ in water.
MATTER. }

[What changes does the food taken up by the plant undergo?]

Many of the chemical changes which take place in the interior of the
pl