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 const.i.tuting 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 plant are well understood, but they require too much knowledge of chemistry to be easily comprehended by the young learner, and it is not absolutely essential that they should be understood by the scholar who is merely learning the _elements_ of the science.

It is sufficient to say that the food taken up by the plant undergoes such changes as are required for its growth; as in animals, where the food taken into the stomach, is digested, and formed into bone, muscle, fat, hair, etc., so in the plant the nutritive portions of the sap are resolved into wood, bark, grain, or some other necessary part.

The results of these changes are of the greatest importance in agriculture, and no person can call himself a _practical farmer_ who does not thoroughly understand them.

CHAPTER VI.

PROXIMATE DIVISION OF PLANTS, ETC.

We have hitherto examined what is called the _ultimate_ division of plants. That is, we have looked at each one of the elements separately, and considered its use in vegetable growth.

[Of what do wood, starch and the other vegetable compounds chiefly consist?

Are their small ashy parts important?

What are these compounds called?

Into how many cla.s.ses may proximate principles be divided?

Of what do the first cla.s.s consist? The second?

What vegetable compounds do the first cla.s.s comprise?]

We will now examine another division of plants, called their _proximate division_. We know that plants consist of various substances, such as wood, gum, starch, oil, etc., and on examination we shall discover that these substances are composed of the various _organic_ and _inorganic_ ingredients described in the preceding chapters. They are made up almost entirely of _organic_ matter, but their ashy parts, though very small, are (as we shall soon see) sometimes of great importance.

These compounds are called _proximate principles_,[G] or _vegetable proximates_. They may be divided into two cla.s.ses.

The first cla.s.s are composed of _carbon_, _hydrogen_, and _oxygen_.

The second cla.s.s contain the same substances and _nitrogen_.

[Are these substances of about the same composition?

Can they be artificially changed from one to another?

Give an instance of this.

Is the ease with which these changes take place important?

From what may the first cla.s.s of proximates be formed?]

The first cla.s.s (those compounds not containing nitrogen) comprise the wood, starch, gum, sugar, and fatty matter which const.i.tute the greater part of all plants, also the acids which are found in sour fruits, etc.

Various as are all of these things in their characters, they are entirely composed of the same ingredients (carbon, hydrogen and oxygen), and usually combined in about the _same proportion_. There may be a slight difference in the composition of their _ashes_, but the organic part is much the same in every case, so much so, that they can often be artificially changed from one to the other.

As an instance of this, it may be recollected by those who attended the Fair of the American Inst.i.tute, in 1834, that Prof. Mapes exhibited samples of excellent sugar made from the juice of the cornstalk, starch, linen, and woody fibre.

The ease with which these proximates may be changed from one to the other is their most important agricultural feature, and should be clearly understood before proceeding farther. It is one of the fundamental principles on which the growth of both vegetables depends.

The proximates of the first cla.s.s const.i.tute usually the greater part of all plants, and they are readily formed from the carbonic acid and water which in nature are so plentifully supplied.

[Why are those of the second cla.s.s particularly important to farmers?

What is the general name under which they are known?

What is the protein of wheat called?

Why is flour containing much gluten preferred by bakers?

Can protein be formed without nitrogen?

If plants were allowed to complete their growth without a supply of this ingredient, what would be the result?]