No doubt plants have the powers to select various materials. Upon the same acre of land a hundred different plants may feed and manufacture as many varieties of products, sweet, bitter, sour, poisonous, nutritious, fragrant, offensive, green, yellow, red, and so on through the entire list. As has already been suggested, many vegetable products which are quite diverse in character, are either identical or quite similar in chemical composition. Starch, whether obtained from the potato, the root of the carrot, the kernel of corn, the leaves of the cabbage, or the cotyledons of the bean, is composed of six atoms of carbon, ten atoms of hydrogen, and five atoms of oxygen. Sago, tapioca, bread fruit, arrowroot, and scores of other plant products have the same proportions. Woody fiber whether from the root, stem or branch, woven into cloth, built into houses, twisted into rope, made into paper, used as fuel, or manufactured into furniture, is C₆H₁₀O₅.
Slight variations in composition often produce marked differences. The introduction of the least ferment into sugar (C₆H₁₂O₆) would break it up into two deadly poisons, alcohol (2C₂H₆O) and carbonic acid gas (2CO₂—two molecules of each). A slight addition of oxygen spoils all the sweetness of the preserves.
The rhubarb manufactures oxalic acid, the grape tartaric acid, the apple malic acid, the lemon citric acid, the oak tannic acid, from carbon, hydrogen and oxygen, by simply varying the number of their atoms.
If we add one atom of oxygen (C₁₀H₁₆O) to the constituents of the volatile oils previously mentioned, we form a new group comprising camphor, wintergreen, spearmint, cinnamon, bitter almonds, and many others.
Notwithstanding the great uniformity in the composition of various vegetable products, it is now well understood that one crop may restore to land what another has removed, hence the modern agricultural doctrine of
ROTATION OF CROPS.
In southeastern Virginia you find many pine forests in which may be traced the ridges of the corn rows. These fields were planted with corn continuously, until the soil became so impoverished that it would not yield a crop. They were then abandoned and allowed to grow up to pines.
A better system would have secured perpetual fertility. The soil of England produces far more than formerly, even after the cultivation of a thousand years. China furnishes a still more remarkable example of productiveness.
The amount of the earth’s crust which is concerned in the support of life is exceeding small. The natural tendency is constantly to diminish this.
Rains and rivers bear away the best of the soil and deposit it in the lakes and seas. Some inhabitant of our earth in the far future, may secure the benefit of these stores, when the beds of the present seas and oceans shall have risen above the waters and become the continent.