1140. What is the difference between an animal, a plant, and a mineral?

The great naturalist, LinnÆus, used to say that animals grow, live, and feel; plants grow and live; and minerals grow.

Animals are here defined to enjoy three conditions of existence; plants two conditions; and minerals one condition.

This definition has, in latter days, been held to be unsatisfactory, since there are a few plants that are supposed to feel, and a few animals that are supposed to have even less feeling than the sensitive plants alluded to.

The concise definition by LinnÆus, nevertheless, is true, as far as regards a vast majority of the bodies constituting the three great kingdoms of nature. And it may be sufficient to say that

Animals—grow, live, feel, and move.

Plants—grow and live.

Minerals—grow, by the addition of particles of inorganic matter.

If we now state the few exceptions that are admitted to this definition, we shall bring the explanation as near to the truth, as the present state of knowledge will permit.

"And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree, yielding seed; to you it shall be for meat."—Genesis i.

1141. Why is it understood that some plants feel?

Because the sensitive plant closes its leaves on being touched; the Venus's fly trap closes its leaves upon flies that alight upon them; others close upon the approach of rain, and at sunset, and open at sunrise, and turn towards the sun during its daily transit.1142. Why is it understood that some plants move?

Because certain sea-weeds throw off undeveloped young plants, which move through the water by the aid of fine cilia, or muscular hairs, until they find a suitable place upon which to attach themselves.

The roots of plants will penetrate through the ground in the direction of water, and of favourable soil.1143. Of what elementary substances are plants composed?

Of carbon, oxygen, hydrogen, and nitrogen.1144. Whence do plants derive those substances?

From the air, the earth, and water.1145. How do plants obtain carbon?

They obtain it chiefly from the air, in the form of carbonic acid gas. The carbon, of the carbonic acid gas, which is thrown out by the breath of animals, and by other processes in nature, is absorbed by the leaves of plants, and the oxygen which had united with the carbon to form the carbonic acid gas, is again set free for the use of animals.1146. How do plants obtain oxygen?

They obtain it from the atmospheric air. But as they do not require a large amount of oxygen for their own use, they throw off the amount which is in excess, after having separated it from the other elements with which it was combined when taken up by them. From the humble blade of grass, to the stately tree of the forest, plants operate to purify the air, and to correct and counteract the corruption of the air, by the myriads of animals inhabiting the earth.

It has been generally stated that plants in rooms purify the air by absorbing carbonic acid by day, and releasing a part of the oxygen; but that, as the presence of light is necessary to produce this action, they do not restore oxygen to the air, by night, but, on the contrary, give off carbonic acid gas. Therefore it has been stated that plants in rooms by night are unhealthy. Mr. Robert Hunt, one of the ablest chemists of the present time, makes the following remarks upon this subject in his "Poetry of Science:"—

"The heavens declare the glory of God: and the firmament showeth his handy work. Day unto day uttereth speech, and night unto night showeth knowledge."—Psalm xix.

"The power of decomposing carbonic acid is a vital function which belongs to the leaves and bark. It has been stated, on the authority of Leibig, that during the night the plant acts only as a mere bundle of fibres—that it allows of the circulation of carbonic acid and its evaporation, unchanged. In his eagerness to support his chemical hypothesis of respiration, the able chemist neglected to enquire if this was absolutely correct. The healthy plant never ceases to decompose carbonic acid during one moment of its existence; but during the night, when the excitement of light is removed, and the plant reposes, its vital powers are at their minimum of action, and a much less quantity is decomposed than when a stimulating sun, by the action of its rays, is compelling the exertion of every vital function."

In hot, swampy countries, where vegetation is very rapid, and the soil loaded with decomposing carbonic matter, the plants absorb more carbonic acid than they require, and they then evolve carbonic acid gas from their leaves. Hence such climates as the West Indies are injurious to life, though favourable to vegetation.1147. How do plants obtain hydrogen?

They obtain hydrogen in combination with oxygen in water, and with nitrogen, in the form of ammonia, as which it exists in animal manures.1148. How do plants obtain nitrogen?

From the atmospheric air, and from the soil, in which it is combined with other elements.1149. How do plants apply these elements to the formation of their own structures?

When those substances which form the food of plants are absorbed, either by their leaves or their roots, they are converted, with the aid of water, into a nutritive sap, which answers the same purposes in plants as blood does in animals.1150. How is the nutritive sap applied to the growth and enlargement of the plant?

Every seed contains a small amount of nutrition, sufficient for the sustentation of the germ of the plant, until those vessels are formed, by which the nutritive elements can be absorbed and used for the further development of the living structure.

The earth, penetrated by the sun's rays, warms the sleeping germ, and quickens it into life. For a short time the germ lives upon the seed, which, moistened and warmed by the soil, yields a kind of glutinous sap, out of which the first members of the plant are formed. And then the tender leaf, looking up to the sky, and the slender rootlet penetrating the soil, begin to draw their sustenance from the vast stores of nature.

"He causeth the grass to grow for the cattle, and herb for the service of man: that he may bring forth food out of the earth."—Psalm civ.

1151. Of what do vegetable structures consist?

Of membranes, or thin tissues, which, being variously arranged, form cells, tubes, air passages, &c. Of fibres, which form a stronger kind of membrane, and which is variously applied to the production of the organs of the plants. And of organs, formed by those elementary substances, by which the plants absorb, secrete, and grow, and fulfil the conditions of their existence.1152. Why are seeds generally enveloped in hard cases?

Because the covering of the seed, like the shell of an egg, is designed to preserve the germ within from the influence of external agencies, until the time for development has arrived, and the conditions of germination are fulfilled.1153. Why does a seed throw out a root, before it forms a leaf?

Because moisture, which the root absorbs from the earth, is necessary to enable the germ to use the nutrition which the seed itself contains, and out of which the leaf must be eliminated. Moisture forms a kind of gluten, in which the starch of the seed is dissolved, and converted into sugar, the sugar into carbonaceous sap, and the sap into cellular tissue and woody fibre, as the leaves present themselves to the influence of the air and light.1154. Why does a plant grow?

Because, as soon as membranes and vessels are organised in the young germ, the nutritive fluid, formed by its first organs, begins to move through the fine structures, and from that time the plant commences to incorporate with its own substance the elements with which it is surrounded, that are suitable to its development.

"Can the rush grow up without mire? can the flag grow without water? Whilst it is yet in his greenness, and not cut down, it withereth before any other herb."—Job viii.