IMPROVEMENTS IN PROGRESS.

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ample guarantees, as I have said, the success of similar operations in the United States of America and in our British colonies; while the now advanced condition, especially of our chemical knowledge, both in regard to the soil which is to be cultivated and to the plants we wish to grow, insures a far more easy and certain advance to the process of restoration in these countries than in past times could take place among ourselves; less waste of time and money in ill-adjudged experiments, and less cost of labour in all the necessary operations of husbandry.

CHAPTER IV.

THE PLANT WE REAR.

A perfect plant, what.-Effects of heat upon it.-Contains carbon, water, and mineral matter. Relations of the plant to the air.-Structure of the leaf.--Its pores absorb carbonic acid, and give off oxygen gas.-Relations to water.-Structure of the root. -Purposes served by water.-Relations to the soil-Plants affect pesty, sandy, loamy, or clay soils.-Effects of the drain, of lime, or of manure.-The art of manuring. How the colours of flowers may be changed.-Effect of culture upon wild plants.-The carrot, the cabbage, the turnip.-Garden fruits, flowers, and vegetables-Origin of wheat and its varieties.-How these changes are produced.Plants which follow the footsteps of man; why they follow him.-Rapidity of growth in favourable circumstances.-The yeast plant in grape juice.-Manufacturo of dry yeast.-Chemical changes within the plant.-Production of numerous peculiar substances-medicines, perfumes, and things useful in the arts.-The green of the leaf, and the poison of the nettle.-The covering of the ripe potato, apple, and young twig.-General purposes served by vegetation.-It adorns the landscape.In relation to dead nature, it purifies the atmosphere, produces vegetable mould, and forms deposits of combustible matter.-In relation to living animals, it supplies subsidiary luxuries and comforts, but its main use is to feed them.-Numerous interesting chemical inquiries suggested by the natural diversities and different effects of the vegetable food consumed by herbivorous and omnivorous races.

A FAMILIARITY With the chemical relations of the plant we rear makes still more apparent the relations of chemistry to the soil we cultivate.

A perfect plant consists essentially of two parts-the stem and the leaf. The root is an underground extension of the stem, as the bark is a downward prolongation of the leaf. The several parts of the flower, also, are only changed leaves.

When any part of a plant is heated in a close vessel, it

THE PLANT AND THE AIR.

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gives off water, vinegar, and tarry matters, and leaves behind a black, bulky, coaly mass, known by the name of wood charcoal; or if billets of wood be heaped up in the open air, covered carefully over with sods, and smother-burned, as it is called, with little access of air, the tar and other matters escape into the atmosphere, while the charcoal remains undissipated beneath the sod. This charcoal is an impure form of carbon. The manufacturer of wood-vinegar collects the volatile substances as the more important products. The charcoal-burner allows them to escape, the black residue being the object of his process. Both experiments, however, are the same in substance, and both prove that carbon and water form large parts of the weight of all plants.

If a piece of wood charcoal be burned in the air it gradually disappears; but when all combustion has ceased, there remains behind a small proportion

of ash. The same is seen if a portion taken from any part of a living plant be burned in the air. Even a bit of straw kindled in the flame of a candle (fig. 14), and allowed to burn, will leave a sensible quantity of ash behind. All plants therefore, and all parts of plants, besides water and carbon, contain also a sensible proportion of mineral inorganic mat

Fig. 14.

ter which is incombustible, and which remains unconsumed when they are burned in the air.

The carbon of the plant is chiefly derived from the air, the water and the mineral matter chiefly from the soil in which it grows. Thus the plant we rear has close chemical relations with the air we breathe, with the water we drink, and with the soil we cultivate. I shall briefly illustrate these several relations in their order.

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First. The plant is in contact with the air, through its leaves and its bark. The surface of the leaf is studded over with numerous minute pores or mouths (stomata), through which gases and watery vapour are continually entering or escaping, so long as the plant lives. In the daytime they give off oxygen and absorb carbonic acid gas. During the night this process is reversed-they then absorb oxygen and give off carbonic acid.

We have already seen that carbonic acid consists of carbon and oxygen.* It is from the large excess of this gas which plants absorb during the day that the greater part of the carbon they contain is usually derived.

The number and activity of the little mouths which stud the leaf are very wonderful. On a single square inch of the leaf of the common lilac as many as 120,000 have been counted; and the rapidity with which they act is so great, that a thin current of air passing over the leaves of an actively-growing plant is almost immediately deprived by them of the carbonic acid it contains.

The gas thus absorbed enters into the circulation of the plant, and there undergoes a series of chemical changes which it is very difficult to follow. The result, however, we know to be, that its carbon is converted into starch, woody fibre, &c., to build up the plant, while its oxygen is given off to maintain the purity of the air.

These pores of the leaf absorb also other gascous substances in smaller quantity—such as ammonia, when it happens to approach them; and especially they absorb watery vapour, when previous heat or drought has dried the plant, and made the leaves droop soft and flaccid. Hence the natural rain enlivens and invigorates the herbage, and the artificial shower gives new life to the tenants of the conservatory. The falling water not only supplies their want of

See THE AIR WE BREATHE.

WATER AND THE ROOTS.

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fluid, but it washes also the dusty surface of the leaves, and clears their many mouths, so that with fresh vigour they can suck in new nourishment from the surrounding air.

The green bark of the young twig is perforated with pores like the green leaf, and acts upon the air in a similar way; but as it hardens and gets old the pores become obliterated, and it ceases to aid the leaves in absorbing carbonic acid, or in giving off oxygen to the atmosphere.

Second. The water which fills the vessels of the plant, though partly derived from the air in seasons of drought, and drunk in by the leaves from the dews and falling showers, is principally sucked up by the roots from the earth in which it grows. These roots, as I have said, are only downward expansions of the stem. At the surface of the ground they exhibit a bark without and a pith within the woody portion. But as they descend, these several parts disappear, and graduate into a porous, uniform, spongy mass, which forms the ends of the fibry rootlets. Upon the surface of these rootlets the microscope enables us to perceive numerous minute hairs which, like hollow horns, thrust themselves latérally among the particles of the soil. Through these hollow hairs, as it is believed, the plant draws from the earth the supplies of water it constantly requires, and which in droughty weather it so copiously pours out from its leaves into the air.

How interesting it is to reflect on the minuteness of the organs by which the largest plants are fed and sustained. Microscopic apertures in the leaf suck in gaseous food from the air; the extremities of microscopic hairs suck a liquid food from the soil. We are accustomed to admire, with natural and just astonishment, how huge rocky reefs, hundreds of miles in length, can be built up by the conjoined labours of myriads of minute insects labouring together on the surface of a coral rock; but it is not less wonderful that,