acid, is retained in the plant to constitute a portion of its bulk, while the oxygen goes off through the pores of the leaf. The pores in the under side of the leaf usually effect the absorption, the decomposition goes on in the interior, and the oxygen is given off through the pores on the upper part. These pores have for their office to give off, while that of the others is to receive. Some plants will live for a long time, if the under surface of the leaves is kept constantly wet; if the upper only be wet, the plant soon dies. If either surface be varnished, so as to stop the pores, great injury results.

During daylight, the leaves are constantly absorbing carbonic acid, and giving off oxygen; but as soon as the sun goes down, a change takes place; an examination will now show that it is carbonic acid which passes off from the leaves, and oxygen that is being absorbed. It is just the reverse of what goes on during the day.

a. This curious fact shows why it is that plants grow so rapidly in the long days of summer. The nights are then comparatively a small portion of the day, so that for by far the greater part of the twenty-four hours the plant continues to absorb carbonic acid, and to build itself up with the carbon thus obtained.

b. In Greenland and Kamschatka the summer is not more than two or three months, but during that time it is always daylight, the sun scarcely going below the horizon at all. Certain plants are thus enabled to grow so fast as to mature and ripen their seed, even in that short sumWe see how this beautiful provision of nature tends to equalize different climates. If the nights of the short Greenland summers were even so long as our shortest, their crops would never ripen; but as they have nearly perpetual day, they can get enough food from their fields to sustain life during a large part of their long winter.

mer.

CARBON ALSO OBTAINED BY PLANTS FROM THE SOIL.

We see that plants are able to obtain much carbon from the air, but it is found that a considerable quantity comes from the soil also. This is all, in one form or another, drawn in through the roots. The rain water which falls upon the surface, and all of the spring water found there already, contains some carbonic acid dissolved. This water entering the roots, carries with it a variety of substances in solution, which the plant seems to use or not as it may require; among these is carbonic acid. This is probably the chief form in which carbon is obtained from the soil; but there exist in contact with the roots, other sources of this important article of food. Every soil contains more or less of organic matter, derived from the decay after death of plants and animals. Where abundant, this gives a black color to the soil, and one containing

a large proportion of it is frequently described by farmers as a vegetable mould. While plants, etc. are decaying to form this mould, various compounds containing carbon are the result. Quite a number of these have been examined by chemists, but it is not necessary to say much of

them here.

a. Humus is a name often given to the black mould of a rich vegetable soil, and this probably because a great part of the mould consists of a substance called humic acid. This acid may be obtained by boiling some rich mould or peat in a solution of common soda, continuing for an hour or two; filtering through a piece of blotting paper, and then making the liquid quite sour with muriatic acid. Little brown flocks will soon begin to appear, and will fall to the bottom: these are humic acid. b. This substance may serve as a specimen of a large class that are contained in the organic part of the soil. They all consist of carbon, oxygen and hydrogen and in many situations are extremely abundant. They do not decay or dissolve very easily, and it is not supposed that plants get a large part of their carbon in this way. It seems certain, however, that they do get some; and it is found that in most cases where soils contain much of this organic matter, they are quite fertile. In all ordinary situations, it is supposed that at least two-thirds of the carbon in plants comes from the air, the remaining third in various forms from the soil. This is shown by the fact that plants cultivated year after year, cause the organic matter of a soil to diminish quite rapidly.

SOURCE OF THE OXYGEN AND HYDROGEN OF PLANts.

Beside carbonic acid, the leaves of plants absorb through their pores a large quantity of water. During the day, when the hot sun is upon them, the evaporation is of course far more than the absorption, and in a dry time the leaves may be seen to droop in the afternoon; but let the sun be obscured and the atmosphere become misty and damp, and they soon absorb enough moisture to strengthen their failing stems. Every farmer knows that a light shower, which only moistens the leaves without wetting the ground at all, will revive his crops for many hours. Nothing in this case can have been taken in through the roots.

Water, as has been said, is composed of oxygen and hydrogen. These two bodies are needed by the plant, and water is consequently not only of service in moistening its various parts and furnishing a circulating fluid, but gives its oxygen or its hydrogen or both, as the plant may happen to require. Water has a peculiar adaptation to this purpose, and to others equally useful in the interior of the plant, in the facility

with which it is decomposed. Carbonic acid and other chemical substances only decompose with great difficulty; but the elements of water, a substance so universally diffused and so indispensable, separate easily, affording hydrogen here, oxygen there, to the necessities of the plant.

SOURCES OF THE NITROGEN OF PLANTS.

We have now seen how the plant gets carbon, hydrogen and oxygen in abundance; but there is yet one more of the organic bodies, which is so necessary to the plant; this is nitrogen: it remains for us to consider the most probable source of this gas. a. As it has been said that the atmosphere consists of oxygen and nitrogen, we might naturally conceive that the leaves absorb this gas, as well as carbonic acid. Experiments have shown that this is not the case to any extent. After many careful trials, it has not yet been certainly proved that any nitrogen at all is obtained by the greater number of plants in this way. If there is, the quantity must be in most cases very trifling indeed.

b. This is one of the most remarkable points connected with the nutrition of plants. Here we have, in the air which surrounds the plant, and presses against every part of it, an immense quantity of the gas nitrogen. It constitutes four-fifths of the whole atmosphere, and yet we cannot find that plants absorb it in any quantity whatever. On the contrary, as we have seen, they select out another kind of gas, carbonic acid, although it is present in so small a proportion as th. This shows conclusively that the leaves do not draw in through their pores every thing that is presented to them indiscriminately, but that they have a power of choosing those kinds of food best adapted to their

wants.

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c. Thus the smallest plant has the power of doing what man by his unaided senses never has been able to accomplish, and which he has only learned to do by artificial means within a few years. Every little worthless weed by the wayside has its leaves spread, its thousands of mouths open, selecting and drawing in from the passing air food best adapted to its wants.

As plants obtain, according to the above statements, little if any of their nitrogen from the air directly through their leaves, they must obviously get it in some way through their roots. There are two bodies which are now considered the chief sources of supply: these are called ammonia and nitric acid.

Ammonia is a gas, composed of nitrogen and hydrogen. We do not find it largely in this shape, however, on account of the strong tendency Assembly, No. 175.]

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which it has to unite with other bodies, such as carbonic acid, sulphuric acid, etc. When it cannot find any thing else, it is at once absorbed by water, which will take up an immense quantity of it before becoming saturated. A pint of cold water will absorb between 600 and 700 pints of ammonia. The aqua ammonia of the shops, is water through which ammonia has been passed until it is very strong. By smelling of it, the extremely pungent and peculiar odor of ammonia is perceived. The strong aqua ammonia is so powerful in its effects as to take away the breath, and cause a momentary suffocation. A more agreeable form of ammoniacal odor is in the ordinary smelling salts. These are usually nothing more than carbonate of ammonia, scented in various ways with other perfumes.

The properties of ammonia ought to be understood by every farmer, because it is a substance of much importance; it does not exist so abundantly in the soil as do many or most other necessary ingredients, and consequently he ought to know how best to increase its amount, and how to keep it on his farm when he has got it there.

Ammonia is very easily lost, because driven from its combinations with great facility. If, for instance, you mix with muriate of ammonia, a compound which has little or no smell of the gas, some quicklime, and rub the two together, there will immediately a strong smell of ammonia be perceived, passing off into the air and disappearing. This is the reason why quicklime should not be mixed with manures containing ammonia, as that gas is driven off by it, and the value of the manure greatly diminished.

Nitric acid, (common aquafortis,) is another important source of nitrogen. This acid is composed of nitrogen and oxygen. It is to be found in druggists' shops, and is a nearly colorless liquid, having a peculiar smell, and being extremely sour and corrosive. a. When strong, it destroys the skin, and in all cases turns it of a deep yellow color, which cannot be removed by washing. b. It eats holes through cloth, turning it to a bright red color. c. Like ammonia, and the acids before mentioned, we do not find it naturally as a pure substance; it is always combined with something else. One of the most common forms is nitrate of potash, or saltpetre. Nitrate of soda is also often found in nature. d. In South America, this latter is so abundant as to be brought away by the shipload. It is in the form of such compounds as these that nitric acid is present in the soil. They are easily dissolved in water, can be received into the circulation of plants through their roots, and can furnish nitrogen as readily as ammonia..

In some situations more nitrogen is received into the plant as ammonia, than from any other source; in others more as nitric acid. I consider that this is owing simply to the quantity of either that may be present in different localities. Both kinds of manure produce remarkable results when applied to the soil of most farms; and these effects are nearly or quite identical in appearance, showing that in both cases, nitrogen caused the improvement, and that between these two forms of applying it there is little choice.

OF THE ORGANIC SUBSTANCE OF PLANTS.

STRUCTURE AND FUNCTIONS OF THE PLANT IN ITS SEVERAL PARTS.

The different external parts of plants are well known; they consist of roots, stems, bark or epidermis, and leaves.

The internal structure and the functions of the roots are not so perfectly understood as that of the other parts, owing to the difficulty of knowing exactly what occurs underground. At a short distance beneath the surface they begin to divide, sending out little rootlets in every direction, and at the extreme end of each is a small bundle of soft, minute, white fibres. These are all so many mouths for the nourishment of the stem. If you place the roots of a growing tree in certain colored liquids, its body will soon become colored. This part of the plant has, to a considerable extent at least, a power of selection, as it is found that certain substances are admitted to the exclusion, either partial or total, of othSome coloring solutions for instance, as above, enter with facility and tinge the whole stem in a short time, while others are scarcely absorbed at all. The same must, in a degree, be true of various kinds of food, as we find that far more of one kind is taken than of another, even when both are present in equal quantities.

ers.

In the stem are numerous little tubes running up and down, which serve to convey the sap absorbed by the roots up to the leaves. It passes up in the interior vessels or tubes, and passes down in the exterior, or just under the bark. This can be shown by the example of the tree and the colored fluid, just referred to; the inner part of the tree will be colored first, and finally the outer, in the descent of the sap, after it has passed out to the extremities of the branches.

There is then a regular circulation between the soil and the plant; sap flows up, having been formed in the roots and stem, out of the various substances drawn in from the soil, and ultimately flows down again next the bark and out into the soil.

During its circuit the sap undergoes many changes, and deposits such