As no seed will germinate unless a certain degree of heat is present, so also does it require that a certain quantity of water be in contact with its outer skin or integument; and this is required, not only to soften this covering, and thus permit the enlargement of the cotyledons (seed lobes) always. preceding germination, but also to afford that water to the internal components of the seed, without which the chemical changes necessary for the nutriment of the embryo plant will not take place.

Pure water, or some other liquid of which it is a large constituent, is absolutely necessary: no other fluid will advance germination a single stage.

Liebig, from actual experiment on a large scale, states that both rain and snow contain ammonia; and if there be only one-fourth of a grain in each pint of water, the annual deposition from the atmosphere would be more than sufficient, on half an acre of ground, to give all the nitrogen contained in the vegetable albumen of 150 cwt. of beet root. Rain water also contains a peculiar organic substance, analogous to the extractive matter and gluten of plants, though differing from them chemically. To this substance Dr. Daubeney has given the name of Pyrrhine. Traces of salts and oxides have also been found in rainwater; but, compared with all other naturally produced, it is so pure, and so abounds with the gases beneficial to plants, that none other can equal it for their service. That obtained from ponds or springs often contains matters offensive or deleterious to plants. Those known as hard water, containing in excess salts of lime or magnesia, are invariably prejudicial, and pond water is scarcely less so. If it be stagnant and loaded with vegetable extract, it is even worse than hard spring water.

These last named, if obliged to be employed for tender plants, should have a pint of the ammoniacal water of the gas works mixed thoroughly with every sixty gallons, an hour or two before they are used.

If pond-water be clear, and not only not loaded with putrid or mineral matters, but containing Conferva, or other growing aquatic plants, it may then be used very beneficially for the watering of plants. This is ascertained from long experience, and it is explained by the fact, that such water contains an excessive amount of oxygen gas. This excess is greater in proportion to the brightness of the sunshine, and the length of time to which the water has been exposed to it. During such bright weather, the aquatic plants give out oxygen most abundantly. M. Morren found, that in the afternoon of a sunshiny day, the oxygen in such water amounted to sixty per cent. of the bulk of the air which it contained.

Water being such an essential application to the seed, as well as to the growing plant, it may be observed that the source from whence it comes is by no means immaterial. The best for the gardener's purpose is rain water, preserved in tanks sunk in the earth, and rendered tight by puddling, or bricks, and cement To keep these replenished, gutters should run round the eaves of every structure in the garden, and communicate with these tanks. Every 100 cubic inches of rain water contain more than four cubic inches of air, of which more than half are carbonic acid gas, and the remainder nitrogen and oxygen, in the proportion of sixty-two of the former to thirty-eight of the last named.

A seed placed in a situation where it is supplied with the desirable degrees of heat, moisture, and air, begins immediately to enlarge in sue. This is occasioned by its absorbing moisture, which, passing into the cotyledons, causes their immediate increase in size. The rapidity of this process is remarkable, and warns the gardener from disturbing the seed after it is once committed to the ground. A few choice peas, from which to raise stock, being sown, accidentally, in ground devoted to another crop, were removed after twenty hours, and were not again committed to the ground for some days. Not one of them produced a fruitful plant, and only two or three vegetated.

Moisture is absorbed, and causes the immediate enlargement of the parts of the seed; and this moisture, though it will and does penetrate through the surface of the integuments, yet is chiefly imbibed through the hilum or scar. It passes to the cotyledons, causing their enlargement, and setting in motion their elaborating powers for the nutriment of the radicle and plantlet; for, if they are removed, or if they have been injured by insects, the seed does not germinate; and if they are removed even after the radicle is developed into a root, the plant's vegetation ceases.

Lastly, if seeds of plants loving a fertile soil be sown along the partition, dividing a vessel into two portions, of which one portion is filled with rich earth, and the other with sand, though both portions are equally moist, equally loose, and equally warm, all the radicles will direct their course into the fertile soil. - The Science of Gardening.