Germination is the sprouting, or first step in vegetation of a seed. To enable it to germinate, it must have a perfectly-developed embryo, and be ripe, or nearly ripe. It must not be too old.

The following list, furnished by the late Mr. Loudon, shows the greatest age at which some of our common garden seeds germinate freely; and this result of experience is quite concurrent with our knowledge of their chemical constitution: -

One Year

Peas, beans, kidney beans, carrot, parsnip, oraches, herb-patience, rhubarb, elm, poplar, and willow. Two years. - Radish, salsafy, scorzonera, purslane, the alliums, car-doon, rampion, alisander, love-apple, capsicum, egg-plant. Three years. - Sea-kale, artichoke, lettuce, marigold, rue, rosemary. Four years. - Brassicas, skirret, spinach, asparagus, endive, mustard, tarragon, borage. Five and six years. - Burnet, sorel, parsley, dill, fennel, chervil, hyssop. Ten years. - Beet, celery, pompion, cucumber, melon".

Mr. Loudon may be safely received as good authority on subjects which he investigated. If the age at which the vitality of certain seeds cease in England as expressed herein, be correct, it proves a result in that climate different from our own. For instance, peas, beans, carrots, etc, vegetate freely in the United States when two or three years old, sea-kale seldom after the first year, and so of other seeds enumerated in the list.

A certain degree of warmth is essential; for no known plant has seed that will germinate below or at the freezing point of water. A temperature above 32° of Fahrenheit's thermometer therefore is requisite. But on the other hand, the temperature must not be excessively high. Even no tropical seed, probably, will germinate at a temperature much above 120° F., and we know from the experiments of M. M. Edwards and Colin, that neither wheat, oats, nor barley will vegetate in a temperature of 113°.

Every seed differing in its degree of excitability, consequently, has a temperature without which it will not vegetate, and from which cause arise the consequences that different plants require to be sown at different seasons, and that they germinate with various degrees of rapidity. The gardener should always bear in mind that it would be a very erroneous conclusion, because a seed does not germinate at the accustomed time, that therefore its vegetating powers are departed. No two seeds taken from the same seed-vessel germinate precisely at the same time; but on the contrary, one will often do so promptly, while its companion seed will remain dormant until another year.

M. de Candolle relates an instance where fresh tobacco seedlings continued to appear annually for ten years on the same plot, though no seed was sown after the first sowing; and the same phenomenon usually occurs for two or three years, when the seed of either the peony or hawthorn are sown. Why one seed is more easily excited than another is as yet unexplained; but the wisdom of this one of many provisions for avoiding the accidental extinction of a species in any given locality is readily discerned. An ungenial spring may destroy the plants from those seeds which first germinated; but this could scarcely occur also to those of the second and third year, or even to those which were only a few weeks later in their vegetation.

It is not possible to enunciate a general rule relative to germinating temperatures, requiring no exceptions; but in general, for the seeds of plants, natives of temperate latitudes, the best germinating temperature is about 60°, and for those of tropical plants about 80°; and the necessity for such temperatures depends upon the same causes that prevent the incubation of eggs, unless they be kept for a certain period at a temperature of about 100°.

As no seed will germinate unless a certain degree of heat is present, so also does it require that a certain quantity of water is 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 internal components of the seed, without which the chemical changes necessary for the nutriment of the embryo plant will not take place. As water is essential to germination, and only a certain quantity is required for its healthy progress, so is it by no means a matter of indifference what matters it holds in solution. Until germination has commenced, no liquid but water at common temperatures will pass through the integuments of a seed.

So soon as germination has commenced, this power to exclude foreign fluids ceases; but the organs starting into activity, the radicle and the plumule are so delicate, that the weakest saline solutions are too acrid and offensive for them. It may be noted as a warning to those who employ steeps for seed, with the hope of promoting the vigour of the future plant, that they must keep the seed in those steeps a very few hours. In forty-eight hours, if the temperature be 60° or more, putrefaction commences, and germination is weakened, or entirely destroyed. M. Vogcl, of Munich, has published an extended course of experiments upon this subject, and they fully confirm my opinion that salts, innoxious when the plant is of robust and advanced growth, are fatal to it at the time of germination.

The presence of one of the constituent gases of the atmosphere, oxygen, is also essential to germination. It is necessary that the oxygen should penetrate to the cotyledonous parts of the seed, as is evident by the changes which take place during germination, and it is further proved by experiment. When healthy seed is moistened and exposed in a suitable temperature to atmospheric air, it absorbs the oxygen only. This power of separating one gas from the others appears to reside in the integuments of the seed, for old seeds lose the power of absorbing the oxygen, and, consequently, of germinating; yet they will frequently germinate if soaked in an aqueous solution of chlorine - a gas which has the power of attracting hydrogen from water, and others of its compounds, and releasing the oxygen, doing so in the case of seeds within their integuments, as well as without-side. Humboldt and Saussure have also shown that the application of chlorine to seeds accelerates its germination; and Cress seed, which under ordinary circumstances requires some days to complete the process, they found effected it in no more than three hours. The late Mr. George Sinclair, author of the excellent Hortus Grami-neus Woburnensis, also informed me that he employed chlorine with singular success.

He obtained it by mixing a tablespoonful of muriatic acid with a similar quantity of black oxide of manganese, and half a pint of water. After allowing the mixture to remain two or three hours, the seed is to be immersed in the liquid for a similar period, and then sown. Another, and I consider the most eligible mode of applying the chlorine was also suggested to me by the same distinguished horticulturist. In this way he said he made tropical seeds vegetate which refused to germinate by other modes of treatment. He placed the mixed ingredients mentioned above in a glass retort, inserting its bulb in the hot-bed, and bringing its beak under the pot in which the seeds were sown, connecting it with the draining aperture of the pot. The chlorine gas is gradually evolved, passing through the earth of the pot to the seeds, with more or less rapidity, according to the heat employed. This absolute necessity for the presence of oxygen is a reason why seeds will not germinate if buried beyond a certain distance from the earth's surface; and why clayey soils often fail of having a good plant, an impervious coat of the clay enveloping the seed, and preventing the air's access.

How oxygen operates in aiding the seed to develope the parts of the embryo plant, we cannot even guess - we only know that most seeds have more carbon (pure charcoal) in their composition than other parts of their parent plant; that the oxygen absorbed by the seeds combines with a portion of that carbon, and is emitted in the form of carbonic acid. These are the attendant phenomena, - but we can penetrate the mystery no farther.

I have never been able to discover that light has injurious influence over germination, and in those experiments apparently proving the contrary, due care was not taken to prevent the seed being exposed to a greater degree of dryness as well as to light.

If seed be placed on the surface of a soil, and other seed just below that surface, and care be taken to keep the former constantly moist, it will germinate just as speedily as the buried seed, and if exposed to the blue rays only of the spectrum by being kept under a glass of that colour, even more rapidly. Therefore the object of sowing the seed below the surface, is for the purposes of keeping it in a state of equable and salutary moisture, as well as to place the radicle iu the medium necessary for its growth into a root, immediately it emerges from the integument of the seed. These facts hold out some beacons worthy of being attended to, as guides for the operation of sowing.

They point out that every kind of seed has a particular depth below the surface at which it germinates most vigorously, as securing to it the most appropriate degree of moisture, of oxygen gas, and of warmth. From a quarter of an inch to two inches beneath the surface, appear to be the limits for the seeds of plants; but they usually vary for the same seeds in different grounds and countries. It must be the least in aluminous soils and dry climates. In general, sowing should be performed in dry weather, especially on heavy soils, not only because of the greater saving of labour, but because it prevents the seed being enveloped with a coat of earth impermeable by the air, "which," says Sir H. Davy, "is one cause of the unproductiveness of cold clayey soils." Perhaps the time at which any ground may be raked with the greatest facility is as good and practical a criterion as any to judge when it is fit for sowing. In general, if clay does not predominate in its constitution, a soil rakes best just after it has been turned up with the spade. If clay does predominate it usually rakes with most facility after it has been dug two or three days, and then immediately after a gentle rain.

But it is certain that the sooner seed is sown after the soil is dug for its reception, the earlier it germinates. In the droughts of summer, water is often required to newly-sown beds. Such application must not be very limited or transitory; for if the soil is only moistened at the immediate time of sowing, it induces the projection of the radicle, which in very parching weather, and in clayey, caking soil, I have known wither away, and the crop be consequently lost from the want of a continued supply of moisture. - Princ. of Gardening.