If I were asked the question, "What point do you consider of most importance in the management of plants?" I would unhesitatingly answer, the application of water; and I believe the answer would be endorsed by all whose experience and observation makes their opinion worthy of notice. It matters not what soil a plant is growing in - how it is situated with regard to temperature or light; if negligently or unskilfully supplied with moisture, it cannot thrive; and I feel warranted in saying that there are more failures in plant cultivation, occasioned by the improper use of this liquid, than by any other cause.

In treating on matters relating to the cultivation of plants, it is difficult to lay down definite rules that will suit all cases alike; neither will any amount of explanatory information compensate for want of that discriminating knowledge obtained through practice ly opposed to each other. This partly arises from the accommodating nature of plants, and the infinite modifications of the various agents by which they are influenced, rendering it extremely improbable for any single observation to be of universal application. A further cause of this diversity of opinion may be found in the lucubrations of those who may be termed speculators in vegetable physiology - "who are apt to draw general conclusions, and raise axioms from every particular they meet with;" and seldom hesitate in promulgating their ideas, and investing their theories, however crude they may be, with all the "pomp and circumstance" of authenticated experiment.

The importance of water to plants, and its influence upon them, may be readily conceived, when it is known to form upwards of one-half of all green vegetable matter, and serves as the means of conveyance for all the nutritive elements required for their food. Mineral ingredients must be in a state of solution, or so minutely divided as to be carried along with water, before they gain admission into the roots of plants. It is also supposed that gaseous elements cannot be absorbed by roots in their aeriform state, and are available only when in solution with water.

Seeing that water is not only a most important agent in vegetation, but is one more under subjection than any other, it is evident that by its skilful application we can, to a great extent, regulate and control the growth of plants. It is true, that light, heat and air, are also essential agents, and their co-operation indispensable, but over the first of these we have no control, and the latter are under subjection only in artificial atmospheres; but even in out-of-door culture, in fields and gardens, it is possible by draining and deep cultivation, to modify the injurious effects resulting either from excess or deficiency of water. Our control over this agent enables us to meet extremes in the others. By its withdrawal at certain seasons, and freer application of it in others, we can induce different developments, and hasten or retard their termination at will.

Our possession of this controling influence is practically of most importance in the cultivation of exotics, and the production of fruits and flowers out of their natural seasons. Success in forcing depends chiefly upon the previous preparation of the plants for that purpose; an early and thorough ripening of the previous growth, with a proper period of rest, are absolutely indispensable. This is effected by a gradual withdrawal of water. If we lessen the supply of water to a growing plant, we of course cut off the means whereby nourishment is conveyed into its system. Alkalies and other inorganic substances held in solution, are constantly conveyed to the plant with the water absorbed by the roots, these co-operate with the carbonic acid absorbed by the leaves in forming new constituents of the plant. When water is withheld from the roots this supply of mineral ingredients is cut off; these are then taken from the plant itself, principally from the juices of the leaves, which soon begin to change color, and ultimately fall off; while the substances by which they were upheld are incorporated into woody fibre, or other matters necessary for the structure of the plant. (Liebig.) And "rest in plants is effected in one or two ways; either by a very considerable lowering of temperature, or by a degree of dryness under which vegetation cannot be sustained." (Lindley.)

The inflorescence of plants is increased by a diminished supply of water while the buds are being formed; anything, indeed, that tends to check luxuriance - so long as the plant is in a healthy state, is favorable for the production of flowers. The rudiments of flower make a large development of leaves and branches without producing any fruit. Taking that familiar flowering plant, the Camellia, as an illustration - during its growth it delights in a free supply of moisture, both in soil and atmosphere; if these conditions are kept up uninterruptedly after the seasonal growth is completed, it will not form flower buds, but immediately start into a second growth. On the other hand, as soon as the young shoots are done growing, let the plant be placed in a dryer atmosphere, and the supply of water to its roots diminished, and almost every bud will be converted into a flower bud.

The ripening of fruit is also accelerated by lessening the supply of water to the roots during this process. This may readily be supposed, seeing that it has the same organic connection with the roots as the flowers and leaves. When in a young, green state, the fruit performs the same functions as the leaves, absorbing carbonic acid and giving off oxygen, thus elaborating matter for itself so long as it remains of a green color. Its principal dependence for nourishment, however, is from the leaves. Fruit formed upon naked branches will rarely ripen to perfection without the assistance of leaves. Its intimate relation and dependence upon the leaves renders it similarly affected by change of circumstances. "One of the most essential of the alterations which occur in fruits during ripening, is the decomposition or dissipation of the water that they attract from the stem. A diminished supply of water will, under equal circumstances, produce an accelerated maturation, because less time will be required to decompose or dissipate this element; and, on the other hand, an excessive supply of water will retard or prevent ripening, in consequence of the longer time required for the same purpose." (Lindley.)

The amount of cold that plants are capable of enduring, depends in a great measure upon the quantity of water contained in their structure. We frequently see plants at one time killed with an amount of frost that in previous occasions did not injure them. The soft, watery, immature points of shoots, are also frequently killed, while the rest of the plant remains uninjured. Mild, moist winters, followed by severe spring frosts, are more injurious to vegetation than those of earlier severity, because in the former case the roots are constantly sucking fluid from the soil and driving it upwards into the system, the whole plant becomes distended with watery particles, and owing to the expansion of water in freezing, the bark is frequently rent asunder, and the plant otherwise injured. Hence the proprirty of inducing early maturity - checking the growth by withdrawing moisture, that the shoots may become less hard and solidified, and enabled to withstand cold; and hence, also, the fact that plants flower and fruit more profusely and withstand the rigors of climate with less injury, on well drained lands, than in those constantly saturated with moisture. A wet soil is truly a cold one.

Where there is water, the rays of heat penetrate very slowly; the more water the greater the evaporation, and evaporation always produces cold.

Attention to these facts is of much importance to those having the management of greenhouse plants during the winter. By a proper preparatory treatment these can be inured to bear a degree or two of frost without injury; and in severe weather it is preferable to allow this falling off of the atmosphere, rather than have recourse to high artificial heat, which is often given to an amount not only useless, but decidedly injurious. I have frequently allowed the thermometer to fall as low as 29o in a house containing not only an average collection of what are usually considered green-house plants, but also many that ordinarily receive hot-house treatment, as Ixoras, Stephanotis, Eschynanthus, Gardenias, etc. In a common wooden frame sunk in the earth, with no other protection than that afforded by the glazed sash, and a close canvass cover elevated a few inches from the glass, I have kept Verbenas, Roses, Carnations, Ac., together with young plants of lettuces and that "best of all flowers," the Cauliflower, although a thermometer in their immediate vicinity frequently fell six and eight degrees below zero.

These were kept scrupulously dry - "dry as dust," and carefully excluded from clear sun-light for some days after fine weather set in.

As we become more familiar with the natural habits of plants, and knowledge in the science of cultivation increases, it may be expected that a largo number of plants which are now considered tender, will prove to be much hardier than is generally supposed. It is a fact that many of the failures in cultivation are caused by a mistaken eagerness to keep plants in an artificial instead of a natural condition. In their native countries, many of the plants we cultivate in our hot-houses, are subjected to extremes of temperature which they cannot endure under ordinary artificial treatment. In Australia, for instance, such plants as Acacias, Bovonias, Mimosas, Eucalypti, Hoveas, Myopoviums, and others, which arc generally considered to be more susceptible of cold than these, are found growing and flowering where the night temperature falls many degrees below the freezing point. No doubt this power of resisting cold is attributable to the high, arid temperature to which they had previously been subjected, evaporating and solidifying the juices so as to bring the plant into a state of comparative dryness. It is a very prevalent opinion that all tropical plants should be constantly kept in a state of active vegetation.

This opinion is unfounded; for there is no climate in the world without its periodical seasons of excitement and repose, as regular as our summer and winter. A knowledge of the natural climates, soils and stations of the plants we cultivate, is a desideratum of the highest importance. Not only should we be acquainted with the climate and physical condition of the atmosphere in the countries to which they belong, but also the altitude at which they are found. A plant growing upon a bleak mountain side, where it can scarcely find root-hold on the rocky surface, exposed to every change of temperature, must require very different artificial treatment from one located in a valley, luxuriating in the rich depositions of the surrounding elevations, where a free current of air rarely visits it. There is a great want of authentic, practically useful information on this subject.

It is questionable whether we do not, in our aim at what is called high cultivation, sometimes overstep the simple means nature employs for her purposes. The closer we can imitate nature, the more likelihood is there of success; and though there is much in nature that the wisest cannot understand, there is also much that is intelligible to the most illiterate. William Saunders, Gardener to Johns Hopkins, Esq.

Clifton Park, Baltimore, Md., Jan. 6,1851.