In approaching this important horticultural subject, it may be well to examine the outward bearings of the case; and, in so doing, we must refer to the causes which govern the currents of air in the external atmosphere. When the air by which we are surrounded becomes heated, it expands, and is specifically lighter, in consequence of which it mounts upwards; and the colder and denser air which surrounds the mass thus rarified, rushes in to supply its place. When the door of a heated apartment is thrown open a current of air is thereby immediately produced, the warm air from the apartment passing out at the top of the opening, and the cold air from the passage rushing in at the bottom, while in the middle it is stationary, or nearly enough so for all practical explanations. When the rays from the sun, by their reflection from the earth's surface, have heated or rarified a portion of the surrounding air, the air so rarified ascends into the higher regions of the atmosphere, and the colder air by which it was surrounded moves forward in a sensible current to fill the vacuity.

From the above observations, then, it is readily understood, that the tendency of heated air is to fly upwards; and, also, if there is free egress from the higher part of any ordinary glass structure, the inside temperature will be prevented from attaining to any considerably higher range than the external air; while a rapid circulation will be secured by the drawing in of colder and denser air from the outside, through the laps of the glass, crevices, and jointings of even the best constructed house, without the necessity of opening any bottom ventilators.

"By far the larger number of gardeners attach great importance to preserving the power of ventilating their houses abundantly, without, perhaps, sufficiently considering the nature of the plants they have to manage; and, as has been justly enough said, by supposing that plants require to be treated like man himself, they consult their own feelings rather than the principles of vegetable growth. There can be no doubt, however, that the effect of excessive ventilation is more frequently injurious than advantageous; and that many houses, particularly hothouses, would be more skilfully managed, if the power of ventilation possessed by the gardener were much diminished.

"Animals require a continual renovation of the air that surrounds them, because they speedily render it impure by the carbonic acid given off, and the oxygen abstracted by animal respiration. But the reverse is what happens to plants; they exhale oxygen during the day, and inhale the carbonic acid of the atmosphere, thus depriving the latter of that which would render it unfit for the sustenance of the higher orders of the vegetable kingdom; and, considering the manner in which glass-houses of all kinds are constructed, the buoyancy of the air in all heated houses would enable it to escape in sufficient quantity to renew itself as quickly as can be necessary for the maintenance of the healthy action of the organs of vegetable respiration. It, therefore, is improbable that the ventilation of houses in which plants grow is necessary to them, so far as respiration is concerned." - Linaley's Theory of Horticulture, p. 150.

It is now a well-known fact, that many plants will thrive better when confined in close glass cases than they would do with an abundance of fresh air, providing light is duly admitted; and it is as fully demonstrated that light is more indispensable in general culture than a constant change of the atmosphere. Under the influence of this agent chemical action progresses, carbonic acid is decomposed, the oxygen is driven off, and the carbon retained, and, consequently, according to the deposit and amount of the latter, so will be the more or less solidity of vascular tissue in the plant, saccharine matter in the fruit, and corresponding concentration of growth. I have, in many instances, proved the truth of this reasoning in practice; and we have also the testimony of the well-known authority Mr. Knight, in support of the same. He says, (Horticultural Trans., ii., 225,) "It may be objected that plants do not thrive, and that the skins of grapes are thick, and other fruits are without flavor, in crowded forcing-houses; but in these it is probably light, rather than a more rapid change of air, that is wanted.

For, in a forcing-house which I have long devoted, almost exclusively, to experiments, I employ very little fire heat, and never give air till my grapes are nearly ripe, in the hottest and brightest weather, further than is just necessary to prevent the leaves being destroyed by excess of heat. Yet this mode of treatment does not at all lessen the flavor of the fruit, nor render the skins of the grapes thick; on the contrary, their skins are always most remarkably thin, and very similar to those grapes which have ripened in the open air." Now, the entire exclusion of "fresh air" is not advocated here, neither would it be judicious during all periods of vegetable growth, and with all kinds of plants; but such argument goes to prove, that in some cases it is absolute, and, under other circumstances, a continual flow is not required. The general expression, then, "Admit plenty of fresh air," demands some more tangible proof than the mere assertion, before the advice is acted upon; and more particularly so, when we take into consideration the fickle and changeable climate we are working in, and where,-we may say, we are often fighting the most adverse elements.

Nearly all the plants that are cultivated in glass structures are natives of more favored climes than ours; besides which, as in the case of forcing fruits, we have them in full activity at a time when, in a state of nature, they would be at rest; and out of doors, our most perverse wintry weather is in existence, which makes it self-evident that great caution is needed with regard to the admission of external influences. A few examples directly from nature, respecting the geographical peculiarities, though, perhaps, a slight digression, may serve to illustrate what is here meant. As all the plants which come from these different regions must be somewhat accommodated accordingly, we have no other resource left than to suit their organism. All that has been said and written about acclimating, and all that may be in future attempted, will never change this; and, further, all that we can gain is only a knowledge of how far the fixed organic structure of any plant is adaptable to our necessary artificial conveniences.