In the West Indies the mean average in the shade of different stations, extending from lat. 13° to 18°, is about 80° 55', and the difference between the hottest and coldest months is not more than 5° or 6°. In the Indian Archipelago, almost under the equator, it is nearly the same, and the thermometer is never known to fall below 60°; and so, comparatively, of other tropical countries. In addition to this uniform temperature, many of the plants from such regions are found growing in the woods and thickets, and are consequently less exposed to the fluctuations of weather, which renders them more susceptible to injury from sudden drafts of cold, or dry air. From the above we may infer that an even, genial, and somewhat moist atmosphere, ranging from 85° by day, with sunlight, and 60° to 70° at night, is suitable for stove-plants. Such in practice has always been the best and most successful with the writer, and all good plant-growers will testify to the same.

A house where stove-plants are grown ought never to have a front ventilator opened, nor yet a door, excepting while it is necessary to pass through.

As we pass away, north or south, from the torrid zone, there is a corresponding difference of climate; it is more temperate, but the extremes of radiation and other circumstances become greater. In the tropics, however, the same conditions exist in the more elevated situations, the sides of exposed hills, and other like differences; and these conditions, in progressive, ratio, are also to be found in the temperate regions of which we now speak. Excepting the higher portions of the loftiest mountains, nearly all the plants which are indigenous to these adjoining latitudes, are, in cultivation, technically known as greenhouse. Here a freer circulation of air may be allowed with advantage. It requires, however, some experience, even in this case, to determine what amount of ventilation is most suitable for the various genera, and at what particular periods of growth and weather it ought to be freely given. This class of plants are not confined to narrow limits, they are spread over a vast geographical range; but in no case are they able to bear many degrees of frost; and while some are very impatient of extreme heat and dry air, and equally so as regards cold, there are others which, during the growing season, will bear with advantage the greatest heat and rarefication of the air. that occurs in any country, and, when at rest, may be exposed to a lower temperature than the former mentioned, provided the growth is fully matured.

Both may be considered as greenhouse plants, and are commonly accepted as such; but the bare mention of these different constitutional habits goes to show that ventilation ought to vary much in the respective treatment. These very opposite subjects are often congregated together in the same house, as a matter of necessity, and with tolerable success if the warmer and colder parts of the apartment be occupied according to the requirements of the inmates. For example, the upper part of any glass-house is generally warmer, while the lower is colder; and these two extremes are mentioned here to show that the evil consequences of bottom ventilation may be prevented at all times when the weather is unpropitious. The best growers of fruits, artificially, are well; aware of these facts, and are very cautious with regard to ventilation, knowing full well that upon this point the greater part of success depends.

To return more directly to our subject, let us take into consideration the general form of glass-houses, how the circulation of air is affected by ventilation and artificial heat, and what effect these have upon the vital action of plants. Whatever style of architecture we may adopt for these houses, it is only comparatively a modification of the common lean-to as respects the circulation of air within. There is always a roof overhead, which becomes one or more angles above the lower cube; the heat, from whatever source it may be obtained, is always moving up into these higher parts, and, as glass is a good conductor, much of it is given off into the outside atmosphere. When we take into account, also, the immense number of small apertures by which the external air can enter from below, it is readily seen, that there is no such thing as a complete quiescence, and more particularly when the heating apparatus is working there must be a tolerably brisk movement going on, although the means for ventilation are closed.

These conveniences for ventilation are generally arranged along the upper part of the roof, and in the upright front; and, supposing they are all opened, the consequence is a quick escape of the heated air and moisture from above, and a violent rushing in of that which is denser, colder, and drier from the outside below, in extreme, or otherwise, according to the condition of the external atmosphere, increased in intensity by the partial vacuum which is being produced. So much is this the case, that the lower stratum of air in a glass-house may be rendered several degrees lower than the outside temperature in the middle of a hot summer day, and a most chilling draft when the weather is colder. If the air has been for some time, and still is, under the influence of artificial heat, these effects are augmented, not so much in degree as the less qualified state of the plants to bear such extremes. Plants are, in this respect, like animals, possessed with life, having perspiring, respiring, and circulating organs of great delicacy, and equally, or more, susceptible to injury, each and all being submissive to its own constitutional entailment; and it behooves us to study how we may best adapt our care of them; for, depend upon it, if we do not humor their peculiarities, they will give us neither pleasure nor profit.

Let us stop for a moment to discuss the relative conditions. As stated above, our climate is extremely changeable; one day hot and dry, the next cool and moist; in summer we have often a torrid heat, and in winter a polar cold, with a frequent change of 30° to 40° in a few hours at any time of year. The plants we cultivate in- glass-houses, either as fruits or for ornament, are either from the torrid zone, (where the extremes of temperature may be only 10p or 12° throughout the year,) or more uniformly mild regions than our own. The object of our artificial structures is, to furnish these subjects with a similar climate to that from which they were imported, and this, too, very often, during the continuance of our arctic winters. Is it not reasonable, then, to conclude, that the admission of such climatic fluctuations as are common to our continent must be nullifying our intentions, and paralyzing in the effects produced upon the plants?