The question of the admission of air, is one of some importance. It is an opinion, which was I believe first publicly brought forward by the late Mr. Knight, that an influx of a large volume of the external atmosphere, to the interior of forcing houses, is by no means requisite, and is often the source of very serious evils. Were it for no other reason, than that of avoiding the chilling influence of cold air on the tender tissue of plants growing in a high temperature, I should feel inclined to support such a view; but when there are facts sufficiently abundant, to prove, that plants do not themselves vitiate the air of such structures to an extent sufficient to render it unfit for their continued growth, or at least, that a sufficient interchange is constantly going on, without opening the sashes of a forcing house, the evidence appears to be overwhelming; and the necessity of continuing a practice so fraught with danger, and so frequently attended with disappointment, appears to be done away.
The injury done to the tender foliage of plants in forcing houses, by contact with cold air, results from the increased capacity of air for moisture, as it become heated. When cold air is admitted to these structures, it cannot contain so great a quantity of aqueous matter, as it is capable of taking up when it becomes warmed: this increase of temperature, is soon in great measure, supplied to it, but rarely is a sufficient quantity of moisture, at the same time within its reach, to enable it to supply its increased capacity for aqueous matter: the consequence is, that on coming in contact with the foilage of the plants, which is of a succulent nature, and contains a great proportion of water, the warmed air continues to abstract a portion of moisture from the plants, until its capacity is satisfied; and hence the plants are robbed of their “life’s blood.” Besides this action, which is the cause of serious evil, the tissue itself is contracted and thereby injured, by reason of the degree of cold, which is at the first gush, liable to come in contact with the warm foliage. These remarks apply to cold air, when admitted in a large bulk, by opening the sashes; and when a draught is produced, by opening them, both at the back and front, and the top and bottom of the house.
Deterioration of the air, by the action of the functions of the plants, could not take place, except in hermetically sealed structures: for by reason of the expansibility and elasticity of air, when it becomes at all heated, it not only gains egress, but also admission through the most minute crevices: that this interchange is sufficient to counteract any deteriorating influence which the plants might have on the internal air, with respect to their continued existence in it, is abundantly proved by the growth of plants in Ward’s cases, from the interior of which the external air is excluded as fully as it possibly can be, without their being actually sealed: if therefore, any injurious effects result to plants, from their being cultivated in a close atmosphere, we must seek for the cause, in some other source, than the plants themselves. If any noxious qualities exist in the atmosphere of structures, to which the external air has not free ingress, they must result from some neglect or ignorance on our part, in suffering extraneous and unwholesome matters to accumulate in such situations, and there to decompose, and enter into combination with those gaseous bodies, which form the volume of the internal atmosphere of our plant structures. The existence of such extraneous matters, may indeed be traced to various sources; and they may be present, even when much vigilance is employed to prevent their accumulation; and therefore, as an inconceivably minute quantity, inappreciable to the senses, would frequently be sufficient to effect deterioration, it is possible that these impurities may often originate in sources which are least of all suspected. The decomposition of organic matter, whether animal or vegetable, may frequently be the source of injurious results in this respect; for although this is principally resolved into those elementary gases, which appear to form the basis of all created objects, yet there are other matters liberated, which may then enter into fresh combinations; and either this, or a disproportionate accumulation, even of these elementary bodies, may reasonably give rise to serious apprehension, and demand the exercise of discretion, in order to prevent them from becoming injurious. Besides this, these decomposing bodies, afford just the very state of things, which appears to be requisite to call into existence, and developement, a numerous phalanx of cryptogamic vegetables: not that such matters, can for a moment be rationally considered to generate, these cellulares; but that they afford a suitable pabulum, and medium of developement for those millions upon millions of sporules, which we may readily conceive to be dispersed in the atmosphere; and with which it may be teeming, though from their buoyancy and minuteness, they may float to us invisibly therein.
The admission of the external air, by the ordinary process of opening the sashes of forcing houses, has been said to be unnecessary, or at least by no means important, in so far as the function of vegetable respiration is concerned, because the buoyancy of the air within all such structures, would enable it to escape in sufficient quantity through their openings and crevices, to counterbalance any thing like deterioration, which might by any means result from the vital action of the plant. The admission of external air, is also directly injurious to forced plants, during the winter and spring months, when a very material difference of temperature exists between it, and the internal volume, by contracting the vessels, impeding the circulation of the juices, and thereby checking the regular course of the growth of the plant. If these reasons fail to stamp it as a practice which ought not largely to be indulged in, it is further objectionable, as being productive of a prodigal expenditure of fuel: there can be little doubt but that generally speaking, a far greater quantity of fuel than is requisite, is expended in maintaining the temperature of forcing houses, solely from this cause; for the cold air when admitted, continues to abstract a portion of heat from the warmed air, until the temperature of both becomes equal, and consequently an increased application of fuel is requisite, in order to raise the newly admitted air to the same temperature as that which has been suffered to escape; and as the buoyancy of heated air is so great, an immense volume must necessarily rush out through a very small aperture, and thus there must also of necessity be an immense waste both of heat, and of fuel. A given portion of fuel, in its combustion, can give off but a certain proportionate ratio of heat, and if this is allowed unnecessarily to escape, the prodigality is self-evident. It is but a weak argument, which would seek to give to the admission of cold air, the office of regulating the temperature of plant houses; this ought to be effected by limiting the degree of heat applied, and not by attending to the abstraction of that which had been previously administered with two lavish an hand. Besides the extravagance of such a course, the constitutional vigour and energy of the plants is at the same time sacrificed by undue excitement. The admission of cold air in large quantities, therefore, brings condemnation in its train, since it is unnecessary, and extravagant, as well as directly injurious.
There are nevertheless some considerations which render the admission of air, when regulated and applied with discretion, an operation of importance to the health of plants: it is productive of beneficial effects in carrying off the noxious vapours, which may although unseen, and guarded against, still float in the atmosphere; and there can be little doubt that another beneficial influence which it exercises, results from the motion which is produced by a body of air changing its position, which probably promotes circulation, and increases the excitability of the plants.
Since therefore a change of the volume of the atmosphere in plant houses, is productive of benefit, and the admission of a large body of cold air, is at the same time so decidedly objectionable, it is important, that in endeavouring to secure the benefits of the practice, the injuries which are liable to result, should if possible be avoided. The regulation for the admistion of air, which is described in the second chapter of this treatise, may be regarded as being of some importance in this respect, as well as in the provision which it includes, of supplying the heated air, with a due proportion of moisture.
Physiologists tell us, that plants derive a considerable proportion of their food, directly from the atmosphere, by a process similar to the inhaling of animals; and that the substances thus derived, are carbonic acid, ammonia, and water, which contain the elements of organic matter in considerable proportions. The influence of the atmosphere is exerted beneficially, by its constituents entering into combinations with other matters, which are taken into the system by the roots, and spread out and exposed in the leaves: this exposure has so far the effect of altering the character of the substance carried up from the roots, that it is no longer a body of crude juice, but is undergoing a process of elaboration, and is being assimilated with the superincumbent tissue of the plant. There seems to be no reason why those particular gaseous bodies which plants appropriate to themselves from the atmosphere, should not to a great extent be supplied to them artificially, at such periods as it may be necessary, or desirable, to accelerate their growth, and induce a more perfect and mature developement. It has been already stated, that the most important of these aeriform bodies, are nitrogen, which plants derive from ammonia; and carbon, which they derive from carbonic acid gas, on the liberation of the oxygen, which is one of its constituents; neither of these, can however be appropriated, when in a free state, but only when in a state of combination, and forming either a gaseous or a fluid body. It is probable that nitrogen might be supplied to plants, through the medium of the atmosphere in an artificial manner, by placing within any structure, a portion of some of the volatile salts of ammonia, which latter being given off, would at once supply the demands of vegetation. Carbon might be applied, by the use of charcoal; and it is worthy of experiment how far the combustion of charcoal, in plant structures, by accelerating the formation of carbonic acid gas, may have a beneficial influence on vegetation. The use of charcoal as an ingredient in the soil, though doubtless partly, and perhaps principally mechanical, is nevertheless in all probability rendered advantageous in this very way; the slowness of its decomposition must however render the quantity applied, very homœopathic in its nature.
A series of experiments with the view of ascertaining the practicability of continually supplying to the atmosphere, those qualities which plants abstract from it, and of determining the manner, and the degree in which they should be applied, would be one of the most interesting and important matters, to which the minds of Horticultural reformers could possibly be directed; but it is most essential, to remember, at the same time, “that these are powerful agents, requiring much skill in their adaptation,” and capable of effecting serious injury and disappointment, if indiscriminately applied.