Part 4

When a moist atmosphere is duly and regularly maintained, there is but little fear need be entertained of the establishment of a colony of insects--such as the thrip, and the red spider, which are perhaps the greatest pests which have to be overcome in the forcing house; nor is there a more effectual method of destroying them, than by applying a high temperature in conjunction with an intense degree of moisture. To the want of a balance of moisture in the composition of the atmosphere, and in the soil, too, rather than as is commonly supposed, to an excess of it in the former, is the appearance called mildew to be attributed; this it occasions by checking the regular action of the perspiratory organs, and thereby inducing an eruption of the cells of the tissue: the extravasated sap lodging on the cuticle, affords a nidus for the germination of the sporules of that particular fungus, which when grown, is the mildew: the remedy consists in avoiding an irregular composition of the atmosphere, as regards heat and moisture; and also an excess or deficiency of moisture in the soil, so that each may be in a condition to exert its proper influence on the constitution and developement of the plants. Canker, another disease, to which Cucumbers are sometimes subject, appears to be produced by too low a degree of temperature, accompanied by an excess of moisture, both in the soil and the atmosphere, and it generally attacks those particular parts, where any check or obstruction is offered to the flow of the sap, such as that occasioned by a wound, or even the ramifications of the stem: this suggests that its remedy, would consist in a due regulation and balance of the constituents of the atmosphere, and the soil.

Moisture is generally applied to the soil by being poured directly on it, and to the atmosphere, by means of the syringe, and the use of evaporation troughs. When applied to the soil only from the upper surface, there is a liability of its failing thoroughly to moisten it, and by reason of this, together with the constant action of the heat from below, by whatever means heat may be applied, the soil is frequently found to be dry beneath, when the appearance of the surface might lead to the supposition that it was sufficiently moistened.

By a reference to the sketch and description already given, it will be seen, that a provision is there made, whereby water can be poured in quantity _beneath the soil_, immediately on the top of the tank, whence in the form of vapour it will rise among the soil, and thus render it thoroughly moist; at the same time, it can be applied to the surface, whenever it may become necessary to do so. The moistening of the atmosphere will also be fully secured by the mode of ventilation which is there proposed, for the air, at the same time that it is warmed, will become charged with moisture in a ratio equal to its temperature, before it enters the house. If it becomes requisite to admit moisture without changing the volume of air, it can readily be effected by opening the tubes or shafts inside the house, without opening the exterior ventilators; and when dry heat may be required, it can be secured by closing entirely the communication with this reservoir of moisture, and the hot-water pipes will then radiate any quantity of dry heat that may be required.

By means of a due application of these provisions, an equable degree of moisture beneath and among the soil, as well as in the composition of the atmosphere, can be secured with perfect ease, and a trifling amount of labour.

CHAP. VII.

ON THE REGULATION OF THE TEMPERATURE.

If we figure to our minds, a plant which in its native habitat enjoys a climate far more genial, and a temperature far more elevated, than our own country affords, it must be obvious that some regulation, and increase of temperature, either positively, by the artificial application of heat, or negatively, by affording shelter and protection, will be required in order to ensure any degree of success in its cultivation. The Cucumber is a reputed native of the East, and we have therefore in this supposed fact, an indication of the nature of the climate, which it should be our object to provide for it; but still it must be borne in mind, that in conducting any system of artificial cultivation, it is not at all times desirable, or even safe, to supply a resemblance to any part of the natural circumstances affecting the growth of a particular plant, unless we have the means of supplying the _greater part_, or _all_ the conditions which exist in a state of nature: this I shall again have occasion to refer to.

By another step we arrive at the conclusion that the standard of temperature, to which the Cucumber is submitted in its cultivation in this country, is a point, varying with the individual opinion of cultivators; as some may take a part of the natural conditions of growth as their rule; others, all these circumstances; and others, again, various combinations of them.

Referring back again to the provisions of nature, we can scarcely hesitate to conclude, that in clear sunny weather, the temperature to which the Cucumber is submitted, _cannot within reasonable limits_, be permitted to rise too high; whilst at other times, when the weather is dull, or cloudy, and always at night, a much lower degree of heat ought to be applied. In sunny weather, the natural agents which cause excitement and activity of the vital functions, are in full action; and consequently at such periods we may rationally indulge in the application of those exciting agents which are under our controul--always however bearing in mind, that we must not unduly apply one agent, when we either cannot, or neglect to apply the others also. On the other hand, in dull weather, and at night, the source of light being in the one case absent, in the other obscured, a comparative state of lethargy or repose is prevalent, and the natural functions of vitality are but feeble in their action, if not in some cases, absolutely in a quiescent state; with such a state of things existing, it is barely rational to apply stimulants, and to induce unnatural excitement. The application of exciting and stimulating agents at such periods, may be compared in its effects to the excitement of a frightful dream acting on the human frame; the vital functions--not the vitality itself--cease during sleep, and both the animal and the vegetable should be at rest; excitement acts on both by deranging the system, at least for a time, and since a succession of these derangements are known to produce injurious results, we may be certain, that each seperate instance must have an evil tendency.

In applying this practically, to the case before us, it may be recommended, that the temperature in which Cucumbers are grown during winter, should not fall much below 60° Fahrenheit, at night; and in the day time it should not rise above 70° in dull weather, by the aid of heat artificially applied; in clear weather, by the influence of that glorious source of light and heat, the sun, it may be safely allowed to rise to 80°, or a little higher, before air is admitted. A somewhat higher range may be permitted, as the days lengthen, and the influence of the sun becomes more powerful; thus at night, it should not rise over 65°, by day 75° to 80°, and by sun heat to 90°. Thus it will be seen, that I have recommended the regulation of the temperature of the internal atmosphere, by that which is external; and it is my firm conviction that inattention to this simple rule, is the source of much of the failure, which is experienced by some of those who attempt the growth of plants, at any other than that, which may be regarded as their natural season of growth. It appears to me, most unreasonable, to aim at attaining any particular point of the thermometer, merely because any particular season of the year may be present, or any particular stage of growth attained. Even if in the sunny climes, from whence the Cucumber has been transmitted to us, there exists such an equality of temperature and atmospheric serenity, as some cultivators attempt in the growth of these plants; it surely cannot be consistent in us to equalize and elevate the temperature of our artificial atmospheres, when we cannot supply them at the same time with the same intensity of light, or provide for them the same serene and unclouded sky. It should rather be our object to adapt the plant to the climate of our country, since we cannot change the climate to supply the natural circumstances, with which the plant is favoured; and acting on this principle, we should never aim at supplying the agents which would induce a premature and therefore debilitated developement, when the whistling wind, and the drifting snow, tell us that Nature, would have, at least the members of her vegetable kingdom, be at rest.

Since however, it is apparent that during the depth of the winter season, at least when wintry weather is present, the progress of plants in an artificially heated atmosphere, ought not to be rapid, or unduly forced; it by no means follows that no progression at all should be made: the elements of growth maybe supplied; but the application of them should be guided by moderation, being lessened at those particular periods when the weather is least propitious, and increased during those periods when it is most favourable. In the works of Nature we may ever learn a lesson of consistency, for they are perfect: they teach us that food is requisite to maintain the life of all those objects which are endowed with it; that that food must undergo a process both of digestion and assimilation, ere its purpose is fulfilled; and that each of these processes depend on the action of natural agents. In the vegetable kingdom, heat and fight as derived from a united source, are the agents appointed to bring about these results, and in order to ensure their proper action, they must both be present in a powerful degree: in artificial schemes of culture, we can command a supply of the one, but the other is not within our power; our consistency therefore depends on our applying so much of the one under our controul, as will secure the united action of it, with the existing degree of the other--consequently, _when light is absent, or deficient, heat should also be diminished; and when light is present and abundant, heat may safely be increased_.

CHAP. VIII.

ON THE ADMISSION OF AIR.

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 homoeopathic 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.

CHAP. IX.

ON THE GROWTH OF MELONS.

It is barely possible to suppose any use to which a structure which during the winter season had been devoted to the growth of Cucumbers, could be so legitimately appropriated in the summer, as that of the growth of the finer Melons of Persia, Cashmere, and the East. The superiority of such as these, in every point of view, over those kinds, which have been long in cultivation, would be an ample recompense for the appropriation of such valuable space to their use; whilst in no other structure could the peculiarities of the treatment they require, be so fully complied with, and be rendered so completely under control, as in that under consideration.

There are some peculiarities in the treatment of these Melons, to the consideration of which, it may be desirable to devote a brief space; the most important of these, are the composition of the soil, the application of moisture at the root, the regulation of atmospheric warmth, and also, of atmospheric moisture; in these particulars, they offer some differences to what has been previously stated, with reference to the Cucumber.