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A DAMP ATMOSPHERE.

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insects, as well as transfusing a wholesome moisture over the yet leafless branches; but which would prove injurious, if permitted to rise in so great a quantity when the leaves have pushed forth. In a few days, the violence of the steam abates as the buds open, and in the course of a fortnight the heat begins to diminish; it then becomes necessary to carry in a small addition of fresh dung, laying it in the bottom, and covering it over with the old dung fresh forked up: this produces a renovated heat, and a moderate exhalation of moist vapour. In this manner the heat is kept up throughout the season, the fresh supply of dung being constantly laid at the bottom in order to smother the steam, or rather to moderate the quantity of exhalation; for it must always be remembered that Mr. French attaches great virtue to the supply of a reasonable portion of the vapour. The quantity of new dung to be introduced at each turning must be regulated by the greater or smaller degree of heat that is found in the house, as the season or other circumstances appear to require it. The temperature kept up is pretty regular, being from 65 to 70 degrees." (Hort. Trans., i. 245.)

In this case, which attracted much attention at the time, it is evident that the success of the practice arose principally out of two circumstances: firstly, the moisture of the atmosphere was skilfully maintained in due proportion to the temperature; and, secondly, a suitable amount of bottom heat was secured. This is, as will be elsewhere remarked, the principal cause of the advantages found to attend the Dutch mode of forcing. The reporter upon Mr. French's practice speaks with surprise of the rudeness of the roof of his forcing-houses, and of the numerous openings into the air through the laps of the glass and the joints of the sashes; but these were points of no importance under the mode of management adopted.

The impossibility of preserving any plants, except succulents, in a healthy state, for any long period, in a sitting-room, is evidently owing to the impracticability of maintaining the atmosphere of such a situation in a state of sufficient dampness.

An excess of dampness is indispensable to plants in a state of rapid growth, partly because it prevents the action of perspiration becoming too violent, and partly because under such circumstances a considerable quantity of aqueous food is absorbed from the atmosphere, in addition to that obtained by the roots. But it is essential to observe that, when not in a

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AXIOMS AS TO DAMP AIR.

state of rapid growth, a large amount of moisture in the air will be prejudicial rather than advantageous to a plant; if the temperature is at the same time high, excitability will remain in a state of continued action, and that rest which is necessary will be withheld, the result of which will be an eventual destruction of the vital energies. But, on the other hand, if the temperature is kept low while the amount of atmospherical moisture is considerable, the latter is absorbed, without its being possible for the plant to decompose it; the system then becomes, in the younger and more absorbent parts, distended with water, and decomposition takes place, followed by the appearance of a crop of microscopical fungi; in short, that appearance presents itself which is technically called "damping off."

A skilful balancing of temperature and moisture in the air, and a just adaptation of them to the various seasons of growth, constitute the most complicated and difficult part of a gardener's art. There is some danger in laying down general rules with respect to this subject, so much depending upon the peculiar habits of species, of which the modifications are endless. It may, however, I think, be safely stated that the following maxims deserve especial attention :

1. Most moisture in the air is demanded by plants when they first begin to grow, and least when their periodical growth is completed.

2. The quantity of atmospheric moisture required by plants is, cæteris paribus, in inverse proportion to the distance from the equator of the countries which they naturally inhabit.

3. Plants with annual stems require more than those with ligneous stems.

4. The amount of moisture in the air most suitable to plants at rest is in inverse proportion to the quantity of aqueous matter they at that time contain. (Hence the dryness of the air required by succulent plants when at rest.)

CHAPTER IV.

OF VENTILATION.

It is probable that no horticultural question has excited more difference of opinion than that of ventilating glass-houses. On the one hand it has been contended that plants in such places require no more air than will necessarily be introduced through crevices, sashes, and doors; on the other it has been insisted by gardeners of great experience that plants require an incessant and abundant supply of fresh air in motion.

Those who support the latter view point to the facts that meet the observer at every step in wild nature, where plants are to be found in the most vigorous health. In the open air the atmosphere that surrounds them is incessantly in motion, even in the calmest day; and by evening or during the night, when they most especially are feeding, in rapid motion. The atmosphere is their pasture, and its ever-varying density is a natural phenomenon most intimately connected with the maintenance of vegetable health. It is a beautiful compensation for their want of locomotion; as plants cannot move to the atmosphere, the atmosphere is ever moving towards them. It is therefore certain, without inquiring into the exact philosophy of the matter, that free access to abundant air must be secured, if the health of plants in glass-houses is to equal that in the open air.

On the other hand the advocates of a confined atmosphere believe that those who attach so much importance to ventilating houses abundantly, scarcely consider the nature of plants, and suppose that they require to be treated like man himself, thus consulting their own feelings rather than the laws of vegetable growth. It is true that animals require a continual renovation

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NECESSITY OF VENTILATION.

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; and, considering the manner in which glass-houses are constructed, the buoyancy of the air in heated houses will 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 so necessary to them as is supposed. So it is said.

There can, however, be no doubt that the latter argument is fallacious, and that gardeners who judge of the requirements of plants by their own, are not so much in error as has been supposed. It is true that ventilation is not required in order to supply plants with food enough to maintain existence; they get from tranquil air as much gaseous food as will support life. But it is one thing to exist, and another to thrive. Moreover, the admission of abundant air is not merely for the purpose of feeding a plant; it enables it to perspire copiously, a function not less indispensable than feeding, for perspiration with plants is only a part of the process of digestion. Let us only watch the effect of allowing a continual change of air to take place among plants in a greenhouse. The best managed house within our knowledge, in which the plants are always dark green, short jointed, and loaded with flowers, is one with a span roof, the lower half of which is moveable and the upper fixed; by raising or lowering the lower sashes a strong current of air can at all times be carried through the plants, among which it incessantly plays. In this place there are no yellow leaves, no mildew, no spot, no languor, no fogging off. At Sion the Clove has borne flowers, the Litchi and Nutmeg ripened their fruit with all their natural aroma, and the Mangosteen is growing as if at Batavia; this has been effected in a stove so constructed as to secure the presence of constant currents of air. The Mango has never flourished more than it did at Walcot, in the days of the late Lord Powis ;

IMPORTANCE OF AIR IN MOTION.

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it grew there in a house in which air was necessarily in constant and very rapid motion. The best flavoured Grapes are ripened out of doors; no one would compare our hot-house Grapes for flavour with those of the climates where they ripen naturally. The best coloured Grapes are ripened out of doors; no one ever saw ripe black Grapes deficient in colour in the open air. The best Peaches, Strawberries, Apricots, are ripened in the open air. The best flavoured Queen Pine I ever tasted was one ripened at Bicton in the open air.*

It is not improbable that one of the advantages of ventilation depends upon a cause but little adverted to, but which certainly requires to be well considered. It was an opinion of Mr. Knight, that the motion given to plants by wind is beneficial to them by enabling their fluids to circulate more freely than they otherwise would do; and in a paper printed in the Philosophical Transactions for 1803, p. 277, he adduces, in support of his opinion, many experiments and observations; of which the following is sufficiently striking:

"The effect of motion on the circulation of the sap, and the consequent formation of wood, I was best able to ascertain by the following expedient. Early in the spring of 1801, I selected a number of young seedling Apple trees, whose stems were about an inch in diameter, and whose height between the roots and first branches was between six and seven feet. These trees stood about eight feet from each other; and, of course, a free passage for the wind to act on each tree was afforded. By means of stakes and bandages of hay, not so tightly bound as to impede the progress of any fluid within the trees, I nearly deprived the roots and lower parts of the stems of several trees of all motion, to the height of three feet from the ground, leaving the upper part of the stems and branches in their natural state. In the succeeding summer, much new wood accumulated in the parts which were kept in motion by the wind; but the lower parts of the stems and roots increased very little in size. Removing the bandages from one of these trees in the following winter, I fixed a stake in the ground, about ten feet distant from the tree, on the east side of it; and I attached the tree to the stake at the height of six feet, by means of a slender pole, about twelve feet long; thus leaving the tree at liberty to move towards the north and south, or, more properly, in the segment of a circle, of which the

* See p. 101. The author regrets to see that the name of Lady Rolle is misprinted Rolfe, at that place.

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