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September, 1° to 2°; October, 2° to 3°; and in November and December, between 3° and 4° must be added to the mean temperature of the air, in these months respectively, in order to obtain the approximate mean temperature of the soil. This will apply to all places having a climate resembling that of Chiswick. But where the temperature of the air falls very low in the end of autumn, and beginning of winter, as at Upsal, where the mean of the air in October is 12° below that in September, 6 or 7° must be added to the mean of the air, for the mean temperature of the earth; and 8 or 9° in each of the two following months. On the contrary, where the winters are milder than at Chiswick, the difference between the earth and air temperatures will be somewhat less than that which appears in the table, and represented in the diagram for that place.

"Within and near the tropics the earth, it appears, is, on the average, always warmer than the air by several degrees. In some months, and at some places, both temperatures are nearly alike; and in other instances they differ as much as 6 or 7o, much depending on the fall of rain, and the nature of the soil. If we add 2° to the temperature of the air, in June, July, August, and September, and 4° in the other months, we shall approach the monthly mean temperature of the soil between 0° and 30° latitude, sufficiently near for all practical purposes; certainly much nearer than the temperature to which plants from that soil have been subjected by artificial treatment in this country." It must, however, be understood that such calculations are necessarily uncertain, and can only be taken as rough approximations to truth, near enough for practical purposes, but nothing more. An infinite multitude of circumstances, reducible within no general rules, modify all such estimates.

As regards the Indian seasons, the greater part of the west and south coast of the peninsula is so damp that the growing and flowering season lasts all the year round. Even of Rice there are, according to Buchanan Hamilton, two crops in Malabar :-one sown in May, transplanted in June, and reaped in July; another sown in August, transplanted in September and October, and reaped in November. In Northern India the growing season for tropical crops-Rice, Millet, &c.—generally

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speaking begins with the rains (May and June), and lasts all the rains; August and September are fruiting months, and these are followed by rest for such crops. Then, however, Oats, Barley, Tobacco, Wheat, Sesamum, Poppy, and all Pulses are put in to be reaped in spring. In Malabar on the average the seasons are the same, but the dry season is so damp that the most tropical crops can be raised all the year round.

Of course in the Southern Hemisphere the seasons are the reverse of those in the Northern, midwinter in Sydney corresponding with midsummer in Europe.

In the tropical parts of America, where Humboldt found the mean temperature of the coldest month not to be lower than 79-16° at Cumana, we shall be justified in concluding that the temperature of the earth's surface never falls permanently below that amount; and as the mean summer temperature of the place was found to be 82.04°, so it is probable that the earth will have something above that degree of warmth, on an average, in the summer.

For the warmest month, this great observer gives 84.38° as the mean, which corresponds remarkably with the temperature a foot below the surface in New Grenada, where, according to a correspondent of Mr. Hay, it is 85° during summer, "as a gentleman, a planter there, wrote home for his information." (See Loudon's Gard. Mag., vi. 437.)

BOOK II.

OF THE PHYSIOLOGICAL PRINCIPLES UPON WHICH THE OPERATIONS OF HORTICULTURE ESSENTIALLY DEPEND.

ALL operations in horticulture depend for success upon a correct appreciation of the nature of the vital actions described in the last Book; for although there have been many good gardeners entirely unacquainted with the science of vegetable physiology, and although many points of practice have been arrived at altogether accidentally, yet it must be obvious that the power of regulating and modifying knowledge so obtained cannot possibly be possessed, unless the external influences by which plants are affected are clearly understood. Indeed, the enormous difference that exists between the skill of the present race of gardeners and their predecessors can only be ascribed to the general diffusion, that has taken place, of an acquaintance with some of the simpler facts in vegetable physiology.

In attempting to apply the explanations of science to the routine of horticultural practice, it appears desirable, in order to avoid frequent repetition, that mere details should be omitted, and that those general operations should alone be adverted to which, under many different modifications, and in various forms, constitute the foundation of every gardener's education.

CHAPTER I.

OF BOTTOM HEAT.

THIS term is, in common practice, made use of only in those cases where the temperature of the soil in which plants grow is artificially raised considerably above that which we are acquainted with in England; and there seems to be a general idea that such an artificial elevation of temperature is only necessary in a few special instances. It has, however, been shown (p. 125) that the mean temperature of that part of the soil in which plants grow is universally something higher than that of the air by which they are surrounded, and consequently it appears that nature, in all cases, employs some degree of bottom heat as a stimulus and protection to vegetation. At the same time, it must be admitted that, in some cases, the amount is extremely small; for Von Baer found Ranunculus nivalis and Oxyria reniformis flowering in Nova Zembla, where the soil was not warmed above 341; and, in Jakutzsk, Erdmann states that Summer Wheat, Rye, Cabbages, Turnips, Radishes, and Potatoes are cultivated, although the ground is not thawed above three feet in depth.

How the warmth of the soil may act as a protection to plants will be easily understood. A plant is penetrated in all directions by innumerable air passages and chambers, so that there is a free communication between its extremities however far they may be apart. It may therefore be conceived that if, as necessarily happens, the air inside the plant is in motion, the effect of warming the air in the roots will be to raise the internal temperature of the whole individual; and the same is true of its fluids. Now, when the temperature of the soil is raised to 150° at noonday by the force of the solar rays, it will retain a considerable part of that warmth during the night: but the temperature

NATURAL TEMPERATURE OF SOIL.

135

of the air may fall to such a degree that the excitability of a plant would be too much and suddenly impaired, if it acquired the coldness of the medium surrounding it; this is prevented, we may suppose, by the warmth communicated to the general system, from the soil, through the roots; so that the lowering of the temperature of the air, by radiation during the night, is unable to affect plants injuriously, in consequence of the antagonist force exercised by the heated soil. It is not improbable that this fact may be hereafter applied in gardening to the acclimatising of half-hardy plants. Were an open border heated artificially in the winter, it is possible that plants might endure an amount of cold upon their stems and leaves, which in the absence of such heat would be fatal to them. An experiment upon this subject was tried some years ago, and although it was conducted so negligently and unskilfully, as not to justify any inference being drawn from it, yet the result, such as it was, was full of promise.

That elevating the temperature of moist soil produces an unusual degree of vigour in plants unaccustomed in nature to such an elevation is a fact which requires no proof; it is attested by the condition of vegetation round hot springs, and in places artificially heated by subterraneous fires; and this has probably been the cause of the employment of tan and hot-beds, by which means bottom heat has been generally obtained for rearing delicate species, and especially seeds. But if this stimulus acts in the first instance beneficially in all cases alike, it soon becomes a source of mischief in those species which are natives of climates where such terrestrial heat is unknown, the latter "drawing up," as the saying is, becoming weak and sickly, and speedily presenting a diseased

appearance.

On the other hand, it is equally well known that, unless the temperature of the soil be raised permanently to at least 75°, the seeds of tropical trees will not germinate; or, if they do, they push forth feebly, and from the first present the sickly appearance of plants suffering from cold. Hence arises the impossibility of making the seeds of tropical plants germinate when sown in the open air in this country, where the mean temperature of the earth seldom rises to 65°, and that for only short periods of time. It is, therefore, obvious that all plants require some bottom heat; but the amount varies with their species, and the only means of determining what the amount should be

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