BOOK IV. ACCELERATING VEGETATION. 421 



provement in the form as well as management of these buildings has, as in every other 

 case, been progressive; and there are now a great choice both of the forms adopted, the 

 materials used in the construction of these forms, and the mode of producing artificial 

 heat. 



2195. There are two leading modes of accelerating plants in hot-houses; the first is by 

 placing them there permanently, as in the case of the peach, vine, &c. planted in the 

 ground ; and the second is by having the plants in pots, and introducing or withdrawing 

 them at pleasure. As far as respects trees, the largest crops, and with far less care, are 

 produced by the first method ; but in respect to herbaceous plants and shrubs, whether 

 culinary, as the strawberry and kidneybean, or ornamental, as the rose and the pink, 

 the latter is by far the most convenient method, and it is also the best adapted for afford- 

 ing very early crops. (2185.) Where large pots are used, the peach, cherry, fig, &c. 

 will produce tolerable crops. Knight has observed, that " vines and other fruit-trees, 



being pruned in autumn, occupies twenty square feet of the roof of a hot-house ; and I 

 have constantly found that vines in such pots, being abundantly supplied with food and 

 water, have produced more vigorous wood, when forced very early, than others of the 

 same varieties, whose roots were permitted to extend beyond the limits of the house." 

 (Hort. Trans, vol. ii. p. 373.) 



2196. When trees are planted for a permanency within, or close to the outside of a hot- 

 house, the soil requires to be prepared of depth and quality according to the nature of 

 the tree ; and a principal consideration is to form, if such does not naturally exist, a sub- 

 soil, which shall be impenetrable to the roots. The depth of soil on such a substratum 

 need not in general be great, provided it be rich. Formerly a depth of three or four 

 feet was recommended ; but Hayward proposes to have his fruit-tree borders only fifteen 

 or eighteen inches deep ; which is conformable to an observation of Hitt, that the finest 

 crop of peaches he had ever seen, grew on trees which were nourished from a border not 

 more than one foot deep, with a compact rock below. Nicol allows from twenty-four to 

 thirty inches of soil. Knight is of opinion, that " a large extent and depth of soil seem 

 to be no farther requisite to trees than to afford them a regular supply of water, and a 

 sufficient quantity of organisable matter ;" and, he thinks, " the rapid growth of plants 

 of every kind, when their roots are confined in a pot to a small quantity of mould, till 

 that becomes exhausted, proves sufficiently the truth of this position." (Hort. Trans. 

 vol. ii. p. 127.) 



2197. The operations of forcing chiefly respect the admission of air, the supply of heat, 

 of light, and of water. The grand effect is produced by heat, and the great art is 

 just to supply as much as will harmonise with the light afforded by the sun and the 

 nature of the species of plant to be forced. All the operations of nature are gradual ; 

 and a good gardener will always follow these as the safest examples. He will never 

 be anxious to apply artificial heat before buds have naturally swoln ; he will then 

 increase the temperature gradually for some weeks ; he will in particular guard against 

 any sudden decrease of warmth, it being most necessary towards success, to con- 

 tinue the course of vegetation uninterruptedly, through foliation, inflorescence, and 

 fructification. 



2198. Heat and light. An error in hot-house culture in general, of very considerable 

 importance, and which has prevailed till lately, consists in not adjusting the heat of art 

 to the light of the sun. In cloudy weather, and during night, the artificial atmosphere 

 is kept hot by fires and exclusion of the external air, while in clear days and during 

 sunshine, fires are left off or allowed to decline, the external air is admitted, and the at- 

 mosphere within is reduced to the temperature of that without. As heat in nature is the 

 result of the shining of the sun, it follows that when there is most light there is most 

 heat ; but the practice in forcing is very generally the reverse. " A gardener, in forcing," 

 Knight observes, " generally treats his plants as he would wish to be treated himself; 

 and consequently, though the aggregate temperature of his house be nearly what it ought 

 to be, its temperature, during the night, relatively to that of the day, is almost always too 

 high." In one of Knight's forcing-houses, in which grapes are grown, he always wishes 

 to see its temperature, in the middle of every bright day in summer, as high as 90 ; " and 

 after the leaves of the plants have become dry, I do not object to ten or fifteen degrees 

 higher. In the following night, the temperature sometimes falls as low as 50 ; and so 

 far am I from thinking such change of temperature injurious, I am well satisfied that it 

 is generally beneficial. Plants, it is true, thrive well, and many species of fruit acquire 

 their greatest state of perfection in some situations within the tropics, where the tempera- 

 ture in the shade does not vary in the day and night more than seven or eight degrees ; 

 but in these climates, the plant is exposed during the day to the full blaze of a tropical 

 sun, and early in the night it is regularly drenched with heavy wetting dews ; and con- 



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