WITH REFERENCE TO HORTICULTURE. 



81 



consequence the earth at their roots is kept in a state comparatively dry ; 

 the only supply of moisture being commonly derived from the pots and the 

 exhalations of the leaves, is not enough to saturate the air, and the conse- 

 quence is a prodigious power of evaporation. This is injurious to the plants 

 in two ways : in the first place, if the pots be at all moist, and not protected 

 by tan or other litter, it produces a considerable degree of cold upon their 

 surface, and communicates a chill to the tender fibres with which they are 

 lined. The danger of such a chill is carefully guarded against in the case 

 of watering, for it is one of the commonest precautions not to use any water 

 of a temperature at all inferior to that of the hot air of the house ; inattention 

 to this point is quickly followed by disastrous consequences. The danger is 

 quite as great from a moist flower-pot placed in a very dry atmosphere. 



256. " The custom of lowering the temperature of fluids in hot climates, by 

 placing them in coolers of wet porous earthenware^ is well known, and the 

 common garden pot is as good a cooler for this purpose as can be made. 

 Under the common circumstances of the atmosphere of a hothouse, a depres- 

 sion of temperature, amounting to fifteen or twenty degrees, may easily be 

 produced upon such an evaporating surface. But the greatest mischief will 

 arise from the increased exhalations of the plants so circumstanced, and the 

 consequent exhaustion of the powers of vegetation. Some idea may be 

 formed of the prodigiously increased drain upon the functions of a plant 

 arising from an increase of dryness in the air, from the following considera- 

 tion. If we suppose the amount of its perspiration, in a given time, to be 

 57 grains, the temperature of the air being 75°, and the dew-point 70°, or the 

 saturation of the air being 849, the amount would be increased to 120 grains 

 in the same time if the dew-point were to remain stationary, and the tem- 

 perature were to rise to 80° ; or, in other words, if the saturation of the air 

 were to fall to 726, 



257. " The cause why plants in living rooms do not thrive so well as those 

 which are kept in plant structures, is chiefly owing to the extreme dryness 

 of the air, while a constant drain upon the leaves and the soil of the pots is 

 maintained for moisture. Hence the fibres in the inside of the pots are alter- 

 nately moistened and dried, and cooled and heated, and the leaves are deprived 

 of their water by evaporation instead of by perspiration. 



258. '* Besides the power of transpiration, the leaves of vegetables exercise 

 also an absorbent function^ which must be no less disarranged by any defi- 

 ciency of moisture. Some plants derive the greatest portion of their nutri- 

 ment from the vaporous atmosphere, and all are more or less dependent 

 upon the same source. 



259. " These considerations must be sufficient," Mr. Daniell imagines, 

 " to place in a strong light the necessity of a strict attention to the atmosphere 

 of vapour in our artificial climates, and to enforce as absolute an imitation 

 as possible of the example of nature. The means of effecting this is the next 

 object of our inquiry." 



260. " Tropical plants require to be watered at the root with great caution, 

 and it is impossible that a sufficient supply of vapour can be kept up from 

 this source alone. There can, however, be no difficulty in keeping the floor 

 of the house and the flues contmually wet, and an atmosphere of great elas- 

 ticity may thus be maintained in a way perfectly analogous to the natural 

 process. Where steam is employed as the means of communicating heat, an 

 occasional injection of it into the air may also be had recourse to : but this 



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