THE PINE-APPLE. 



651 



most of the best cultivators of exotic plants, 

 and practical experience has long ago proved 

 the correctness of the principle. Professor 

 Daniell, whose opinions on these matters are 

 entitled to the utmost respect, observes, in 

 " Transactions of the Horticultural Society," 

 vol. vi. p. 1, "If the hygrometer were consulted 

 in hothouses as commonly managed, it would 

 be no uncommon thing to find in them a differ- 

 ence of 20° between the point of condensation 

 and the air, or a degree of moisture faUing short 

 of 500°." Dryness in the atmosphere of hot- 

 houses, although accompanied with sufBcient 

 heat, has as disastrous effects upon the plants 

 as the evaporating east winds in spring, accom- 

 panied by their coldness, has upon tender trees 

 in the open air. To provide against these effects, 

 Dr Daniell says ; " The danger of over-watering 

 most of the plants, especially at particular 

 periods of their growth, is in general very justly 

 appreciated, and in 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 consequence is a prodigious power of evapo- 

 ration. This is injurious to the plants in two 

 ways : in the first place, if the pots are 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 

 at 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. 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 depression of temperature, amounting to 15° 

 or 20°, 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 ex- 

 haustion 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 consideration : 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 temperature 

 were to rise to 80°, or, in other words, if the 

 saturation of the air were to fall to 726. 



" The cause why plants in living-rooms do not 

 thrive so well as those that are kept in plant 

 structures, is chiefly owing to the extreme dry- 

 ness of the air, while a constant drain upon the 

 leaves and the soil of the pots is maintained. 



Hence the fibres in the inside of the pots are 

 alternately moistened and dried, and cooled and 

 heated, and the leaves are deprived of their 

 water by evaporation instead of by perspiration. 

 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 nutriment from the 

 vaporous atmosphere, and all are more or less 

 dependent upon the same source. These consi- 

 derations must be sufficient 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 pos- 

 sible of the example of nature." 



Professor Lindley, in " Theory of Horticul- 

 ture " says : " The skilful balancing of the tem- 

 perature and moisture of the air, in cultivating 

 different kinds of plants, and the just adaptation 

 of them to the various seasons of growth, con- 

 stitute the most complicated and difficult part 

 of a gardener's art. There is some danger in 

 laying down any general rules with respect to 

 this subject, so much depends upon the peculiar 

 habits of species, of which the modifications are 

 endless. It may, however, I think, be safely 

 stated, that the following rules deserve especial 

 attention : Most moisture in the air is demanded 

 by plants when they first begin to grow, and least 

 when their periodical growth is completed. The 

 quantity of atmospheric moisture required by 

 plants is, cceteris paribus, in inverse proportion to 

 the distance from the equator of the countries 

 which they naturally inhabit. Plants with an- 

 nual stems require more than those with lig- 

 neous stems. 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." 



John Rogers, Esq., on the same subject, ob- 

 serves in " The Gardeners' Magazine" that "the 

 causes, whose constant operation renders our 

 artificial climates unnaturally dry, are principally 

 two — the condensation of moisture on the glass, 

 and the escape of heated and damp air through 

 the crevices of the building, the space which it 

 occupied being constantly supplied by dry ex- 

 ternal air. A third drain of moisture formerly 

 existed in the absorbing surface of brick flues, 

 which drank up the moisture of the air in con- 

 tact with them, and carried it off with the smoke 

 into the outer air. The very general use of hot 

 water in iron pipes has removed this nuisance, 

 and we have now only to contend with the two 

 first mentioned. Some idea of the drain of mois- 

 ture, by the escape of heated air, may be formed 

 from the following considerations. The capacity 

 of air for moisture^that is to say, the quantity 

 of water which a cubic foot of air will hold in 

 invisible solution— depends upon its tempera- 

 ture, and increases with it in a rapid ratio. It 

 is doubled between 44° and 66°. The conse- 

 quence is, that every cubic foot of air which 

 escapes at the latter temperature carries off with 

 it twice as much moisture as it brought in. 

 Where the difference of temperature is greater. 



