224 
ON THE CLIMATE OF HOT-HOUSES. 
those integuments, and other wonderful provisions, by which nature guards her 
first embryo in more uncertain chmates. Comparatively speaking-, they shoot 
naked into the world ; and are suited only to that enchanting mildness of the 
atmosphere, for which the whole system of their org-anisation is adapted. In the 
tropical climates the sap never ceases to flow ; and sudden checks or accelerations of 
its progress are as injurious to its healthy functions, as they are necessary in the 
plants of more variable climates, to the formation of those hybernacula which are 
provided for the preservation of the shoots in the winter season. 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 sup- 
pose the amount of its perspiration, in a given time, to be fifty-seven grains, the 
temperature of the air being seventy-five degrees, and the dew point seventy ; or 
the saturation of the air being eight hundred and forty-nine, the amount would be 
increased to one hundred and twenty grains in the same time, if the dew point were 
to remain stationary, and the temperature were to rise to eighty degrees ; or, in 
other words, if the saturation of the air were to fall to seven hundred and twenty- 
six. Besides this power of transpiration, the leaves of vegetables exercise also an 
absorbent function, which must be no less disarranged by any deficiency of moisture. 
Some plants derive the greatest portion of their nutriment from the vaporous atmo- 
sphere ; and all are more or less dependent on the same source. The Nepenthes 
distillatoria lays up a store of water in the cup formed at the ends of its leaves, 
which is probably secreted from the air, and applied to the exigencies of the plant, 
when exposed to drought ; and the quantity, which is known to vary in the hot- 
house, is no doubt connected with the state of moisture of the atmosphere. 
These considerations must be sufficient, I imagine, 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 our next object of inquiry. Tropical plants require to be 
watered at the root with great caution ; and it is impossible that a suflScient supply 
of moisture can be kept up from this source alone. There can, however, be no dif- 
ficulty in keeping the floor of the house and the flues continually wet ; and an 
atmosphere of great elasticity may thus be obtained, in a way perfectly analogous to 
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 
method would require much attention on the part of the superintendent, whereas 
the first cannot easily be carried to excess. It is true, that damp air, or floating 
moisture, of long continuance, would also be detrimental to the health of the plants; 
for it is absolutely necessary that the process of transpiration should proceed ; but 
there is no danger that the high temperature of the hot-house should ever attain the 
point of saturation by spontaneous evaporation. The temperature of the external 
air will always keep down the force of the vapour; for, as in the natural atmosphere, 
the dew point at the surface of the earth is regulated by the cold of the upper 
regions, so in a house the point of deposition is governed by the temperature of the 
