136 Mr. Danidl on Climate. [Ffift. 



and other wonderful provisions by which nature guards her first 

 embryo productions in more uncertain climates. 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 organization 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 func- 

 tions as they are necessary in the plants of more variable 

 climates to the formation of those hybernacula which are pro- 

 vided for the preservation of the shoots in the winter season. 

 Some idea may b(i 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 76 , 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. 



Besides this power of transpiration, the leaves of vegetables 

 exercise also an absorbent function, which must be no less dis- 

 arranged 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 upon the same source. 

 The Nepenthes distiUatoria lays up a store of water in the cup 

 formed at the end 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 atmo- 

 sphere 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. 



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 

 continually wet, and an atmosphere of great elasticity may thus 

 be maintained in a way perfectly analogous to the natural 

 process. Where steam is employed as the means of communi- 

 cating heat, an occasional injection of it into the air may also 

 be had recourse to ; but this method would require much atten- 

 tion on the part of the superintendant, whereas the first cannot 

 easily be carried to excess. 



It is true that damp air or floating moisture of long continu- 

 ance would also be detrimental to the health of the plants, for it 



