20 



On Climate, $c. 



so circumstanced, and the consequent exhaustion of the 

 powers of vegetation. The flowers of the torrid zone are 

 many of them of a very succulent nature, largely supplied 

 with cuticular pores, and their tender buds are unprovided 

 with those integuments and other wonderful provisions by 

 which nature guards her first embryo productions in more 

 uncertain climates. Comparatively speaking they shoot na- 

 ked 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 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 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. 



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 de- 

 rive the greatest portion of their nutriment from the vaporous 

 atmosphere, and all are more or less dependent upon the 



