256 ON PRUNING TIMBER TREES. 



short off 500. 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 ; the exhalations of the 

 leaves is not enough to saturate the air, and the consequence 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 air of the house ; inattention to this 

 point is quickly followed by disasterous 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 cli- 

 mates, 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 hot-house, a depression of temperature 

 amounting to 15 or 20 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. 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 

 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 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. 



{To be continued) 



