AGITATION OP THE ATMOSPHERE CONSIDERED. 83 



totally inconsistent with the welfare of tropical vegetation. The chill which 

 is instantaneously communicated to the glass by a fall of rain and snow, and 

 the consequent evaporation from its surface, must also precipitate the internal 

 vapour, and dry the included air to a very considerable amount, and the 

 effect should be closely watched." (Hort. Trans., vol. vi. p. 23.) 



262. " The skilful balancing of the temperature and moisture of the air," says 

 Dr. Lindley, " in cultivating different kinds of plants, and the just adapta- 

 tion of them to the various seasons of growth, constitute 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 : 



(1.) Most moisture in the air is demanded by plants when they first begin 

 to grow, and least when their periodical growth is completed. 



(2.) The quantity of atmospheric moisture required by plants is, cateris 

 paribus, in inverse proportion to the distance from the equator of the coun- 

 tries which they naturally inhabit. 



(3.) Plants with annual stems require more than those with ligneous 

 stems. 



(4.) 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." (Theory of Hort., p. 153.) 



SECT. III. The Agitation of the Atmosphere considered with reference to 

 Horticulture. 



263. The motion of the atmosphere., known as wind, and varying in grada- 

 tion from the gentlest breeze to the most furious tempest, is, to a certain 

 extent, under the control of the horticulturist. He cannot, indeed, agitate 

 the air at pleasure, but he can lessen the agitation, when it is produced by 

 nature, by shelter ; and he can take advantage of it when it is wanted, by 

 exposure ; and, in hothouses, he can even create agitation. The use of wind 

 in the economy of nature seems to be to carry off impure exhalations from 

 particular spots, and to equalize in the atmosphere the diffusion of gaseous 

 matters, and of moisture and temperature. The free action of the wind on 

 the surface of the ground assists in drying it when moist, because air has a 

 great capacity for water ; and it promotes the vigour of plants, and especially 

 of trees, by the motion which it produces in their stems, branches, and leaves. 

 In some cases it may do good by carrying off insects, and in others injury 

 by bringing them. The fact that the motion of the stems and leaves of trees 

 by wind increases their diameter, is doubted by some, though according to 

 others it is easily proved by observing what takes place in fruit-trees trained 

 against walls, as compared with the same species growing as standards. If, 

 say those who are of the latter opinion, the deposition of woody matter in the 

 stems and branches depended on the number and exposure to the sun of the 

 leaves, then wall-trees must necessarily have a thicker stem than standard- 

 trees ; but as the contrary is the case, and as the only difference in the circum- 

 stances in which standard and wall-trees are placed is, that the standards are 

 subject to be put in motion by the wind, to that cause we must attribute the 

 greater thickness of their stems and branches. It is added, that tying the 



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