33 CIRCULATION OF THE ATMOSPHERE, CONSIDERED 



protection would be so effective as double glass, including a stratum of 

 air. When it is considered that a temperature at night of 20 is of 

 no very unfrequent occurrence in this country, the saturation of the 

 air may, upon such occasions, fall to 120; and such an evil can only 

 at present be guarded against by diminishing the interior heat in pro- 

 portion. But, by materially lowering the temperature, we communi- 

 cate a check which is totally inconsistent with the welfare of tropical 

 vegetation. It is, however, more natural, and consequently better for 

 the health of the plants, to maintain a night temperature of from 10 

 to 15 lower than the day. This checks the loss of heat, and conse- 

 quently of moisture, and brings down a natural dew on the leaves. 



" The skilful balancing of the temperature and moisture of the air," 

 said the late Dr. Lindley, " in cultivating different kinds of plants, and 

 the just adaptation 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 sub- 

 ject, 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 special 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 of the countries 

 which they naturally inhabit from the equator. 



(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.) 



The Circulation of the Atmosphere, considered in its relation 

 to Horticulture. 



The motion of the air is essential to life. It is the great carrier of 

 the world's food and drink, the great remover of its filth and poison. 

 It brings the food of plants from afar, and places it within reach of the 

 open mouths of roots and leaves. Many have thought that by the 

 motion it imparts it likewise helps plants to digest or assimilate their 

 food, while the mechanical force of moving air is supposed to give 

 strength and stability to their closely packed fibres and tissues. Moving 

 air likewise sucks up the watery matter of plants, and increases the power 

 and activity of their perspiring and absorbing functions. The wind 

 dries and moves all bodies exposed to it, and doubtless each of these in 

 moderation is essential to the full development of the life and beauty 

 of plants. But, while exposure to air in motion is beneficial, the air 

 very often moves faster, and the wind proves stronger, than desired. 

 For this reason shelter may often be needed to break the force or 

 change the direction of large volumes of air in motion. The most 



