122 GROWTH OF PLANTS IN AIR. 



and power obtained by commerce, and for the materials of his 

 various manufactures. 



172. From what has been stated, it would appear that water, 

 carbonic acid, and a minute quantity of ammonia, supply the 

 ingredients of the new compounds, which are formed in the 

 living Plant ; but that, in most cases, mineral substances of some 

 kind are required in addition. There are some plants which 

 derive a sufficient quantity of all these elements, from the atmo- 

 sphere alone; so as to be able to maintain life, and even to flourish, 

 without any other kind of supply. The water is absorbed by 

 the general surface, but especially by the roots, which in such 

 plants are usually long, and of soft tissue throughout ; the car- 

 bonic acid is taken in through the green parts from the atmo- 

 sphere alone, in the manner which will be described in the next 

 Chapter ; and the minute quantity of ammonia, also contained in 

 the atmosphere, which is probably dissolved in water and taken 

 up with it, affords a sufficient supply of nitrogen. Such aerial 

 plants usually contain but a very small quantity of mineral 

 matters ; and these, too, are probably derived from the atmo- 

 sphere, in which, as will be hereafter mentioned, their particles 

 are suspended. 



173. These aerial plants, clustering round the branches of 

 lofty trees, and hanging to a great depth beneath them, are ex- 

 tremely common in tropical climates ; in which the atmospheric 

 moisture is much greater, and where they constitute an important 

 part of the vegetation ; and they are not wanting in this country. 

 Many trees and plants, which do not ordinarily grow in this 

 manner, may be caused to do so by accident or design ; and may 

 even thrive extremely well. At New Abbey, in Galloway shire, 

 in the year 181 7 5 there was growing on the top of a stone wall 

 which measured ten feet in height, a Plane tree, which measured 

 twenty feet in height ; and, as it soon exhausted the bare and 

 scanty soil in which the young plant grew, it sent down roots 

 which clung to the side of the wall, and threw out neither bud 

 nor branch until they reached the ground, which was not until 

 several years had elapsed ; during all this time, the tree must 

 have lived upon the materials supplied by the atmosphere alone. 



