34 Public Parks. 



The fluid absorbed by the roots, is thus carried from cell to cell, 

 rising principally in the wood, and is attracted to the leaves, or 

 other parts of the plants exposed to sun and light, by the exhalation 

 which takes place from them, and the consequent inspiration of the 

 sap. Here the crude sap is exposed to sun and light, and assimilated 

 and converted into organizable matter. Carbon, hydrogen, oxygen, 

 and nitrogen, are indispensable to vegetation, and make up at least 

 from 88 to 90 per cent, of every vegetable substance ; the proper 

 vegetable structure, however, is only composed of carbon, hydrogen, 

 and oxygen. 



Plants also receive nitrogen in the form of ammonia, which is 

 always produced when any animal, and almost when any vegetable 

 substance decays, and which, being very volatile, continually rises 

 into the air from these and other sources. Ammonia is soluble and 

 is greedily absorbed by aqueous vapor, and is brought to the ground 

 by rain and snow. The carbon of plants is derived wholly from the 

 carbonic acid of the atmosphere, and it makes up jaV-g- ^^ its bulk, 

 from which it is directly absorbed by the leaves. It may then be said 

 that the atmosphere contains all the essentials to plant growth, viz., 

 water in a state of vapor, which is not only food itself as it supplies 

 oxygen and hydrogen, but is likewise the vehicle of the others, 

 carrying to the roots what it has gathered from the air, namely, the 

 requisite supplies of nitrogen, either as such, or in the form of 

 ammonia, and of carbon in the forin of carbonic acid. 



In fact, all of the essential elements of plants or proper food 

 may be absorbed by the leaves directly from the air, and no doubt 

 most plants take in a great part of their food in this way as droop- 

 ing foliage may be revived by sprinkling with water, or exposure to 

 a damp atmosphere. 



Air plants live on the atmosphere, and a branch of the common 

 "•Live Forever" will grow when pinned to a dry and bare wall. 

 All leafy plants derive their carbonic acid from the air, and many, 

 as has already been stated, derive their whole food from the air 

 or part of it. It is found, that when a current of carbonic acid 

 is made to traverse a glass globe containing a leafy plant exposed to 

 full sunshine, some carbonic acid disappears, and an equal bulk of 

 oxygen gas supplies its place. Now since carbonic acid gas contains 

 just its own bulk of oxygen, it is evident that what has thus been 

 decomposed in the leaves has returned all its oxygen to the air. 



