INFLUENCE OF AIR ON PLANTS. 871 



In many respects, they closely resemble each 

 other. In the first place, they are both formed 

 of the same elementary constituents, (oxygen, 

 hydrogen, carbon, and nitrogen,) though in differ- 

 ent proportions. They are both furnished with 

 organs of circulation, secretion, nutrition, and 

 generation, all of which are maintained in a state 

 of vital activity by the same principle. In other 

 respects, however, they differ essentially. 



For example, we have seen that plants have 

 no breathing apparatus,* and depend wholly on 

 the agency of external temperature for the mani- 

 festation of their powers. The consequence of 

 which is, that they have no complex organs of assi- 

 milation and absorption, no nervous and muscular 

 tissues, no sensation, perception, or power of loco- 



* It is true that in germination, atmospheric oxygen unites 

 with a portion of the carbon contained in the starch of seeds, by 

 which sugar is formed for supplying the germ with nourishment, 

 until its leaves are developed. The same process takes place 

 during the flowering or fecundation of plants, by which a portion 

 of the farina is converted into sugar for sustaining the embryo 

 bud, carbonic acid being formed, and caloric evolved, as during 

 germination. Botanists have observed, that when the sugar of 

 flowers has been extensively rifled by honey bees, they are ren- 

 dered unfruitful. Oxygen also unites with a portion of the carbon 

 contained in the gum and lignin of green fruits, during the pro- 

 cess of ripening, by which sugar is formed. The leaves of plants, 

 however, must be regarded as organs of assimilation rather than 

 of respiration, in which crude sap is converted into cambium by 

 the decomposition of carbonic acid, or the appropriation of its 

 carbon, and the liberation of oxygen. Nor is it unworthy of 

 notice, that mushrooms, mosses, algae, lichens, and other plants 



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