508 Analogy between Vegetables and Animals, 



constructed in different animals. In man, and the higher 

 classes, it consists of the windpipe and its ramifications, of 

 the lungs, and of the diaphragm. The windpipe is an elastic 

 tube, which runs down the fore-part of the neck. It divides 

 first into two, and then into smaller branches ; which at last 

 terminate in small cells, that form the minute structure of the 

 lungs. The lungs are organs of a light spongy texture ; and 

 composed of bloodvessels, air-cells, and cellular substance. 

 In fishes, the organs of respiration consist of branchiae or 

 gills. In many of the invertebrated animals, they consist of 

 a number of tubes called tracheae, into which the air is con- 

 stantly entering. There are numerous tribes in which no 

 distinct respiratory apparatus can be found : in this case, the 

 air acts on the blood through the medium of the skin. 



The sap of plants, in the crude and indigested state in which 

 it is absorbed from the soil, contains a large proportion of 

 water; and before it is fit for the purposes of nutrition, it 

 undergoes in its nature and properties a general change, 

 which bears a striking resemblance to that which the blood 

 undergoes in the lungs. If plants be exposed to the light, 

 and particularly to the direct rays of the sun, they exhale an 

 abundance of vapour, and likewise of oxygen gas. This 

 watery exhalation is supplied from the sap, and the oxygen 

 from the decomposition of carbonic acid gas, which vegetables 

 imbibe from the atmosphere and from the earth. As soon as 

 they have absorbed it, it is decomposed by the light : the 

 oxygen is set free, and the carbon is retained. Hence it 

 appears that carbon is the proper food of plants ; and thus 

 vegetation destroys the deleterious effects arising from the 

 respiration of animals. In the dark, however, plants, instead 

 of exhaling, absorb oxygen. The reason of this I will explain. 

 The sap, besides containing a quantity of carbonic acid gas, 

 contains likewise, in solution, carbon combined with animal 

 and vegetable matter. Before this carbon can be assimilated, 

 it must be combined with oxygen, and converted into car- 

 bonic acid. For this purpose, then, oxygen is absorbed in 

 the night ; in the day, the carbonic acid thus formed is de- 

 composed, the carbon is deposited, and the oxygen flies off. 

 But, even during the day, when placed in the shade, vege- 

 tables absorb oxygen, and evaporate carbonic acid gas. Plants 

 not only exhale moisture, but they absorb it : they likewise 

 absorb a portion of nitrogen. All these processes are performed 

 in the leaves ; which may be considered the respiratory organs 

 of plants. Their structure resembles that of the lungs ; con- 

 sisting of bloodvessels, air-tubes, and cellular tissue. If the 

 leaves be plucked off, so that the sap cannot be properly 



