103 ELEMENTS Ol'" STRUCTURAL riOTANY. 



found ou the upper surface, and iu vertical leaves they 

 occur pretty equally ou both surfaces. Immersed leaves 

 are without stomates. 



174. The crude sdp, then, which ascends into the 

 leaves is concentrated by the evaporation of its super- 

 fluous water. When so concentrated, the action of sun- 

 light, in connection with the green colouring matter 

 existing in the cells of the leaves, and known as chloro- 

 phyll, decomposes the carbonic acid, contained in the 

 sap, into its carbon and oxygen. The latter gas issues 

 from the leaves into the air, whilst the carbon is retained 

 and combined with the remaining elements to form 

 dahorated sap, out ui which the substance of new 

 cells is constructed. 



175. It thus appears that the chemical action which 

 goes on in the leaves of plants is precisely the reverse 

 of what takes place iu the lungs of animals. Tlie latter 

 inhale oxygen, combine it with the carbon of the blood, 

 and exhale the resulting carbonic acid. The former take 

 in carbonic acid, decompose it in the leaves and other 

 green parts, and exhale the oxygen. Plants may there- 

 fore be regarded as purifiers of the air. 



17G. It remains to be added, that besides the four 

 substances, carbon, oxygen, hydrogen, and nitrogen, 

 which are called the organic elements, many others 

 are found in the fabric of plants. When a piece of 

 wood is burnt away, the organic elements disappear, 

 but a quantity of a.s/t remains behind. This contains 

 the various mineral substances which the water absorb- 

 ed by the plant has previously dissolved out of the soil, 

 but which it is not necessary to our present purpose to 

 enumerate here. 



