OF AGRICULTURE, 21 



stili in a crude form, and too diluted to be adapted to 

 the purposes to which it is designed ; it must, there- 

 fore, undergo certain changes, These changes take 

 place in the leaves* The sap is condensed, that is, 

 the surplus moisture is thrown off, This takes place 

 through the pores of the leaf, and is similar to the 

 perspiration of animals* This is called inhalation* 

 The pores or mouth of the leaf are open in the light 

 and closed in the dark, 



Plants derive a large proportion of their nourish- 

 ment from the air, through their leaves, in the form 

 of carbonic acid gas, and they at the same time 

 throw off oxygen, This inhalation of carbonic gas 

 and exhalation of oxygen is called respiration. It is 

 different from the breathing of animals, as the ani- 

 mal inhales oxygen and exhales carbonic gas. The 

 respiration of plants goes on chiefly by day, the 

 mouths or pores being opened by the influence of 

 light. When the carbonic acid gas enters the leaf it 

 is dissolved by the sap, and carried through the veins 

 of the leaf, when it is decomposed, its carbon being 

 retained by the plant, and its oxygen exhaled into 

 the air. 



The food taken up by the roots of plants and car* 

 ried by the sap to the leaves, these meet the gaseous 

 food of the air, forming by their solution " crude sap.'* 

 This is changed by its circulation through the leaf, 

 if an abundant supply of light be present, The 

 changes which plant food thus undergoes is called 

 digestion, because of its resemblance to animal diges- 

 tion. When the sap has thus been prepared for 

 nourishing the plant it is called the "true sap^ It 

 is then conveyed by the circulating organs to the 



