134 BOTANY OF THE LIVING PLANT 



of materials, yielding chemical energy for the cell-processes. The 

 term Respiration is applied to any such energy-releasing processes 

 in plants, some type of which is carried on in every living cell. 



The type of respiration normal for the great majority of plants 

 consists in the oxidation of sugars to carbon dioxide and water ; 

 it may be summarised by the equation : 



C 6 H 12 6 + 60 2 = 6C0 2 + 6H 2 + 674,000 calories. 



A considerable amount of energy is evidently produced by the com- 

 bustion of sugar : it is equal to that required for the building-up of 

 sugars (see p. 114). The process represented by the above equation 

 is in effect that of photosynthesis reversed, though the intermediate 

 stages are very different, as we shall see later. The sugars oxidised 

 in respiration have been previously manufactured in the plant by 

 photosynthesis. Since the sugar molecules are broken down in the 

 process, we may see in respiration a phase of metabolism which is 

 destructive in nature, as compared with the constructive phases 

 which we have so far considered. In photosynthesis the plant 

 stores up potential chemical energy within the sugar molecules, 

 while it is able by means of the respiratory process to release that 

 energy as required and at any point in the plant body. 



The equation indicates that respiration in the plant, as in the 

 animal, is attended by the absorption of oxygen, which is obtained 

 from the atmosphere (or from photosynthesis) and by the evolution 

 of carbon dioxide. While in the animal there may be mechanical 

 inhalation and exhalation of the gases involved the respiratory 

 interchange in the higher land-plants depends on simple gaseous 

 diffusion between the atmosphere and the air spaces of the tissues, 

 in aerial organs via the stomata and lenticels. In roots oxygen 

 enters dissolved in the water which the root absorbs, while the 

 carbon dioxide produced in respiration escapes from the roots in 

 solution. Similar considerations apply to submerged aquatic plants. 



The evolution of carbon dioxide by plants is most readily demon- 

 strated in the case of germinating seeds or of flowers, in which respira- 

 tion is specially active ; while photosynthesis, which tends to mask 

 respiration, is absent. If a stream of air, from which the carbon 

 dioxide initially present has been removed by passage through 

 caustic soda, is drawn through a flask containing such respiring 

 material, and then through a further flask containing lime-water, a 

 copious precipitate will soon be formed in the latter. The evolution 

 of carbon dioxide is thus demonstrated. 



