NUTRITION. 149 



replaced by the formation of new protoplasm from the foods. 

 Ultimately, therefore, respiration results in a diminution of 

 the reserve food. 



207. A vital function. — Respiration is a function of the 

 protoplasm, and does not occur simply because substances 

 are present in the plant which are destroyed when oxygen is 

 brought into contact with them, as fuel is in a furnace. On 

 the contrary, the oxygen seems to enter into loose combina- 

 tion with protoplasm, forming an extremely unstable com- 

 pound. This, under unknown conditions, and often some 

 time after its formation, breaks down into simpler substances, 

 so setting free energy. Some of these materials are again 

 used in building protoplasm, while others break down still 

 further, ultimately into water and carbon dioxid. The sup- 

 ply of oxygen is so necessary that if a plant cannot obtain 

 oxygen from the air or water, it will secure it by the destruc- 

 tion of part of its own substance for a time, thus burning the 

 candle of life at both ends. 



208. Heat. — While this decomposition of the protoplasm 

 in ordinary respiration is not a true combustion, it neverthe- 

 less results, as combustion does, in the evolution of heat. 

 The amount of heat produced is usually not great enough, 

 and its loss too rapid, to make it readily perceptible. Any- 

 thing which prevents the loss of heat will make its measure- 

 ment possible. The germination of large quantities of seeds 

 or the blossoming of a number of flowers in a confined space 

 may raise the temperature as much as 15 or 20° above that of 

 the air. 



The heating of hay, grain, and similar substances, which have been 

 stored when moist, is due partly to the respiratory activity of bacteria 

 and fungi, which grow rapidly under these conditions. Fermentation, 

 which also occurs under the same conditions, adds largely to the evolu- 

 tion of heat. 



