1^4 PLANT LIFE. 



into contact with them. On the contrary, the oxygen seems 

 to enter into loose combination with protoplasm, forming 

 an extremely unstable compound which under unknown con- 

 ditions breaks down into simpler substances, setting free 

 energy. Some of these materials are again used in building 

 protoplasm, while others break down still further, ultimately 

 into water and carbon dioxide. The supply of oxygen is so 

 necessary that if a plant cannot obtain oxygen from without, 

 it will secure it by the destruction of part of its own sub- 

 stance for a time, as shown by intramolecular respiration. 



248. Heat. — While this decomposition of the protoplasm 

 in ordinary respiration is not a true oxidation, it nevertheless 

 results, as oxidation 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. Anything 

 which prevents the radiation 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. Fermentative changes, which also 

 occur under the same conditions, add to the evolution of 

 heat. 



249. Light. — A few plants also produce light. This light 

 is like that seen when phosphorus is exposed to the air in 

 darkness, or when the end of a match is lightly rubbed. 

 Phosphorescence occurs only in some bacteria and fungi. 

 When it is seen upon decaying meat, fish, or wood, it is 

 because these organisms are present. It does not arise from 

 the decaying substance itself. Several of the larger fungi, as 

 certain toadstools, have a mycelium capable of emitting this 

 phosphorescent light. 



