XII. RESPIRATION, FERMENTATION AND DIGESTION 



318. Derivation and Conversions of Energy. Solar radiations 

 constitute the ultimate source of all energy in the organic world. 

 The waves of light act only upon the external layers of the body 

 of a plant and with intermittent periods of darkness. The activity 

 of protoplasm is almost continuous however, so that it has become 

 necessary for it to absorb energy from light during periods of 

 illumination, and store it up for use when needed. In order to 

 accomplish this the kinetic energy of light is converted into 

 potential in the complex chemical compounds formed in photo- 

 synthesis, and these may be translocated to any part of the body 

 and stored for indefinite periods, and it has been pointed out in 

 previous sections of this book that the amount of energy accumu- 

 lated in this manner is generally much greater than that used by 

 the individual plant itself. 



Any other organism such as an animal, or another plant that 

 can assimilate these compounds without previous disintegration, 

 may acquire and use their contained potential. This is accom- 

 plished by most bacteria, fungi and chlorophylless seed plants. It 

 is also possible that some forms acquire energy from heat radiations. 



Material built up in this manner may be used in construction 

 with no disintegration or liberation of energy, or it may be broken 

 up to obtain the energy which it contains in potential form. 

 Thus with a given amount of wood some of it may be used to 

 form the timbers of a house or bridge, while the remainder is 

 burned in an engine, to obtain energy to cut the boards and hoist 

 them into position. 



The evaporation of water in transpiration, and the accompany- 

 ing physical processes use more than 98 per cent, of the energy 

 absorbed from sunlight by a plant, and all of the other work of 

 the organism is accomplished by means of the remaining 2 per 



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