THE BREATHING OF THE PLANT 79 



expending, in its growth and in its varied activities, the 

 energy which it possesses within itself ; and, unless this 

 energy can be restored, it must die. 



As a result of the absorption of food, new plant— or 

 .animal — tissues are constantly being built up, and these 

 tissues form a storage of energy in a potential form. 



The meaning of the word " potential" in this connec- 

 tion will be readily understood from an illustration. 

 Water, stored in a reservoir, has " potential " energy. 

 It can be used to drive an engine or to turn a mill- 

 wheel. But, before it can be of any use whatever, it 

 must be released from the reservoir and allowed to 

 expend its energy in movement to a lower level, i.e. the 

 "potential" energy must be converted into "kinetic" 

 energy, or, in other words, the energy of power must 

 be changed into the energy of movement. 



Similarly, plant substances which are formed during 

 assimilation must be broken down in order that the 

 energy which is stored up in them may be liberated. 

 The oxygen that is taken in in respiration acts as the 

 destroying agent. It enters into combination with 

 chemical compounds, formed in the plant, and breaks 

 them down; as a result of this, energy is liberated. 

 Some of this energy is dissipated in the form of heat, 

 as was shown by the rise in the temperature of the air 

 surrounding germinating peas (Experiment 49), but 

 the greater part of the liberated energy is available for 

 use ill the further growth and activities of the plant. 



It is thus seen that breathing is a destructive process 

 by means of which energy is liberated. For this reason 

 it is continuous, taking place in darkness as well as in 

 the light. 



Exactly opposite is the process of assimilation. Here, 

 from the carbon-dioxide taken in from the air and the 

 water absorbed from the soil, a plant manufactures 

 substances such as starch. This is a building-up process, 

 and, for this reason, energy must be supplied. It 

 follows, therefore, that it can take place only in the day- 

 time, in the light of the sun, the earth's one source of 

 energy. 



