184 THE NATURE AND WOEK OF PLANTS 



222. Physical attraction as a source of energy. 

 Compounds often exert an attraction for each 

 other, and the resulting union does not change the 

 composition of either of them. Thus sugar and 

 salt attract water, and will even draw it from the 

 air; but when sugar and water come together, they 

 form a solution, and the composition of neither is 

 disturbed. The water may be driven off and the 

 sugar will remain as before. Examples of this 

 were seen in the experiments illustrating the action 

 of the root-hair. 



223. Release of energy. After energy has been 

 acquired by the plant, it may be transformed or 

 released and made to do various kinds of work. 

 The principal method of releasing energy is the 

 same as that used in the steam engine, and con- 

 sists in oxidizing or burning the compounds which 

 contain it. 



224. Respiration or breathing. The slow burn- 

 ing of material goes on almost constantly in all 

 living substance, and it is essentially the same kind 

 of a process in both plants and animals. It is most 

 rapid in growing tissues and slowest in resting seeds 

 and spores. It is still maintained, however, and it 



