THE EVOLUTION OF PLANTS 87 



brought about by the reaction of carbon dioxide with 

 the lime. As all carbon dioxide was kept from entering 

 the flask, and as the lime-water shows by becoming 

 milky that some of this gas reached it in the stream of 

 air pumped through it, it becomes perfectly clear that 

 the plant has exhaled it. 1 



Besides carbon dioxide^ certain amount of water vapour 

 is exhaled by the plant in the process of respiration. 



To understand the purpose of respiration we must go 

 back a little and consider some of the features of sugar 

 manufacture. We saw that the energy for this work is 

 derived from the sun. The work it does is to build up 

 sugar and subsequently other compounds, which remain 

 in the plant. These substances retain in themselves the 

 energy which was expended in making them. When 

 they are decomposed or reduced to simple compounds 

 like those from which they were made, this energy can 

 be liberated again. If we burn them, for instance, we 

 get the liberation of a large amount of heat, which can 

 be made to do work in other directions. So we see 

 that the construction of the plant, both living and non- 

 living substance, involves the fixation of large amounts 

 of energy as well as of material. 



The purpose of respiration, which involves the break- 

 ing down of the living substance incidental to its wear 

 and tear and the work it does, is in this way to liberate 

 the energy without which these operations could not 

 take place. 



CHAPTER XI 



THE EVOLUTION OF THE FORMS OF PLANTS ALG^ 



THE considerations set out in the foregoing chapters 

 apply in great part only to the higher terrestrial plants. 

 But the whole range of vegetation is much more exten- 



1 A form of the apparatus is shown in the primer of Biology, p. 56. 



