86 



Elementary Botany 



oxygen, COj. It is produced wherever breathing, burning, and 

 decay of organic bodies are taking place. 



The composition of a thousand parts, by volume, of at- 

 mospheric air is as follows : — 



Nitrogen 



Oxygen 



Water (variable) 

 Carbon dioxide 

 Various gases 



779'5o- 



2o6'IO. 



i4"oo. 



•40. 



traces. 



I,000'00. 



That is to say that a thousand cubic feet of air contain 

 not quite half a cubic foot of carbonic acid. Small as this 

 quantity may appear to be, it is the source of the carbon of the 

 plant. 



The carbon dioxide enters with the other constituents of 

 the air by means of the stomata into the interior of the leaf, 

 there to serve for the nourishment of the plant. 



2. To assimilate the food absorbed. — The food materials 

 which are taken up by the root are not changed as they are 

 carried up to the leaves. The sap becomes thicker by evapora- 

 tion, but when it enters the leaves it still contains simply 

 mineral substances. In the leaves, however, it is brought into 

 contact with the carbon dioxide, and a marvellous change takes 

 place. In the presence of the chlorophyll and under the action 

 of light, the carbonic acid gas is decomposed : its oxygen is 

 restored to the atmosphere, whilst the carbon is chemically 

 united with the water and the mineral substances, to build up 

 new and organic compounds. These changes will only take 

 place in the light, and go on in all the green parts of the plants. 

 It is owing to this that the amount of carbonic acid in the 

 atmosphere is kept down, and the air purified and rendered fit 

 for man to breathe ; hence the importance of open spaces with 

 trees in the midst of our cities. 



One of the first substances formed is generally oxalic acid, 

 which unites with the lime of the sulphate of lime, setting free 

 the sulphur which is required for the manufacture of the various 

 albuminoids. Other organic substances soon follow, and the 



