44 THE VEGETATIVE ORGANS 



a few days and note what happens. The one in the 

 air has turned downwards, as we should expect from 

 our previous experiment, but the one in airless water 

 has continued to grow in the direction in which it 

 was placed in the water. In other words, the root 

 only possesses this power of turning when supplied with 

 air containing oxygen. This suggests a further question. 

 Do plants utilize the oxygen of the air in the ordinary 

 process of growth ? 



Respiration. A simple experiment will enable us to 

 understand the important role played by plants in changing 

 the composition of the air. Soak a number of peas in 

 water for a day, and then place them in two jars (a and b) 

 lined with wet blotting-paper. Put on the stoppers and 

 keep them on for a day or two. A similar jar may be 

 prepared, but without peas, for comparison. Then test the 

 air in the jars as follows : (a) insert a lighted taper, 

 and note whether the air in the jar containing the peas 

 will now support combustion ; (b) pour in a little lime- 

 water and note the result. From these tests we see that 

 the germinating peas have removed from the air the gas 

 which supports combustion, viz. oxygen, and have given up 

 to the air a gas which turns lime-water milky, viz. carbon 

 dioxide. If we breathe on to lime-water we observe a 

 similar effect. In other words, Peas, during their growth, 

 are using up oxygen and giving off carbon dioxide, just as 

 we are when breathing. This process, which is called respira- 

 tion, is necessary to the existence of plants ; they would soon 

 die if kept wet in a closed bottle and without air. 



The oxygen taken into the tissues of the plant during 

 respiration acts chemically upon the complex organic sub- 

 stances which constitute the plant, with the result that 

 they are converted into simpler compounds such as carbon 

 dioxide and water. During these changes energy is set 

 free, and thus the work necessary to the life of the plant 



