LIVING MATTER 



19 



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stance which gives plants their green color. If green 

 water plants are placed in a jar of water in the sunlight, 

 bubbles of gas may be seen passing away from them. If 

 this gas is caught in a test tube and touched with a 

 lighted splinter its behavior proves at once that it is 

 oxygen. Its presence is a 

 sign that the plant is rapidly 

 making starch. (Fig. 9). 



Transfer of Starch. — As 

 starch is a solid that does not 



dissolve in water it cannot 

 pass out of the leaf where it 

 is made until it is changed to 

 a soluble form. Through the 

 activity of certain complex 

 substances (see enzymes) 

 which will be described later 

 it becomes converted into 

 sugar. In this form it passes 

 into the sap, which carries it 

 throughout the plant. When 

 the sugar reaches the potato 

 the complex substances re- 

 verse their action and turn 

 the sugar into starch, in 

 which form it is stored for 

 future use. Though there is an obvious advantage to 

 the plant in this transformation it takes place not for 

 this reason but as a result of chemical activity. 



Purification of Air. — The work that plants do in keep- 

 ing alive is of inestimable benefit to man. In making 



Fig. 9.— Alga giving off oxygen. 

 (After Bailey.) 



