HOW PLANTS GET THEIR CARBON fOOD. 



73 



Synopsis. At temperatures suitable for growth, green plants in the sun- 

 light are constantly giving off a gas. 



In the case of water plants this gas can be seen in the form of bubbles. 



This gas is oxygen. 



At the same time that oxygen is being given off by green plants carbon 

 dioxide (carbon and oxygen) is being absorbed by the plant. 



A chemical change in the carbon dioxide takes place in the plant and 

 some of the oxygen is thus liberated. 



Material. Fresh mats of some alga, either spirogyra, zygnema, or vau- 

 cheria. 



Fresh shoots of one of the higher water plants like elodea (found in the 

 shallow water of ponds, lakes, or streams near low ground). 



Beakers with fresh spring or hydrant water to hold the plants. A funnel 

 and large test tube for demonstration 23. The demonstration should be 

 started several days in advance. 



