THE VARIOUS FORMS OF PLANTS 



147 



up by little bubbles of a gas which seems to arise from the body 

 of the plant. If we collect some of this gas, we can easily prove that 

 it is oxygen. The person who sees a pond with a covering of slimy 



A, jar of water containing pond scum ; B, same jar after an hour in the sunlight ; 

 the pond scum has risen to the top of the jar, buoyed up by the oxygen formed 

 within it. 



pond scum, knowing this fact, should no longer feel that the pond 

 is a menace to health, unless it is a place where mosquitoes live and 

 breed. 

 Under the low power of the microscope, the body of a pond scum 



Spirogyra : n, nucleus ; s, chlorophyll bands. 



is seen to be a thread made up of elongated cylindrical cells, each 

 of which contains a spirally wound band of chlorophyll within it. 



