FLOWERLESS PLANTS 



175 



and leave their remains in our water supply. Much of the un- 

 pleasant taste and odor of drinking water comes from this cause. 

 Pond Scum (Spirog7jra) .— This alga is well known to every 

 boy or girl who has ever seen a small pond or sluggish stream. It 

 grows as a slimy 

 mass of green 

 threads or fila- 

 ments. Frequent- 

 ly it is so plentiful 

 as almost to cover 

 the surface of the 

 water, buoyed up 

 by little bubbles 

 of a gas which 

 seems to arise from 

 the body of the 

 plant. If we place 

 some of the Spiro- 

 gyra in a deep dish 

 of water, taking 

 care to put the 

 dish in the sun, 

 the green plant will be found to give off enough gas to cause the 

 plants to float near the surface. 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 pond scum, knowing this fact, 

 should no longer feel that the pond is a menace to health, un- 

 less, indeed, it is a place where mosquitoes live and breed. 



n 



A 



B 



A , jar of water containing pond scum; B, same jar after an 

 hour in the sunlight. 



Spirogyra: n, nucleus; s, chlorophyll bands. 



Suggestions for Observational Work} — Under the low power of the micro- 

 scope, try to make out the length of one of the filaments of Spirogyra. Do 



* For fvdler directions, see Hunter and Valentine. Manual, page 79. 



