WATEE SILK 69 



water in the dish. At the end of the time stated 

 examine some of the filaments under the micro- 

 scope, and note the changes in the chlorophyll 

 bodies and starch grains. What has become of 

 the starch ? 

 h. Expose some of the filaments used in the preced- 

 ing experiment to the bright sunlight for fifteen 

 minutes to two or three hours according to the 

 intensity of the fight, then test with iodine. Is 

 starch present? Explain. Note the bubbles of 

 gas given off by plants exposed to the sunlight. 

 Compare these with the plants growing in ponds. 

 What causes the plants to float at the surface of 

 the water when exposed to the sunlight ? What 

 advantage in this ? Why do they sink down in 

 the water at night ? What advantage in this ? 

 Is it possible that the slimy excretion on the fila- 

 ments may assist in any way to float the plant ? 

 Why? 

 What conclusions do you draw from these experi- 

 ments ? 



//. Effect of ca/rhon dioxide. 



a. Boil some distilled water to drive off any carbon 

 dioxide it may have absorbed, cool the water, 

 place some of the filaments used in I. a.m. a, vial, 

 fill the vial completely with the cooled water, 

 and set it in the sunlight for fifteen minutes 

 to an hour, then test for starch as in /. 5. What 

 results ? 



h. Take some more of the filaments used in La., 

 and put them in a vial of hydrant water, which 

 usually contains carbon dioxide, and set the vial 

 alongside of //. a. for the same time. Compare 



