234 CRYPTOGAMS 



another hour. What change has now taken place in each 

 jar? Why? 



Arrange an apparatus as in Experiment 145, putting the 

 pond scum below the funnel. Now place it in strong sun- 

 light. What comes from the plant? When a test tube is 

 full, remove and test with a lighted stick. Is it nitrogen? 

 Hydrogen? Carbon dioxide? Oxygen? Give reason for 

 your answer. 



Now select a small portion of the plant. Remove it 

 with your fingers and place in a white earthen dish with 

 enough water to cover it. How does the plant feel? 



Examine the threads of pond scum as they lie in the dish. 

 Of what do they consist? Do the threads branch or fork? 

 Examine with a magnifying glass and you will see that the 

 threads are built up of little cylinders placed end to end. 

 These are cells, and the fiber is made up of a string of such 

 cells. Try to find the end of one of the threads. Does this 

 end cell look like the others ? If not, how does it differ ? 



To what is the green color due? In coarse plants the 

 arrangement of this coloring matter (chromatophores) 

 may be made out. 



At certain seasons, one may find places where the threads 

 are of different color, and sometimes no green color will be 

 found for several cells. Two threads may also be found so 

 joined as to form a sort of ladder. Look for these things. 



What effect does the presence of many such plants have 

 on the value of drinking water ? 



Suggestion. Mount a thread of pond scum in water 

 and examine with a low power. Note the cell wall, the clear 

 contents of each cell, and the green spiral within (Fig. 115). 

 Examine many such threads and search for the end cell. 

 Is it like all the others? If not, how different? 



