EXERCISE 38 



IS THERE A WASTE PRODUCT GIVEN OFF DURING FOOD MANUFACTURE? 



Materials. An aquarium containing some submerged water plants (the following may be 

 grown in such aquaria: water weed (ElodecC), the pondweeds (Potamogeton), water milfoil 

 (Myriopliylluni), and horn wort (Ceratophyttum), these being common in most parts of the country). 



Directions. Place the aquarium with submerged water plants in the sunlight and examine it 

 after fifteen or twenty minutes. Under these conditions the manufacture of foods should be going 

 on in the green parts of the plants. Observe whether a gas is given off from the plants. Darken 

 the aquarium and after a little while examine again to see whether the gas is still coming off from 

 the plants. 



Almost any kind of submerged water plant will answer for this experiment. During the growing 

 season they can be obtained in most bodies of shallow and rather quiet water, such as the margins 

 of lakes and rivers. They grow readily in aquaria. Every laboratory should carry several such 

 aquaria through the winter. 



Additional problems. In drinking troughs for domestic animals where water stands for a long time, 

 small, threadlike green plants (the pond scums) often are found growing. Observe these plants when 

 the sun is shining upon them. What do you think is the origin of the bubbles seen among these plants 

 and arising from them ? Similar conditions may be observed when pond scums are found floating at the 

 top of the water of ponds and sluggish, streams. 



Heat water until the air in it has been driven to the surface. Cool the water as rapidly as possible 

 until it is at air temperature. Place water plants in this water and place in the light. Are gas bubbles 

 thrown off by the plants in this case ? By means of a glass or rubber tube blow your breath, into the 

 water at the bottom for a few minutes. Does this seem to affect the quantity of gas bubbles later produced 

 by the plants ? 



An interesting modification of the experiment of this exercise is possible if an arc-light stereopticon 

 is available. In this case a sprig of one of these plants may be placed in water in one of the thin-glass 

 tanks which are supplied for the purpose, and inserted in place of the slide. The light will be sufficiently 

 intense to enable the plant to carry on photosynthesis, and bubbles will be seen to rise from the plant. 



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