WORK DONE IN MAKING STARCH 117, 
Now place it in the brightly lighted window again. 
The bubbles soon start up afresh. ) 
Do the ‘‘pond scums’’ do the same kind of work? 
Many of you have seen the green-looking “scum,” as 
some people call it, which floats on the surface of ponds 
or on the water of ditches, and which is so abundant in 
the spring and autumn. This pond scum deserves a 
better name, for it is really made up of beautiful tiny 
plants, often consisting of silk-like threads, which we 
can see by lifting a bit of it from the water. To see 
that 1t does the same kind of work as the leafy water 
plant, place some in a bottle of water and set it in the 
window by the side of the other plants. The tell-tale 
bubbles show themselves here also. Now you 


have perhaps noticed that this pond scum, as it 
floats on the water, has a great 
many bubbles in it, caught in the 
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tangle of threads. If you take up 
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some of this tangle, rinse it in the 
water to remove all the bubbles, 



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and then replace it in the water; 
it does not float well, but tends to | fp aes 27 
sink to the bottom. But when the ===> 
bubbles of gas begin to form again a eo 
and are caught in the meshes of 
the tangle, they are so much lighter than water that 
they buoy up the plant and lift it once more to the 
