WEEDS OF PONDS, RIVERS, DITCHES 343 



1 02. This is a long flexible blade with saw-like teeth, 

 which quickly severs the stems of water weeds. 



FIG. 102. Ziemsen's Weed-cutting Saw : A, the saw-like teeth ; B, torpedo- 

 shaped sinkers ; C, clamp ; D, wire ; E, rope ; F, handle. 



In shallow and wide streams weeds are often cut by 

 men who wade in the water and use hand scythes. 



As far as possible the cut weeds should be removed 

 from the river, and not sent down stream to be a 

 trouble and annoyance to neighbours below. 



2. In addition to cutting weeds by means of different 

 tools, some of them may be kept down to a certain 

 extent by swans. These birds nip off the young tops 

 among water plants, and check their rampant growth, 

 as in the case of Elodea canadensis (p. 332). 



3. For the destruction of floating slimy "scum," 

 particularly in fresh-water lakes, copper sulphate has 

 been used with success. This substance in exceedingly 

 minute amounts kills Desmids and many microscopic 

 green and brown slimy algae composing the " scum." 

 (One part in a million is sufficient to destroy the growth, 

 and does not harm fish nor prevent the water from 

 being used for drinking purposes.) 



One and a half ounces of copper sulphate dissolved 

 in two gallons of water and applied as a spray to the 

 " slime " on the surface of water has been found to 

 destroy it. A second spraying a week later makes 

 success more certain. 



