DARK-GREEN SCUM. 



Oscillatoria. 



THALLOPHYTES; ALG^J CYANOPHYCE^. 



PRELIMINARY. 



THE color of Oscillatoria, almost any species of which 

 may be used, is generally sufficient to enable one to 

 distinguish it at sight. Its dark blue-green cast, like 

 that of Nostoc, is in marked contrast with the yellow- 

 green of most other plants which form scums. It is 

 very common on stagnant water, often forming patches 

 of scum thirty centimeters or more in diameter, which, 

 becoming loaded with dust, finally sink to the bottom. 

 It is also very common about watering- troughs, along 

 street gutters, at the outlet of drains, on wet rocks, giving 

 them a slippery surface, on the earth of undisturbed 

 pots in the greenhouse, and in all localities in which 

 the water contains a small amount of decaying organic 

 matter. It can usually be grown indefinitely in an 

 open jar, by supplying water as it evaporates, or with 

 less trouble, when once established, in an unstoppered 

 bottle, in which a small twig or flower-stem is inserted 

 to provide nutriment. The plants are often to be found 

 in winter in as good condition as in summer. 

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