DARK GREEN SCUM. 



Oscillaria tennis Ag. 



PRELIMINARY. 



THE color of Oscillaria, almost any species of which 

 may be used, is generally sufficient to enable one to 

 distinguish it at sight. Its dark blue-green 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 (a foot) or more in diameter, which becom- 

 ing 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, in the greenhouse, and especi- 

 ally in water containing a small amount of garbage. 

 It can usually be grown indefinitely in an open jar, by 

 supplying the water as it evaporates, or with less 

 trouble, when once established, in an unstoppered bottle, 

 in which a small twig or flower stem of some sort is 

 inserted to provide nutriment. The plants are often 

 to be found in winter in as good condition as in sum- 

 mer. The study should be made upon growing plants 

 when possible, but specimens dried on paper or mica 

 will serve quite as well, except to show the oscillating 

 movements, which are characteristic of the group to 

 which Oscillaria belongs. 



