palmer] JUST OOZE AND SLIME 29 



Few of us in the latitude of New York would expect to find 

 apple-blossoms in August, hepaticas in October or golden rod in 

 January. We recognize that there are definite times in the year 

 for each of these plants to bear flower and fruit and we expect them 

 at those times. Fewer still realize that the slimes and plant 

 materials covering a rock in a brook in January are mostly different 

 from those growing on the same rock in July. There is a definite 

 succession of species in the aquatic environment just as there is 

 on land. Temperature seems to be the most important factor 

 governing this succession. The plants growing in a spring 

 may be practically the same throughout the year because the 

 temperature of the water is practically uniform, but in the open 

 stream, — ice-bound in winter and luke-warm in summer the varia- 

 tion is great as is the temperature. The groups found in slowly- 

 flowing water also vary with the seasons and so the story goes. 



Probably there are three ways in which these organisms may be 

 of greatest interest to us. First, there is their influence on the 

 quality of the water in which they grow; next, their relation as a 

 food source to other forms of life in the water and out of it ; and 

 lastly, their influence upon the topography of the earth and 

 consequently upon geography. 



It is not difficult to say which of these is the most important as 

 it cannot be disputed that without pure water life for us would be 

 impossible. When we look at a stagnant pool and see it congested 

 with floating pond scum or when we stand on a bridge over a 

 river in the spring and see masses of a dirty jelly-like substances 

 rise to the surface and float off down stream, it is hard to connect 

 the presence of these plants with water purification. Yet that is 

 exactly what their presence indicates. An increased growth of 

 plants of a somewhat similar type near where sewage is dumped 

 into a clean stream is due to the fact that there is a superabundance 

 of waste-material which to these plants is food. As a result they 

 multiply abnormally. In taking this food they give off oxygen — 

 one of the very best purifying agents. Sometimes they do this 

 so rapidly that the oxygen is not used up in purification of the 

 water. Instead it forms into bubbles sufficiently large to break 

 masses of slime loose from the bottom and bring it to the 

 surface. 



The stones in brooks are covered with the plants whose presence 

 adds to the amount of oxygen in water. Of course when the 



