CHAPTER V 

 A POND SCUM 



59. Pond Scums. — The appearance of a green layer on 

 the surface of pools, ponds, and lakes is a familiar one, es- 

 pecially during the warm months of the year. This scum is 

 made up of green plants which belong, like Protococcus, to 

 the group of algae. Of the many species that may be found 

 in such a green layer, some, Hke Protococcus, are one-celled, or 

 are made up of small groups of cells ; others have the form of 

 long threads, and one of the commonest of these thread- 

 shaped scum-forming algae is Spirogyra. Sometimes we find 

 several species of algse growing together on the surface 

 of a pond; sometimes the scum is composed largely or 

 wholly of one species. Spirogyra may be distinguished from 

 most of the other thread-shaped algae found in similar places 

 by the slippery feeling of a mass of the long, unbranched 

 threads. 



60. The Spirogyra Plant. — The simple plants that we 

 have studied thus far (bacteria, Protococcus, and yeasts) 

 are sometimes found in colonies of various sizes ; but often 

 their cells are separate, and the single cells seem to get along 

 just as well as do those that are parts of colonies. Under 

 certain special conditions and at particular stages, the cells 

 of Spirogyra may occur singly; but practically we always 

 find the plant in the form of a row of cells firmly attached to 

 one another. That is, the colony habit has become firmly 

 fixed in Spirogj^ra, whereas in the cases of the simpler plants 

 that we have studied a colonA' was a temporary and a more or 

 less accidental arrangement. 



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