The Algae 237 



the others. The plant, therefore, consists of a single cell 

 which carries on all the essential processes of life and is able 

 to reproduce itself. Moreover, it is a highly successful plant, 

 for Protococcus occurs in all parts of the world, from the 

 tropics to the polar regions, in habitats of many difTerent 

 kinds. 



The pond scums. If examined in the spring or fall, al- 

 most every pond and little stream will be found to contain 

 many kinds of algce. Some of these are merely masses of 

 rounded cells like the cells of Protococcus. Others have the 

 cells arranged in rows, forming simple filaments. In still 

 others the filaments are highly branched and the plant body 

 may be several feet in length. Some of the forms are im- 

 bedded in a gelatinous matrix. All these various kinds of 

 algae taken together are called the " pond scums." They are 

 the algae that a person who has not studied botany is most 

 likely to know. 



Many of the pond scums are at first attached to under- 

 water objects, but during warm weather they break loose and 

 come to the surface. All cells carrying on photosynthesis 

 give off oxygen, and the bubbles of oxygen that come from 

 the filaments cling to them and help to buoy them up. Fur- 

 thermore, bubbles of air which are given off from the water 

 when it becomes warm collect under the masses of alga? and 

 cause them to come to the top of the water, where they form 

 a green or yellowish-green surface layer. The pond scums 

 are generally considered unsightly, and not a few persons 

 think them poisonous. In reality, they are quite as harm- 

 less as lettuce. The danger in drinking from ponds lies not 

 in the green scums, but in the presence of certain disease- 

 producing bacteria that may have been carried into the ponds 



