448 General Botany 



in moist places in all parts of the world, from the tropics to the 

 polar regions, in habitats of many different kinds. 



Chlamydomonas. This alga is one of the simplest unicellular 

 plants and differs from Protococcus in being free-swimming 

 during its vegetative phase. Chlamydomonas occurs in fresh- 

 water ponds, ditches, and roadside pools. Reproduction occurs 

 both by forming resting cells, by swimming spores, and by 

 gametes. This genus is of interest because it seems to present 

 better than any other the characteristics of the primitive plants 

 from which all the green algse and possibly the higher plants 

 may have been derived. 



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

 every pond and little stream will be found to contain many kinds 

 of algae. 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 embedded in a gelatinous matrix. All 

 these various kinds of algae taken together are popularly called 

 the " pond scums." They are forms of algae most commonly 

 observed. As pond scums they are most unattractive, but seen 

 through a microscope they present varied and beautiful examples 

 of cell architecture. 



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, where they form a green or yellowish-green surface 

 layer. 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. Furthermore, bubbles of air 

 from the water when it becomes warm also collect in the masses 

 of algae and help to support them at the top of the water. The 

 pond scums are generally considered unsightly, and not a few 

 persons think them poisonous. In reality, they are quite as 

 harmless as lettuce. The danger in drinking from ponds lies, 



