THE ALG^E 



221 



Osdllatoria grows in the same kind of region as Nostoc, but 

 is more abundant. It appears as floating mats or as slimy coat- 

 ings upon objects in the water. Frequently sticks and stones 

 on the bottom are covered with a beautiful velvety layer con- 

 sisting of multitudes of these simple plants. The individual 

 plants are thread-like (fig. 173) and often have a swinging, glid- 

 ing, or oscillating movement, from which the name of the plant 

 is derived. In stagnant or foul 

 water Oscittatoria plants grow 

 vigorously, being able to thrive 

 throughout a wide range of tem- 

 perature. The cells of the plant 

 divide, thus lengthening the 

 plant. Later the threads break 

 into two or more new threads, 

 each of which continues to grow 

 as an independent plant. The 

 cells of one plant are inclosed 

 in jelly-like sheaths, but there is 

 no jelly mass which holds many 

 plants together, as in Nostoc. 



It is evident that blue-green 

 algse live and reproduce them- 

 selves in very simple ways. 



205. The green algae : Pleurococcus. This green alga is very 

 widely distributed and is often called green slime, because of 

 the appearance it presents when growing upon partially shaded 

 tree trunks, fences, rocks, and old buildings. It is not usually 

 recognized as being a plant. When there is abundant moisture 

 in the air, the coating of plants presents the appearance of a 

 coating of green paint upon the surface which supports them. 



When examined under suitable magnification, the green slime 

 is seen to be composed of many thousands of single-celled 

 plants. A careful measurement of a number of plants showed 

 their average diameter to be about -^-^-^ inch (.014 mm.). In 

 other words, it would require approximately 500 of these 



FIG. 173. Oscillatoria 



A, tips of several plants; B, part of 



one plant, enlarged to show cellular 



structure. Both magnified, B much 



more than A 



