I04 AQUATIC MICROSCOPY FOR BEGINNERS. 



Fig. 84. — Oscillaria. 



dark purplish or almost blackish color, or they are 

 found entangled among other plants in a dark green 

 film. Under the microscope 

 they consist of filaments com- 

 posed of very many short ceils 

 that vary a good deal in width 

 according to the species, of 

 which there are several. They 

 can usually be recognized by 

 the bluish-green color and by 

 their characteristic motions. 

 Some are like straight rods of cells bending slowly 

 from side to side; others twist and writhe, and coil 

 themselves into circles, only to uncoil slowly and re- 

 peat the movements. Some deliberately glide for- 

 ward, the tip gradually and gracefully bending and 

 curving. The movements, when the plants are in a 

 healthy condition, are incessant. The beginner need 

 never be at a loss to recognize one of the several 

 species of Oscillaria. Three forms are shown in the 

 figure. (Fig. 84.) 



Oscillaria is not in a sheath. A similar alga is not 

 uncommon whose filaments are singly enclosed within 

 a colorless sheath plainly visible at the extremities of 

 the plant, where it projects as an empty, membranous 

 tube. This is Lyngbya and may be readily mistaken 

 for Oscillaria unless the sheath is determined to be 

 present or not. 



4. Spirogyra (Figs. 85, 86). 

 The Spirogyra are easily recognizable by the beauti- 

 ful spiral bands of green within each cell, as shown .in 

 Fig. 85. There may be one, two, or several of these 



