100 MICROSCOPY FOR BEGINNERS. 



face. Frequently, however, the student will find iso- 

 lated filaments under his microscope, and not know how 

 they were placed there, or he will find single threads 

 adherent to other objects which he may be examining. 

 The color of the visible masses is usually bright green ; 

 it may be brownish if the plants are in fruit, or the nat- 

 ural tint of the individual alga may be brownish or 

 purplish. Many species are coated with a mucous or 

 slimy material that makes them very slippery and diffi- 

 cult to handle, or to remove from the water unless a 

 dipper or spoon be used. 



They are seldom found in any abundance in deep 

 water. They seem to prefer shallow ponds and slowly 

 flowing streams, where they may have plenty of warmth 

 and light. Few of the species are free - swimming. 

 Most kinds are adherent to leaves, stones, or sticks in 

 the water ; some form feathery clusters of branching fil- 

 aments, others surround themselves by little balls of 

 translucent jelly often attached to leaves of grass or to 

 other submerged objects. 



The following have been partially described in the 

 Key on page 71. 



1. SCENEDESMUS (Fig. 79). 



The cells are usually four, attached together by their 

 long sides. The spines on the narrow ends of the two 

 terminal cells are curved towards each other, and a spine 

 sometimes grows from the centre of one of the middle 

 cells. The plant is quite common. S. quadricduda, 

 Fig. 79. 



