DES.MIDS, DIATOMS, AND FRESH-WATER ALGiE. 97 



bands smooth and conspicuous. Common (Fig. 78). 

 The cruciform smooth band is the characteristic by 

 which it may be at once recognized. 



For a collection of illus- 

 trations of the diatoms of 

 this country the reader 

 should refer to Wolle's „.. „ ~. ,. , , , 



Fig. 78. — Staiironeis phoenocenternn. 



"Diatomaceas of North 



America, illustrated with twenty-three hundred figures." 



III. FRESH-WATER ALGvE. 



The Algae often collect together and form green 

 clouds in the water or a scum-like growth on the, sur- 

 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 aquatic objects which he 

 may be examining. The color of the visible masses is 

 usually brijtht green; it may be brownish if the plants 

 are in fruit, or the natural tint of the individual alga 

 may be brownish or purplish, or, when massed to- 

 gether, almost black: Many species are coated with a 

 mucous or slimy material that makes. them slippery, 

 and difficult to handle or to remove from the water 

 unless a dipper or a spoon be used, sometimes not even 

 then. 



They, are seldom found in any abundance in deep 

 water, appearing to prefer shallow ponds and slowly 

 flowing streams, where they may have plenty of 

 warmth and light. Few of the species are free-swim- 

 ming. Most kinds are adherent to submerged leaves, 

 stones, or sticks; some form feathery clusters of 

 branching filaments; others surround themselves by 



