DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 6l 



CHAPTER III. 



DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 



The desmids and diatoms are two closely related 

 groups of minute aquatic plants which the beginner 

 will at first probably have some trouble to distinguish 

 from each other; yet after a very little experience he 

 will be able to recognize them at a glance. Both are 

 plants formed of only a single cell, but their beauty 

 and variety of form, their peculiar movements and 

 wonderful structure, place them amongst the most at- 

 tractive of living microscopic objects. And they are 

 among the most frequent. Scarcely a drop of water 

 from a pool in spring or summer can be examined with- 

 out showing a desmid or a diatom. 



The desmids are usually found in the freshest and 

 sweetest water. In salt or brackish marshes, where 

 diatoms flourish as well as in a mill-pond, desmids 

 never occur. They also seem to prefer open pools on 

 which the sun shines brightest and the shadows are 

 fewest, where they probably seek warmth rather than 

 the strong light, for they seldom form patches on the 

 mud as the diatoms do, but adhere to the stems of 

 other plants as a green film; or conceal themselves 

 among the' dissfected leaves of the aquatic vegetation, 

 or among tangled masses of Algae. 



