64: MICROSCOPY FOR BEGINNERS. 



CHAPTER III. 



DESMIDS, DIATOMS, ASTD FRESH-WATER ALG.E. 



THE desraids 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 wonder- 

 ful structure, place them among the most attractive of 

 microscopic objects. And they are among the most fre- 

 quent. Scarcely a drop of water from a pool in spring 

 or summer can be examined without showing a desmid 

 or a diatom. 



The desmids are usually found in the freshest and 

 sweetest water. In salt or brackish marshes, where di- 

 atoms flourish as well as in a mill - pond, desmids never 

 occur. They also seem to prefer open pools on which 

 the sun is brightest and the shadows 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 in a green 

 film, or conceal themselves among the dissected leaves 

 of the aquatic vegetation, or among tangled masses of 



