HOW TO KNOW THE FRESH-WATER ALGAE 



ested does not have to look exactly like the forms which are figured. 

 This is true for many of the genera in the Chlorococcales of the Chlo- 

 rophyta {Scenedesmus, Oocystis, Tetraedion, etc.) and of the desmids 

 (Cosmarium, Euastrum, Micrasterias, e. g.) also in the Chlorophyta. 



For a study of most freshwater algae a compound microscope is 

 needed which has a 10X ocular, and a 10X and 40X objective. For the 

 study of the larger forms such as the stoneworts (Characeae) a binocu- 

 lar dissecting microscope is highly desirable. Best illumination for the 

 microscope is obtained from daylight because colors of the algal pig- 

 ments appear more naturally. In lieu of good daylight (light from a 

 northern window preferred), artificial light from a microscope lamp 

 fitted with a daylight blue bulb is used, or a lamp which has a blue 

 filter. Naturally, all optical parts of the microscope should be kept 

 free of dust, moisture, and finger prints, using rice lens paper for 

 cleaning. It is difficult enough to see algae clearly when optical con- 

 ditions are perfect. An eye-piece micrometer is desirable. 



In preparing mounts for the study of algae under the microscope, 

 small amounts of material should be used, and spread out evenly in 

 a thin layer. Dense clumps and opaque masses of algae in a micro- 

 scope mount produce only disappointment and headaches. 



One of the primary difficulties with which the inexperienced stu- 

 dent is confronted when first using a general key to the algae is that 

 of detecting and identifying colors, green, "blue-green," "yellow-green," 

 etc., to which the key makes reference. Pigmentation in the differ- 

 ent algal groups is a fundamental characteristic and one which is 

 very helpful in making identification. But yellow-green algae may at 

 times appear decidedly grass-green, and the brown-pigmented algae 

 may have a distinct tinge of green, especially when artificial light is 

 used for the microscope. Hence, other characters or a combination of 

 characteristics excluding or in addition to color must be employed to 

 make a choice in the key. Suggestions are given in appropriate places 

 in the following key for making certain tests to help differentiate genera 

 on the basis of color. Although it is a combination of characters which 

 differentiates algae in the final analysis, the key can select these char- 

 acters one by one only. 



The three major phyla of algae (those which are the most com- 

 mon) are the green algae, the blue-green, and the yellow-green (yel- 

 low-brown). It is suggested that in order to facilitate the differentiation 

 of a green, a blue-green, and a yellow-green alga, that (if possible) a 

 known green alga (SpirogyraJ, a blue-green (Anabaena), and a diatom 

 (yellow-brown) be mounted on a microscope slide so that their color 

 and the details of cell structure might be compared. Then a series of 

 illustrations depicting these groups of the algae should be examined 

 so that the gross morphology and the details of cell structure will 

 become associated with the respective pigmentations. 



