THE FRESH-WATER ALGAE 123 



tion with other species which may be found in connection with it. 

 Aside from this, too, cultures are useful in determining what species, 

 especially of the unicellular forms, are present in any collection. 

 Many of these are so minute that they could easily be overlooked 

 unless they exist in great masses, which is rarely the case. So if 

 all forms of a locality are desired, it is well to put a small portion 

 of material gathered, bits of moss, earth, lichen, or washings from 

 higher aquatic plants into a culture medium and allow it to stand 

 3 to 4 weeks, when it may be examined; the chances are that 

 many forms will appear which could not be detected before- 

 hand. Indeed this is the only way in which certain species may 

 be obtained. 



When a pure culture is desired bacteriological methods for pure 

 cultures are most useful, but one who is skillful in working under 

 the low power of a microscope can often, by means of a tiny capil- 

 lary pipette, isolate a single cell, or a cluster of cells, which he knows 

 to be all of one kind. If the medium in which the form was grow- 

 ing contained many other species, the chances are that the first 

 time that the cell or cluster is transferred, a cell of some other 

 minute form such as Chlorella or Stichococcus, too small to notice 

 under that power, may be transferred with the desired form; so 

 to prevent this impurity from being carried to the final culture, thus 

 making the culture worthless, the better way is to transfer the cell 

 first to a drop of distilled water on a slide, then sterilize the pipette 

 in boiling water and, allowing it to cool, transfer the cell again to 

 a drop of distilled water; the process should be repeated three or 

 four times, and the cell finally transferred to the receptacle in 

 which the culture is required. 



For this purpose small low glass preparation dishes with loosely 

 fitting covers are the best. A receptacle that will admit a little 

 air is better than one that excludes air entirely. These small 

 receptacles may then be placed directly on the stage of a micro- 

 scope and the forms studied from time to time without disturbing 

 the growth in the least. 



The bacteriological method for obtaining pure cultures employs 

 gelatine or agar-agar plates. These plates are prepared by spread- 

 ing a thin layer of gelatine or agar-agar mixed with some good 



