124 FRESH-WATER BIOLOGY 



nutrient solution over the bottom of a petri dish or a small glass 

 culture dish. This must then be sterilized before the culture is 

 made. In preparing the culture a very minute portion of the me- 

 dium containing the desired form is mixed with a large drop of 

 distilled water and then this is scattered at intervals over the surface. 

 The material must be diluted with enough water so that each cell 

 will be by itself. 



In the course of a few days the single cells will have increased, 

 and then, while the culture is on the stage of a microscope, the little 

 colony of cells may be transferred to a liquid medium by means of a 

 sterilized needle, the tip of a fine brush, or a very fine pipette. 



To a very large extent the culture medium must be adapted to 

 the species to be cultivated. No one medium is favorable to all 

 species of algae, and the form must be taken into consideration 

 before a medium is prepared. If the species be a new form, various 

 different media must often be tried before the right one is deter- 

 mined. If a quantity of different forms from any collection be 

 placed in one medium and a second quantity in another, the prob- 

 abilities are that in the course of three or four weeks but few of 

 the same species will be found in both cultures. Certain forms will 

 have died in one while perhaps those very forms have found in the 

 other medium the substances and conditions for their development. 



The media to which the greatest number of forms are adapted 

 are Moore's solution and Knop's solution: 



Moore's solution: 



Ammonium nitrate 0.5 gram. 



Potassium phosphate 0.2 gram. 



Magnesium sulphate 0.2 gram. 



Calcium chloride o. i gram. 



Iron sulphate trace. 



These amounts should be dissolved in one liter of distilled water. 



Knop's solution: 



Potassium nitrate i gram. 



Potassium phosphate i gram. 



Magnesium sulphate i gram. 



Calcium nitrate 4 grams. 



Chloride of iron trace. 



The first three substances are dissolved in the required amount of water to make from i to 

 5 per cent of the solution, then the calcium nitrate is added. This solution may then be 

 diluted as needed; usually a 0.2 per cent or a 0.4 per cent solution is favorable for ordinary 

 cultures. 



