and Laboratory Methods. 2313 



cess. In any case, the tubes should be carefully tested by filling with water and 

 being allowed to stand for some time. 



4. Incomplete dialysis. It is usually sufficient to dialize in tap water for 

 twenty-four hours, then in distilled water, changed several times, for the same 

 length of time. Only small quantities (100 c. c.) should be dialized in one tube 

 and the silver nitrate test always used. 



Careful attention given to all the above points will result in the production 

 of a perfectly transparent liquid, which, after sterilization, may be kept for con- 

 siderable length of time in a cool place, if closely stoppered. Upon the addition 

 of the nutrient salts desired, the fluid will harden in an hour or two, or if placed 

 in the steam sterilizer, coagulation will take place in a short time. This medium 

 is particularly serviceable in growing blue green algae, and by careful selection 

 of nutrient salts, a solution may be obtained which is so unfavorable to either 

 fungi or bacteria that a pure algal culture may be maintained, even though sub- 

 ject to occasional sources of contamination. 



In addition to the ordinary methods of growing alga; upon solid media, 

 Chodat and others have recommended the use of pieces of half baked porous 

 porcelain. These pieces, after being sterilized by dry heat, are put in Petri 

 dishes partially filled with nutritive solution, and inoculations may readily be 

 made by means of a sterilized pipette. The advantage of such a method is that 

 algae, which require thorough aeration, thrive well on the porcelain, but as a rule 

 all organisms develop very slowly when grown in this way, it being sometimes 

 six or eight weeks before the colonies appear. 



For general purposes it has been found that Erlenmeyer flasks of about 100 

 c. c. capacity are the most suitable for growing cultures. These, when filled 

 with nutrient agar to a depth of from 1 cm. to 3 cm., and tightly plugged with 

 cotton, will last for a long time. If it is desirable to retain the cultures for future 

 inoculations, the cotton can be covered with the thin rubber caps which come 

 for the purpose, and thus the alga; will be kept alive for years without the risk of 

 contamination. When large jars or bottles are used for cultures, the insertion 

 of a small vial partially filled with sterile water, which will be held in place 

 when the agar hardens, is of considerable service in insuring a moist atmos- 

 phere and preventing the drying up of the medium. 



As a rule, the necessity for plating out a gross culture of alga: is not so im 

 perative as with bacteria. The color and size of the alga; enable one to select a 

 single plant under the microscope with which to inoculate, and the large amount 

 of time saved in this way makes it undesirable to resort to other methods unless 

 necessary. Repeated dilutions with a final and thorough examination under 

 the microscope, is usually quite sufficient to insure a pure culture. It is even 

 possible in this way to obtain cultures from single zoospores, and while one 

 cannot always be certain that the culture will grow, a sufficient number of dilu- 

 tions and care in the use of perfectly clean pipettes will usually result success- 

 fully. 



Marshall Ward, in the Annals of Botany, Vol. xiii, pp. 563-566, gives some 

 ingenious means of separating gross cultures, which may be resorted to when 

 necessary. One of the most satisfactory, and often used with success in separating 



