124 Journal of the Mitchell Society [February 



pure cultures were thus secured. These colonies also were trans- 

 planted to Petric dishes. 



Such methods might serve well where the great majority of the 

 Diatoms are of one species and where there is no great contamination 

 of other forms, but would hardly be successful when used with the 

 ordinary fresh water plankton. The method which we propose here 

 is based upon these previous methods but has a number of variations. 

 In the first place we used artificial media. It was made after the 

 formula suggested by Moore (as given by Kiister) for the culture of 

 Algae, as follows: 



Ammonium Nitrate 5 gr. 



Dipotassium Phosphate 2 gr. 



Calcium Chlorid 1 gr. 



Magnesium Sulfate 2 gr. 



Ferric Sulfate trace 



Distilled water 1000 cc. 



(Special low conductivity) 



This differs from the formulae usually given for Diatoms in that 

 it contains no added Sihcon compound. Chemical analysis of our 

 agar, however, showed Silicon to be present and an examination of 

 the agar filtered through cotton showed the presence of some marine 

 Diatom shells. A 2% solution was now made up with this nutrient 

 and washed agar. The material was sterilized in test tubes and 

 retained in them until needed. It was then melted and poured into 

 Petrie dishes. When it had cooled somewhat, but not hardened, a 

 drop of pond water was placed on it and washed around. The plate 

 was then hardened and was turned up-side-down upon the cover and 

 placed under a bell jar in the green house. After from three to four 

 weeks, there were colonies of various organisms, large enough to be 

 spaded out. This was accomplished by the use of a platinum needle. 

 The colony thus dissected out was examined under the microscope 

 and if not too badly contaminated, it was stirred up in sterile water 

 and replated on new agar plates. These plates were likewise placed 

 under culture conditions and in a few weeks had well formed colonies. 

 In case all the colonies were not of the same species, colonies were 

 dissected out and replated. Thus far this second plating in all our 

 cases, has given us pure cultures. In this manner we have secured 

 four species of Diatoms.* 



♦ In tliis manner we secured also a Blue Green, a unicellular Green and Scenedesmus, 

 the latter of which has bee" thoroughly worked out by the pure culture method by (J. M. 

 Smith (1916). The four species of diatoms thus secured have been identified by Dr. J. J. 

 Wolfe as 



Navicula atomus Naog. 



Navicula minuscuia Gnm. 



Nitzschia amphioxijs (Ehr.) Grun. 



Nitzschia palea Wm. Smith. 



