ON THE ARTIFICIAL CULTUKE OF MARINE PLANKTON ORGANISMS. 46o 



Number of Geonu'tric 



Day. fnistnles. progies&ioii. 



nth . . 59 . . 63 



14th ... 62 . . 68 



19th . . 85 . . 85 



27th . . 140 . . 120 



34th . . 170 . . 160 



41st . . 190 . . 220 



Tlie curve obtained by plotting the number of diatoms against tlie 

 number of days approximates the curve of an ordinary geometric 

 progression, where the ratio is two and the periods are equal to sixteen 

 days. To show this the figures read oft' from the curve at the same 

 intervals as the diatoms are appended in the table. From this it will 

 be seen that, after a start had been made and before exhaustion 

 set in, the numbers obtained agree fairly closely with tlie assumption 

 that every diatom divided once in a period of sixteen days. Probably 

 in normal cultural conditions the rate of multiplication greatly exceeds 

 this figure on account of better lighting, etc. (cp. Miquel, 12). 



Besides diatoms, many other organisms appear in tliese cultures. 

 We are indebted to Prof. Gr. S. West for the identification of a form of 

 unicellular alga, which is very common and difficult to avoid when 

 attempting to obtain persistent ciiltures of the Diatouiacecc, namely 

 Fkurococcus mucosns (Kntz.), Eabenh. This small green alga, if once 

 introduced into a culture of a plankton diatom, will soon miiltiply at 

 the expense of the latter with its ultimate extinction. It is \ery 

 hardy and cultures of it in almost every medium seem to last in- 

 definitely. Multiplication beyond a certain point probably does not 

 occur, but tlie cells retain their colour and normal shape and will start 

 active reproduction if suitable nutrient material is provided. 



In cultures inoculated from plankton many other forms of unicellular 

 and filamentous alga? thrive. Several species belonging to the classes 

 IihodopJn/cca' and Myxophycccc commonly occur, but we h.ave not 

 been able to identify them. The most usual filamentous forms of 

 Chloroph}jcecc are Entcroinorpha, Vancheria, Bhizocloniuiii, etc. It is 

 interesting to note that it was the unintentional appearance of young 

 plants of Laiiiinaria dujitata in some of our Petri dishes that led 

 Mr. Drew (4) to cultivate this alga in Miquel sea-water and so discover 

 its early life history. Cultivations of marine algffi by these methods 

 would without doubt yield many new species, and would also provide 

 rich material for the study of their modes of reproduction. 



Many forms of flagellates live either together with diatoms or alone. 

 Among these is an unidentified species of Chiloiiionas which we have 

 obtained in persistent culture. It multiplies very rapidly, colouring 



