444 ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



off, the filtrate was nterilized by heat, and then treated by various. 



methods. 



In one typical experiment the following was the result : — 



Water from an exhausted culture of Skelctonema costatum in Miquel 



sea-water, reinoculated with the same diatom : — 



A. Filtered and sterilized. 



N'o growth obtained. 



B. Ditto + sol. A (nitrates only). 



Good culture, but did not last very long ; further addition of 

 nitrates made no improvement. 



C. Ditto + sol. B. 



No growth. 



D. Ditto + sol. A + sol. B. 



Very good growth, lasting considerably longer than B. 



E. Ditto + an. char. 



No growth. 



Exhausted cultures in animal-charcoal water gave the same general 

 results on treatment and reinoculation. In an old culture of Biddul- 

 l)Ma mobiliensis in outside water + sol. B only, which was in a very 

 exhausted condition (nine months old), the addition of KNO3 gave a 

 very rapid regeneration, and the diatoms became of normal colour 

 and form. This renewed growth, however, did not last very long, and 

 a further addition of KNO3 did not give any result. The addition of 

 sodium phosphate also failed to stimulate growth. The same rapid 

 regeneration, on the addition of potassium nitrate, has been obtained 

 with almost every medium, but a second attempt has always failed. 



Silica. A very noticeable character of the true plankton species of 

 marine diatom is, that their skeletons are very markedly less silicious. 

 than the great majority of other forms. Their valves are only feebly 

 marked, if at all, and they will not stand the vigorous treatment of 

 cleaning with acids and heat that is commonly used in the case of 

 fresh-water diatoms. In cultural forms, this absence of silica is still 

 more obvious, and no marking can usually be seen on even those 

 forms which, under natural conditions, are the most silicious, e.g. 

 Coscinodiscus excentricus. Deformed and distorted frustules are the 

 rule in certain stages of growth in our cultures, and it is often very 

 hard to make out more than the thinnest coating of silica. It is quite 

 probable that this deformity can be accounted for simply by the 

 absence of a strong silicious skeleton. As a rule, the more rapid the 

 growth, the more teratological forms will 1)6 found. In untreated 

 outside water little deformity will take place, but in normal Miquel, 

 where very rapid growth takes place, the diatoms may assume almost 



