ARTIFICIAL CULTURK OF MAEINE PI,ANIvTON ORGANISMS. 391 



No growth, 



D. Ditto + solution A + solution B. , 



Very good growth, lasting considerably longer 

 than B. 



E. Ditto + animal charcoal. 



No growth. 



Exhausted cultures in animal charcoal water gave, the same 

 general results on treatment and reinoculation. In an old 

 culture of Biddulphia niobiliensis in outside water + 

 solution B only, which was in a very exhausted condition 

 (nine months old), the addition of KN0;5 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 KNO- 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 diatoms is that their skeletons are very 

 markedly less siliceous 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 mai'king can usually be seen on even those forms, 

 which, under natural conditions, are the most siliceous, e.g. 

 Coseinodiscus 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 siliceous skeleton. As a rule, the more rapid the 

 growth the more teratological forms will be found. In 

 untreated outside water little deformity will take place, but 

 in normal Miquel, where very rapid groAvth takes place, the 

 diatoms may assume almost any conceivable shape. The 



