456 ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



food-material substances wliicli the diatoms are incapable of using in 

 their metabolism, e.g. nitrites into nitrates. The absence of any 

 increase in phosphates, when using HoOo, may possibly be the reason 

 why better results were not obtained with this medium. The action 

 which, in addition to any nutritive value, we must assume that sol. B, 

 animal charcoal, and HoOo can all effect, would appear to fall into the 

 class of " protective " actions (p. 439). It is quite conceivable that, with 

 different samples of sea- water, this " protective " action is not neces- 

 sary in every case, and this would account for the anomalous results 

 met with when using sea- water + nitrates + phosphates only, in which 

 medium sometimes good cultures, but more often the reverse, are 

 obtained. The effect of Miquel's sol. B, animal charcoal, and HoOo on 

 the " alkalinity " of the sea- water, also points to some chemical change, 

 which does not directly enter into the metabolism of the plants. 



It may be pointed out that the action of such substances as finely 

 powdered carbon, and ferric oxide precipitates, has been shown to pro- 

 dvice a favourable effect on nutrient solutions used for the culture of 

 certain higher plants, and it has been suggested tliat the beneficial 

 action of these substances is the removal of toxic elements from the 

 media (Breazeale, 3). Such removal of toxins would fall under our 

 definition of " protective " action. 



Of nutritive substances, other than those already mentioned, we 

 have still to consider (1) silica and (2) dissolved oxygen and carbonic 

 acid. Having regard to the conditions under which our cultures have 

 been grown, i.e. in glass flasks, tlie question of silica does not seem to 

 enter into the problems which we have discussed. A few words must, 

 however, be said as to the dissolved gases. Whipple (62) and Bald- 

 win (44) have drawn attention to the observed relations, which are 

 found in natural waters, between algal growths and the amounts of 

 dissolved oxygen and carbonic acid. That these factors are of great 

 importance cannot be doubted, but in our cultures it seems reasonable 

 to suppose that the conditions of saturation of these gases are the 

 same in all, since series of cultures in standard media, such as Miquel 

 sea-water or Berkefeld water, can be set up with the certainty that, if 

 not every one, at least a very high percentage, will give normal results. 



Of the purely physical factors, light is by far the most important. 

 Within limits, the rate of growth in a suitable medium seems to 

 depend directly on the intensity of the light (Whipple, 60). Absence 

 of light, as would be expected, soon completely kills the diatoms. 



Temperature also seems to affect the rate of growth to a certain 

 ■extent, l>ut for those temi)eratures at wliich we have experimented it 

 sdoes not appear to alter tlie quantity of growth. 



