400 E. J. ALLEN AND E. W. NELSON. 



animal charcoal may, and probably do, act as " nutritive " sub- 

 stances ; but, since the addition of pliospliates alone does not 

 yield cultures comparable with those produced by either of 

 them, ,'ind sin'co, excepting phosphates, there is no possible 

 common nutritive substance in their composition, we are led 

 to conclude that, in addition to any nutritive effect, they 

 must exert some other action. 'J'iiis view is supported by the 

 results obtained by using HoO^. Tiiis substance cannot l)e 

 directly ''nutritive," although it may be so indirectly, by 

 oxidising into useful food-material substances which the 

 diatoms are incapable of using in their metabolism, e.g. 

 nitrites into nitrates. The absence of any increase in phos- 

 phates, when using H^Oo, may possibly be the reason why 

 better results were not obtained with this medium. The 

 action, whicli, in addition to any nutritive value, we must 

 assume that solution B, animal charcoal, and H^Ci can all 

 effect, would appear to fall into the class of " protective " 

 actions (p. 38-i). 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 + phos- 

 phates only, in which medium sometimes good cultures, but 

 more often the reverse, are obtained. The effect of Miquel's 

 solution B, animal charcoal, and HoOo on the "alkalinity" of 

 the sea-water, also points to souje 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, have 

 been shown to produce a favourable effect on nutrient solu- 

 tions used for the culture of certain higher plants, and it has 

 been suggested that 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 



