Livingston : Chemical stimulation of a green alga 21 



No doubt all the salts tested here would produce death at 

 sonic concentration, but those whose stimulating power or toxicity 

 is weakest do not bring about this response until their concentra- 

 tion is high enough so that the osmotic pressure of the solution 

 may begin to play its role. The cations Ca, Li, Mg, K, Na and 

 Sr belong to this class, as do also probably NH 4 , Ba, and perhaps 

 Rb. The toxicity of these elements cannot be studied in a nutri- 

 ent medium of as low pressure as the one here used. Perhaps it 

 is impossible to determine it for the filamentous form of this plant. 



What may be the nature of the killing power of the elements 

 for which this property was determined is difficult to conjecture 

 without more data. It is instructive to note, at any rate, that 

 their effect in this regard is exactly similar to that of a drying 

 medium. We see the same sort of death phenomena in these 

 poisoned solutions of low osmotic pressure that was found for 

 solutions of high pressure. In very strong solutions the cells die 

 as filaments, in somewhat weaker ones they round off and assume 

 the palmella form before death ensues. The relation of this toxi- 

 city to the physico-chemical nature of the elements will be con- 

 sidered in a later paragraph. 



2. The response in phenomena of growth. — All of the elements 

 tested produce the palmella response at some concentration. In 

 case of Ca, K and Sr and perhaps of Mg and Na, this response 

 may be in part due to osmotic pressure. For the other cat- 

 ions there can be no doubt that it is purely a chemical stimulus 

 which is acting. The fact to be emphasized here again is that we 

 have exactly the same growth changes brought about by the 

 presence of toxic cations as are produced by physical extraction 

 of water. In the poisoned cultures (whose osmotic pressure, it is 

 to be remembered, is very low, much lower than is necessary for 

 the retention of the filamentous form in an unpoisoned nutrient 

 medium), there are observed exactly the same rounding up of 

 cells, the same thickening of walls, and the same alteration in 

 rapidity and direction of cell-division, as was found to take place 

 when filaments are converted to the other form by the action of a 

 concentrated solution. 



These observations agree with those of the death responses, 

 and it seems possible that in both cases we have to deal with a 



