Livingston : Chemical stimulation of a green alga 19 



From the above statement of responses, it will readily be seen 

 that the action of sulfate and nitrate is the same in practically all 



This means, of course, that according to the dissociation 

 hypothesis, the anions are comparatively without effect and that 

 the responses just described are due to the presence of the metal 

 ions in the medium. This is what should be expected from the 

 fact that in the case of all the more poisonous salts the difference 

 between experiment and control in concentration of the SO^ or NO^ 

 ions is negligible. As has been said, these anions were chosen 

 because they were already present in the nutrient medium. They 

 are two out of the three which the plant uses most extensively in 

 its metaboh'sm, and thus it is not surprising that its protoplasmic 

 system is of such a nature that slight, or even rather great, changes 

 in their concentration are without visible effect upon the hTe-proc- 



It will be remembered in this regard that it was shown in a 

 previous paper * that a decrease to one-tenth its normal amount of 

 any one of the nutrient salts used in Knop's solution is without 

 effect upon this plant, so long as the osmotic properties of the 

 solution are not altered by the change. 



In order to study the question of the relation of stimulating 

 power to the other properties of the metals studied, the following 

 two lists were constructed. One is based on the lowest fatal con- 

 centrations, the other on the lowest concentration producing the 

 palmella form. In the first column of each list is given the symbol 

 of the cation (since anions play no part, they need not be consid- 

 ered), and in the second the concentration with regard to which 

 the list is made. The elements are arranged in the order of their 

 stimulating power, the w^eakest ones first. In cases where the 

 least fatal concentration could not be determined with sharpness 

 but where there are indications as to its position, slightly above or 

 below the concentration given, the signs + and — are used to 

 denote *' greater than'* and ''less than." Where the sign is 

 double it denotes ** much greater than." 



These lists are presented here in order to have the varying 

 degrees of toxicity or stimulating power in mind before taking up 

 the discussion of the three forms of response mentioned in a pre- 



form 



* Livingston, B. E. On the nature of the stimulus which causes the change in 

 in polymorphic green algae. Bot. Gaz. 30 : 289-317. 1900, 



