25 



lyzing) effect of a salt solution. He experimented with Spirog^*ra in 

 order to obtain means of making such distinction, comparing its 

 behavior in a solution of cane sugar, which is believed to possess no 

 chemically toxic properties, with that in solutions of sodium chloride 

 and potassium nitrate. The maximum concentration of the sugar 

 solution in which life could be maintained was determined to be 0.75 

 normal. Allowing for differences of dissociation, 0.46 normal should 

 then be the maximum endurable concentration of a sodium chloride 

 solution if only its osmotic pressure were involved. In fact, however, 

 0.1 normal was found to be the actual limit, so that a definite toxic 

 action of sodium chloride must be admitt-ed (loc, cit., p. 410).^ 



Were the injurious action of these solutions attributable to plas- 

 molysis alone, an approximately equal amount of elongation should 

 take i)lace in solutions of different salts, if each solution contain an 

 equal fraction of a gi*am equivalent to a given amount of water, grant- 

 ing that the dissociation of each salt was equally great at the given 

 concentration, as would be approximately true for strong electrolytes 

 at the concentrations here used; for elongation and growth in general 

 are intimately connected with the turgor conditions of the tissues,^ 

 which, in turn, depend upon the osmotic force exerted by the sur- 

 rounding solution. That force being equal for each of two solu- 

 tions, the turgor and the amount of elongation of the roots immersed 

 in each should also be equal if osmosis were the only factor involved. 

 That this is not the case is sufficiently established by the figures given 

 in Table 11. 



RESULTS WITH LESS SOLUBLE SALTS. 



Besides the easily soluble alkali salts a few others were used in 

 experiments, i. e., calcium sulphate [CaS04] calcium carbonate 

 [CaCOg], calcium bicarbonate [CafHCOg)^], and the carbonate and 

 bicarbonate of magnesium [MgCOg and Mg(HC03)2]. These were 

 found to be either toxic in a ver}' slight degree, indifferent, or posi- 

 tively stimulating to growth. 



- From True's results it is clear that at the concentrations involved in our experi- 

 ments with pure solutions the toxic effect observed must in everj' case be referred 

 to action of a chemical rather than a purely physical nature. In some of the 

 mixed solutions, such as the very concentrated ones containing calcium sulphate, 

 it may be that their osmotic pressure determines the limit of endurance of the 

 plant roots. 



■ For example that, except perhaps in rare instances, gi'owth can not be resumed 

 after an interruption i such as is occasioned Ity transference of plants from one 

 medmm to another i unless the turgor of the plant or the organ concerned is nearly 

 or quite normal, was shown by Curtis [Bui. Torr. Bot. Club, 27, 1 (1900)] in the 

 case of mycelia of Mucor. Botrytis. and Penicillium. grown in a plasmolyzing 

 solution (4 per cent potassium nitrate I. As this author expresses it. " there is a 

 necessity of a certain turgor force before growth is possible, and growth can not 

 occur until a turgor pressure has been reached which is normal to the plant grow- 

 ing in the given solution."' ^Loc. cit., p. IL) 



