52 



In another work^ Pfeffer emphasizes the idea of counter reactions, 

 suggesting that "one has to do in this accelerating stimulation with 

 one of the manifold reactions which serve, through more intensive 

 activity, to counteract as far as possible an injurious influence or to 

 compensate injuries." And again r^ "Probably this [stimulating] 

 effect results from a general reaction of the organism against injurious 

 substances, since similar effects are induced by ether, alkaloids, etc., 

 effects which also find expression in an increase of fermentation and 

 respiration. * * * It is easy to understand * hc * that further- 

 more such substances as are poisonous onl}^ in higher concentration 

 generally occasion no obvious [stimulating] effect." 



If it can be shown that such stimulating effect is sufficiently 

 permanent to express itself in "a marked increase in the yield of a 

 crop, its economic importance would be obvious. That the pres- 

 ence of a certain amount of calcium salts in the soil may reall}^ act 

 as a chemical stimulant to growth, apart from the value of the salts 

 as plant food, or in rendering soluble other nutritive soil comj^onents 

 there appears to be some reason to believe. It is not imj)ossible that 

 other substances, even perhaps those salts of magnesium and of sodium 

 which constitute the most noxious components of alkali soils, when 

 present in quantity too small to be harmful, may be activel}^ stimula- 

 tive rather than merely indifferent to plants. That several of them 

 are likewise valuable as sources of nutritive material is well known. 

 Whether, after all, the distinction between the chemically stimulating 

 effect and the utility as food of certain mineral salts be always as sharp 

 as is commonly supposed, is a question which can not yet be regarded 

 as decisively answered. 



ECONOMIC IMPORTANCE OF THE RESULTS. 



Some of the facts ascertained by these experiments with salt solu- 

 tions in their effect upon plants have a direct practical bearing upon 

 agricultural conditions and methods in regions where alkali salts are 

 frequent. Attention is particularly directed to the effects obtained 

 by the addition of lime salts to others. Each of the common soluble 

 alkali salts is found to be very injurious when alone, but usually 

 much less harmful when two are mixed, especially when a salt of lime 

 is one component of the mixture. This is strikingly the case with sul- 

 phates of magnesium and of sodium, the noxious effects of these salts 

 being enormousl}^ lessened by the application of lime, particularly in 

 the form of gypsum or land plaster (the dihydrate of calcium sul- 

 phate). Contrary to the general impression, this corrective effect was 

 found in water-culture experiments to be considerably less for "black 

 alkali" (sodium carbonate) than for any of the "white alkali" salts, 

 although even the harmf ulness of black alkali can certainly be greatly 

 diminished by the use of gypsum. 



'Pflanzenphysiologie (Zweite Auflage), 1, 374 (1897). 

 2Ibid.,p. 409. 



