22 



and is no higher in fractions of the reacting weight than it is for 

 sodium chloride. It should be mentioned, however, that the plants 

 survive in a solution of the bicarbonate of the strength given in the 

 table in much better condition than in the corresponding concentra- 

 tion of the chloride, so that the latter must be regarded as the more 

 harmful of the two salts. The wide distribution of sodium bicarbon- 

 ate and its abundance as a comj)onent of many alkali soils renders 

 the demonstration of its marked poisonous effects upon vegetation, 

 even when present in comparatively dilute solutions, a matter of no 

 little importance. Although much less injurious than is the normal 

 carbonate or ' ' black alkali, " the presence of this salt can not be 

 neglected in future estimations of the value of western soils. 



An explanation of the harmful action of sodium bicarbonate which 

 at first suggested itself was that by its dissociation free hydrogen ions 

 are liberated, though the weight of evidence on chemical grounds is 

 rather against this view.^ It has been shown by recent investigators^ 

 that it is probablj^ the hydrogen ions dissociated bj^ certain acids 

 (especially the strong mineral acids) which make them so injurious to 

 organisms, even in extremely dilute solutions. If this were the reason 

 for the toxicity of sodium bicarbonate it would follow that water heavily 

 charged with carbon dioxide, as in the check experiment described 

 above (p. 20), would prove similarly injurious to plant roots by reason 

 of the dissociation of hydrogen ions by the carbonic acid (HCO3), 

 which is supposed to be formed when carbon dioxide is dissolved in 

 water. But, as has already been noted, no toxic effect was obtained 

 with an aqueous solution of carbon dioxide.^ 



' Walker and Cormack, Journ. Chem. Soc, 47, 5 (1900) and Bodlander, Zeit. fttr 

 physik. Chem., 35, 25 (1900). 



'-'Kahlenberg and True, Bot. Gazette, 22, 87 (1896); Heald, 1. c, p. 134; Loeb, 

 Pfltiger's Archiv f. die gesammte Physiologie, 69, 4 to 9 (1898); Kahlenberg and 

 Austin, Journ. of Physical Chem., 4, 553 (1900); True, Amer. Journ. Sci. ser. 4, 9, 

 183 (1900). 



^ There exists among plant physiologists some diversity of opinion as to the direct 

 effect of large quantities of carbon dioxide upon the growth of roots. The sub- 

 ject is evidently one which needs a more thoroughgoing investigation, not only 

 from a scientific standpoint, but from economic reasons also, as it is intimately 

 connected with tillage and drainage problems. For an extended discussion of this 

 question see Lopriore in Jahrb. fiir wiss. Botanik, 28, 531 (1895). The author men- 

 tions that Boehm found roots of the bean (Phaseolus vulgaris), when exposed to 

 an excess of carbon dioxide, to be shorter, and the lateral roots fewer, than is 

 ordinarily the case. Jentys [Bui. Internat. Acad. Sci. Cracovie, 1892, 306 (1893)], 

 found that by passing atmospheric air to which had been added 4 to 12 per cent of 

 carbon dioxide through the soil of culture pots, an injurious effect upon the roots 

 of the bean and the yellow lupine could be detected, although the injury was 

 less than in Boehm 's experiments. On the other hand, wheat was practically 

 unharmed. Lopriore (1. c.,p. 623), concludes that carbon dioxide in excess has a 

 hindering but not a permanently injurious influence upon the functions of proto- 

 plasm. This effect is not ascribable to the absence of oxygen, but is specific. 

 Plant cells can gradually accommodate themselves to a quantity of carbon dioxide, 

 which, if applied directly, would injure them. Lopriore 's experiments were made 

 chiefly with Mucor, yeast, and pollen grains and tubes. 



