26 



In a (necessarily dilute) solution of gypsum, which contained a con- 

 siderable quantity of the undissolved salt in suspension, the plants 

 grew decidedly more vigorously than in pure water. ^ 



In a saturated, but necessarily very dilute, solution of normal cal- 

 cium carbonate [CaCOg], roots of Lupinus elongated nearly twice as 

 much and remained in decidedly better condition during twenty-four 

 hours than in distilled water. This solution gave a faint reaction for 

 hydroxyl (with phenolphthaleine) at the beginning of the experi- 

 ment, but none at the end of twenty-four hours, doubtless because of 

 the production of carbonic acid through the excretion of carbon 

 dioxide by the roots. But a solution of calcium bicarbonate 

 [Ca(HC03)2], made by saturating a portion of the same calcium car- 

 bonate solution with carbon dioxide, permitted only about one-third 

 as much growth of ^tlie roots as took place in distilled water. Their 

 condition was decidedly abnormal at the end of twenty-four hours, 

 even the turgor being poor.- 



Magnesium carbonate [MgCOg], in a solution which gave a strong 

 hydroxyl reaction with phenolphthaleine, allowed the roots to grow 

 about as rapidlj^ as in distilled water and to remain in about normal 

 condition. On the other hand, a portion of the same solution to 

 which an excess of carbon dioxide was added and in wliich no free 

 hydroxyl could be detected (either before or after the experiment) 

 exerted a strongly toxic action upon the roots. These made practi- 

 cally no growth during twenty- four hours; their turgor became 

 decidedly inferior, and there occurred a marked discoloration of 

 brownish spots, such as is produced by the readily soluble mag- 

 nesium salts. Here it is obviously a case of a greater amount of 

 magnesium in solution, owing to the presence of carbon dioxide in 



' The stimulating effect which lime often exercises upon the growth of plants is 

 too well known to require illustration. The presence of calcium salts in consid- 

 erable quantity leads to a more vigorous production of root hairs than is normally 

 the case, as can easily be demonstrated by culture experiments, in which only the 

 tip of the root is immersed in the calcium salt solution. On the surface of the root 

 above the solution a great number of unusually long root hairs appear. To this 

 effect of the presence of lime, and the consequent readier absorption of potassium 

 and ammonium salts from the soil, Loew attributes in part the benefits obtained 

 by liming. (Bui. No. 18, Div. Veg. Phys. and Path. , U. S. Department of Agricul- 

 ture, p. 43.) That calcium salts directly stimulate growth, apart from the produc- 

 tion of root hairs, is, however, shown by cultures with the root entirely immersed 

 in an aqueous solution, thus precluding any important development of these organs. 



^ Schloesing's investigations [Comptesrendus, 74, 1552 (1872)] showed that 100,000 

 parts of pure water, i. e., free from dissolved carbon dioxide, would dissolve about 

 1.3 parts of calcium carbonate. Treadwell and Renter [Zeit. fiir anorg. Chem., 

 35, 28 (1900)] showed that by increasing the pressure of the carb'on dioxide in 

 the gas phase in contact with the solution until it was one atmosphere, the solu- 

 bility was increased so that 100,000 parts of water would dissolve 116 parts of 

 calcium carbonate. Even at this extreme solubility there would be but 46 parts 

 of calcium per 100,000 of water, as against about 60 parts in a saturated solution of 

 calcium sulphate, in which plants thrive well. 



