January 6, 1922] 



SCIENCE 



either solution, or into solutions containing 

 both salts mixed in various proportions and 

 above which the roots would absorb ions. It 

 was shown that Ca differed essentially from 

 Mg ions in being harmless in concentrations 

 that proved fatal in the case of magnesium. 



The conductivity method was next applied 

 to the problem of absorption by phanero- 

 gamic seedlings from solutions of the ordinary 

 nutrient salts; these being studied singly in 

 various concentrations^" and mixed in a variety 

 of proportions and concentrations.'^'' I will 

 not try to deal here with the results gained 

 beyond presenting a brief summary of such 

 points as bear on the question now in hand. 

 As a result of the study of several sorts of 

 seedlings grown in solutions of single salts it 

 may be said that in solutions of potassium and 

 sodium salts no concentration was observed in 

 which the seedlings were able to carry on sus- 

 tained absorption, in the end yielding markedly 

 more ions to the medium than they were able 

 at any time to appropriate. 



In solutions of Ca and Mg salts there was a 

 well defined equilibrium concentration below 

 which the roots were not able to absorb and 

 in these sub-minimal solutions ions leached 

 out into the medium. In solutions stronger 

 than this equilibrium concentration, absorption 

 took place in gi-eater or less measure. It 

 appeared that Ca was more favorable gen- 

 erally than Mg. At no concentration tried, 

 the strongest being about 900 X 10 ~ '' gram 

 norm, per liter, was there any evidence of 

 injui'y. Where the concentration of Mg was 

 raised in order to ascertain the maximum 

 quantity of absorption, characteristic injury 

 appeared and death more or less promptly 

 thereafter. A similar injury appeared tardily 

 in weaker solutions on longer duration. 



In mixtures again, absorption or leach de- 

 pended on the presence of Ca or Mg ions. 

 Again Ca in high proportion never brought 

 injury. Mg injury appeared less often than 



isTrue, Rodney H., and Bartlett, Harley Harris, 

 Am. Journ. Bot., 2: 255-278. 1915. 



i*True, Eodney H., and Bartlett, Harley Harris, 

 Am. Journ. Bot., 2: 311-323. 1915. 



i"True, Eodney H., and Bartlett, Harley Harris, 

 Am. Journ. Bot., 3: 47-57. 1916. 



in simple solutions. There was little evidence 

 that any such thing as a very definite Ca-Mg 

 ratio exists. In mixtures containing Ca and 

 other nutrient ions, especially when all or a 

 large proportion of the required ions were 

 present, the total quantity of ions absorbed far 

 exceeded the quantity of Ca ions present. This 

 indicated that in such mixtures Ca ions in 

 some way secured conditions that bring about 

 the absorption of ions, that, offered in umnixed 

 solutions, would be unabsorbed, or would cause 

 an active leach of other ions from the plant 

 cells. 



Thus we may fairly say that the presence 

 of Ca ions in some way makes those absorbable 

 that would otherwise be unabsorbable and en- 

 ables the plant to retain ions that it would 

 otherwise be unable to retain. The Ca ions 

 may be truly said to make the others physi- 

 ologically available to the plant. Stating this 

 in terms of the soil, we may say that when the 

 required minimum of Ca ions is not present 

 in the soil solution other nutrient ions present 

 are largely out of reach and such a deficient 

 soil solution may finally leach mobilized nutri- 

 ents from seedlings. If the required minimum 

 of Ca ions is not present, other nutrient ions 

 may be present in abundance but be physi- 

 ologically unavailable because of the inability 

 of the plant to appropriate them. 



Having thus far established the relation 

 between Ca ions and the ability of the roots 

 of the seedlings studied to retain ions gained 

 by the mobilization of their reserves and to 

 absorb others from the nutrient medium out- 

 side them, let us turn to a somewhat m.ore 

 detailed study of this phase of calcium action. 



Analytical data have long since indicated a 

 close chemical relation between the calcium- 

 content of higher plants and the cell wall. 

 The calcium content is relatively low in young 

 meristematic tissue and increases gi'eatly in 

 those parts characterized by mature cell walls. 



A more critical study of cell walls by Fremy, 

 Mangin, Bertrand and others has shown that 

 these are by no means homogeneous structures, 

 either chemically or structurally speaking, but 

 consist characteristically of an outermost layer 

 lying on the boundary line between adjoining 

 cells and other layers lying between it and the 



