1920] 
DUGGAR—H-ION CONCENTRATION AND NUTRIENT SOLUTIONS 49 
, (18). A comparative study of salt requirements for young and for 
mature buckwheat plants in sand cultures. Soil Science 6: 1-32. f. 1-3. 1918. 
'18a). Toxicity of monobasic MUR towards soy beans grown in 
soil Aud solution cultures. Ibid. 5: 87-122. 1918. 
, and Martin, W. H. (18). A comparison of salt requirements for young 
and for mature buckwheat plants in water cultures and sand cultures. Am. 
Jour. Bot. 5: 186-191. 1918. 
Sörensen, S. P. ((09—10). Études enzymatiques. II. Sur la mesure et l'importance 
de la concentration des ions hydrogéne dans les réactions enzymatiques. Lab. 
Carlsberg, Compt. rend. des Trav. 8: 1-168. f. 1-7; 396-401. 1909-1910. 
Stewart, G. R. (18). Effect of season and crop growth in modifying the soil extract. 
Jour. Agr. Res. 12: 311-308. pl. 14. f. 1-24. 1918. 
Stiles, W. (15). On the relation between the concentration of the nutrient solution 
and the rate of growth of plants in water culture. Ann. Bot. 29: 89-96. 1915. 
—— — —, (16). On the interpretation of the results of water culture experiments. 
Ibid. 40: 427-436. 1916. 
Toole, E. H., and Tottingham, W. E. (18). The influence of certain added solids 
upon the composition and efficiency of Knop’s nutrient solution. Am. Jour 
Bot. 5: 452-461. i 
Tottingham, W. E. (14). A quantitative chemical and physiological E of 
nutrient solutions for plant cultures. Physiol. Res. 1: 133-245. f. 1-15. 1914. 
Trelease, S. F., and Free, E. E. (17). The effect of renewal of culture E. on 
the sra of young wheat plants in water-cultures. Johns Hopkins Univ. 
Circ. 293: 227-228. 1917. 
True, R. H. ('00). The toxic action of a series of acids and of their sodium salts on 
Lupinus albus. Am. Jour. Sci. IV. 9: 183-192. 1900. 
Truog, E. (18). Soil acidity: its relation to the growth of plants. Soil Science 5: 
