433 
Results of Water Culture Experiments. 
statement of Dr. Brenchley mentioned above (p. 430), that when plants are 
growing slowly the concentration of the nutrient solution is without influence 
on the rate of growth over a wide range. As the plant grows larger it will 
grow more rapidly because both the assimilative and absorptive systems are 
larger. If these two systems increase at the same rate, it should follow that 
if concentration of the solution is without influence on the rate of growth 
when the plant is small, so it should also be without influence on it when 
the plant is large, for although the plant will require more salts, the 
absorbing system will be larger and so can obtain more. 
The whole question therefore resolves itself into the problem of the 
relationship between the rate of absorption and the concentration of the 
medium. It is quite conceivable that below a certain point concentration of 
the nutrient solution might be so low that the root could not absorb enough 
salts from the solution simply because of the dilution of the latter. No very 
definite evidence has, however, yet been obtained of this, although the 
researches of True and Bartlett 1 are suggestive in this regard. They 
indicate that this limiting concentration is very low. 
4. Toxic Action of Substances in the Nutrient Solution. The presence 
of certain substances in the nutrient medium may act as a factor limiting 
growth. This has been shown to be the case with salts of a number of 
metals such as copper, zinc, and manganese. An account of such toxic 
action may be found in Dr. Brenchley’s monograph on Inorganic Plant 
Poisons and Stimulants. The limiting action of such substances is no 
doubt due to their reaction with substances of the plant which disturb 
the normal reactions which make up the life processes. 
5. The Ratio between the Concentration of different Substances in the 
Nutrient Solution. Numerous writers have urged as a result of water 
culture experiments the need for a definite ratio between the constituents 
of the culture solution for the production of optimum growth. Any wide 
departure from this ratio causes depression of growth, owing to one of 
the constituents, being present in excess, entering the plant too rapidly 
and producing a toxic action. The whole question has been summed 
up under the name of Antagonism, a general account of which is given 
in a paper by Mr. J0rgensen and the writer. 2 
There may be something in the idea of balanced solutions, but the 
evidence in favour of it will not bear analysis. All the experiments 
adduced in support of it are open to one or two fatal criticisms, and 
most of them to both. These are : 
1 True, R. H., and Bartlett, H. H. : The Exchange of Ions between the Roots of Lupinus 
albus and Culture Solutions containing one Nutrient Salt. Amer. Journ. Bot., vol. ii, 1915, 
pp. 255-78. The Exchange of Ions between the Roots of Lupinus albus and Culture Solutions 
containing two Nutrient Salts. Ibid., pp. 311-23. 
2 Stiles, W., and Jorgensen, I. : The Antagonism between Ions in the Absorption of Salts by 
Plants. New Phyt., vol. xiii, 1914, pp. 253-68. 
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