Absorption of Salts by Plants. 257 
From these experiments Osterhout derives the conception of 
physiologically balanced solutions. He supposes that a number of 
toxic substances present together in a solution are rendered non¬ 
toxic if they are present in a certain ratio. Such a solution is a 
physiologically balanced one. 
Osterhout also points out that a nutrient solution may he used 
in which the dissolved substances are present in such dilution that 
none of the components would exert any toxic action if the others 
were removed. It is obvious that in this case the question of a 
physiological balance does not enter; only when the concentration 
is increased beyond a certain strength will the solution become 
toxic to plants and a physiological balance be required between the 
various solutes if the solution is to do no harm. 
In a third series of experiments the antagonism between 
potassium and magnesium alone is dealt with (26). Species from 
most of the great classes of the plant kingdom were tried and it 
was shown that a solution containing both potassium and magnesium 
chloride in a definite ratio is less toxic than pure solutions of either 
salt in the same molar concentration. 
It was also shown that in the case of the nitrates and sulphates 
the addition of the potassium salt to the corresponding magnesium 
salt reduces the toxicity of the latter. 
Osterhout thus disposes of the theory of Loew and Aso (18) 
that the antagonism is due to the formation of a double salt of 
potassium and magnesium chloride, for the nitrates do not form 
double salts. Moreover in the case of chlorides the salt would 
dissociate into its constituent ions, and the result in solution would 
be much the same. 
In these experiments it will be observed that Osterhout 
generally determines the relative toxicity by measuring the length 
of life of the structure under consideration ; the death point was 
easily noted by observations of the colour reactions and microscopic 
appearance of the cells. 
In some cases, as for instance in those experiments on Equisetum 
prothallia and Wheat seedlings, he uses the amount of growth as a 
criterion of relative toxicity. This method is elaborated in a further 
paper (27) on the protective action of sodium on plants in which 
the mutual action between sodium and potassium, sodium and 
ammonium, sodium and magnesium, and sodium and calcium is 
investigated by means of measuring the growth in length of roots 
of Wheat seedlings and other plants. He worked principally with 
