nd the toxic effect of metallic compounds upon low forms
of plants is quite different from the effects of the same substances upon
the more complex tissues of higher plants, a fact which is utilized to
advantage in the application of fungicides for the control of parasitic
growths on common farm crops.

Among the elements whose physiological effects upon higher plants, such as
the cereal crops, etc., when their soluble compounds are present in the
soil, have been carefully studied, there are three fairly distinct types of
injurious mineral elements. The first of these, represented by copper,
zinc, and arsenic, apparently exert their toxic effect regardless of the
proportion in which they are present in the nutrient solution which is
presented to the plant; although the degree of injury varies with the
amount of injurious substance present, of course. The second type, of which
boron and manganese are representatives, apparently exerts a definite
stimulating effect upon plants when supplied to them in concentrations
below certain clearly defined limits; but are toxic in concentrations above
these. The third includes many soluble salts of magnesium, sodium,
potassium, etc., which while either innocuous or else definite sources of
essential plant foods when in lower concentrations, become highly toxic, or
corrosive, when present in the soil solution in concentrations above the
limits of "toleration" of individual plants for these soluble salts. The
tolerance shown by the different species of plants toward these soluble
salts (the so-called "alkali" in soils) varies widely; indeed, there seems
to be considerable variation in the resistance of different individual
plants of the same species to injury from this cause.

With reference to the toxic effect of the third type of substances, i.e.,
the common soluble salts, it is known that single salts of potassium,
magnesium, sodium, or calcium, in certain concentrations, are toxic to
plants, while mixtures of the same salts in the same concentrations are
not. Thus, solutions of sodium chloride, magnesium sulfate, potassium
chloride, and calcium chloride which, when used singly, killed plants whose
roots were immersed in them for only a few minutes, formed when mixed
together a nutrient solution in which the same plants grew normally. The
remarkable remedial effect of calcium salts in overcoming the injurious
effects of other soluble salts has already been mentioned. One explanation