in the physiological
effect of the substance upon the protoplasm of the tissues which it enters,
and others may be due to differences in the resistance of the protoplasm,
or of its protective coverings, to penetration by the toxic material.
Indeed, the possibilities of different types of toxic action, and of
resistance to it by individual plants and species, are so varied that it is
not possible to divide toxic agents into specific groups according to the
nature of their injurious action upon the plant cell. They are, therefore,
more commonly grouped into classes according to their chemical nature and
economic significance as fungicides, as follows: inorganic and organic
acids; caustic alkalies; salts of the heavy metals; hydrocarbon gases;
formaldehyde; alcohols and anaesthetics; nitrogenous organic compounds; and
miscellaneous decomposition productions of organic origin. The following
brief review of some of the results of the experimental studies of the
toxicity of different compounds belonging to these several groups will
serve to indicate the general trend of the investigations of these matters
which have thus far been made.

=Acids.=--The common inorganic acids (hydrochloric, nitric, and sulfuric)
kill the rootlets of common farm crops when the latter are immersed for
twenty to twenty-four hours in solutions of these acids containing from
three to five parts per million of free acid. Acetic acid must be about
five times as concentrated as this, and other organic acids may be much
more concentrated still before they produce the same injurious effects. The
toxic effect of all these acids is greatly reduced in soil cultures, or if
particles of sand, graphite, clay, filter paper, etc., are suspended in the
solutions containing the acids, the reduction in toxic effect being
probably due to the adsorption of the acids upon the solid particles.
Hence, the concentrations which limit the toxic effects of these acids in
water solutions cannot be taken as representing the condition with which
the same plant will have to contend when growing under normal cultural
conditions.

=Alkalies.=--The caustic alkalies must usually be present in from five to
ten times as great concentrations as those of the mineral acids, in order
to produce the same injurious effects upon the rootlets of common plants.
The so-called "alkali" of soils is not alkali at all, but is neutral
soluble salts present in sufficient concentration to exert a toxi