c effect.

=Salts of the heavy metals= are especially toxic to rootlets of plants.
Salts of copper, mercury, and silver, have been found to kill the roots of
seedlings immersed in them for twenty-four hours when present in
proportions of less than three parts per ten million, while salts of many
other heavy metals are toxic when present in concentrations of less than
one part per million. The salts of the alkali metals are considerable less
injurious than are those of the heavy metals, but even these exert their
familiar injurious effect if present in concentrations which, measured by
the ordinary standards, would still be regarded as very dilute solutions.

=Illuminating gas=, and similar hydrocarbon gases, kill plants when present
in the atmosphere in as little as one part per million. Leaves, buds, and
roots are all alike sensitive to this toxic effect, the nature of which is
not yet understood.

=Formalin=, or formaldehyde, is a penetrating toxic agent for nearly all
plant cells, and is commonly used as a fungicide for the destruction of
parasitic fungi. It probably affects the colloidal condition in some way
similar to its hardening effect upon gelatin, etc.

The toxic effect of many different =organic compounds= is so varied in its
nature and extent that it is impossible to give any satisfactory brief
review of its manifestations. Recent investigations appear to indicate that
organic products of decomposition of plant residues in the soil may exert
powerfully toxic effects upon succeeding generations of the same, or of
different, plants growing on the land. But the experimental data and
conclusions concerning these matters are not yet accepted without question
by all students of plant science or of the problems of the productivity of
the soil. In fact, it is yet an open question whether toxic soil
constituents are really an important factor in the so-called
"unproductivity" of certain soils.

Alkaloids, and even the amino-acids which are produced in the tissues of
some species of plants, while not toxic to the plants or organs which
elaborate them, sometimes exhibit strikingly toxic action upon other plant
organs with which they are brought into contact. There is, as yet, no
satisfactory explanation of this difference in behavior between plant
tissues toward various organic toxic substances.

In fact, the whole subject of the toxic action of various substances upon
plants needs much more study before it is br