s.

There are, however, a large number of water-soluble chemical substances
which are injurious to all living protoplasm, even at concentrations
considerably less than the point of osmotic equilibrium in the juices of
the protoplasm. These substances may act either directly or indirectly upon
the protoplasm, but at certain concentrations they always affect it
injuriously. In the main, these toxins are _external_ agents of other than
plant origin; although chemical substances developed by one plant may be
toxic to other plants, or even to other organs of the same plant than those
in which they are elaborated.

Toxins may be either _general_ (i.e., injurious to all types of plants), or
_specific_ (i.e., injurious to only certain species) in their action.
Examples of specific toxicity are of only minor importance in plant
studies. They seem to be generally explainable on the basis of some unusual
lack of resistance or failure of the susceptible plants to be able to
exclude the entrance of these injurious substances into the protoplasm by
"selective adsorption," or to convert the injurious substances into
insoluble and non-injurious forms, as is done by other plants which are not
susceptible to injury by these "specific" poisons. Hence, particular
attention need not be given to this type of toxins.

Toxic substances may act injuriously upon plant tissues in a variety of
ways. Many electrolytes, especially the salts of the heavy metals of high
valency, coagulate protein material and the entrance of such substances
into the protoplasm causes disturbances in the colloidal condition which
cannot be otherwise than injurious to its normal activities. Similarly,
formaldehyde and many other organic compounds may affect the colloidal
properties of the protoplasmic gel in such a way as to injure the plant
tissues.

The same substance is sometimes much more injurious to the tissues of one
part of a plant than it is to those of another part of the same plant.
Thus, the rootlets of a young growing plant are much more susceptible to
injury by many mineral salts than are the vegetative parts of the same
plants; while anaesthetics of various kinds generally exhibit their greatest
injurious effects upon the leaves, or synthetizing cells. Again, the
mycelia of fungi are much more easily killed by toxic agents used as
fungicides than are the spores of the same fungi. Some of these observed
differences in toxicity may be due to differences