rhaps, turnips and some other root crops. Whether the
benefit which these crops derive from the application of common salt to the
soil in which they are growing is due to the direct food value of either
the chlorine, or the sodium, or to some indirect effect, is not yet known.
The presence of chlorine in the sap of plants is undoubtedly due to the
inevitable absorption of soluble chlorides from the soil and apparently has
no connection with the nutritional needs of the plant.

=Silicon= is always considered as a non-essential element, although it
occurs in such large proportions in some plants as to indicate that it
cannot be wholly useless. It accumulates in the stems of plants, chiefly in
the cell-wall, and has sometimes been supposed to aid in giving stiffness
to the stems. But large numbers of analyses have failed to show any direct
correlation between the stiffness of straw of cereal plants and the
percentage of silicon which they contain. Further, plants will grow to full
maturity and with erect stems when no silicon is present in the mineral
nutrients which are furnished to them. On the other hand, certain
experiments appear to indicate that silicon can perform some of the
functions of phosphorus, if soluble silicates are supplied to
phosphorus-starved plants. But under normal conditions of plant nutrition,
it seems to have no such function.

INORGANIC PLANT TOXINS AND STIMULANTS

Much study has been given during recent years to the question of the
supposed poisonous, or toxic, effects upon plants of various soil
constituents. There seems to be no doubt that certain _organic_ compounds
which are injurious to plant life are often present in the soil, either as
the normal excretions of plant roots or as products of the decomposition of
preceding plant growths. A consideration of these supposedly toxic organic
substances would be out of place in this discussion of mineral soil
nutrients. But there seems to be no doubt that there may also be mineral
substances in the soil which may sometimes exert deleterious influences
upon plant growth. In fact, most metallic salts, except those of the few
metals which are required for plant nutrition, appear to be toxic to
plants. The exact nature of the physiological effects which are produced by
these mineral toxins is not clearly understood; indeed, it is probably
different in the case of different metals. Further, it is certain that both
the stimulating a