Wheat is evidently peculiarly sensitive to the effects of zinc compounds under differing conditions. [Javillier (1908 c)] pointed out that while wheat is very susceptible to the toxic action of zinc, yet it can benefit by the presence of sufficiently small quantities of the compounds of the metal. Rice is another cereal that is said to respond to the action of zinc sulphate, as [Roxas], working in pot cultures with soil both with and without the addition of nutritive salts, obtained an acceleration of growth on the addition of m/1000 zinc sulphate, a quantity so remarkably great that it might be expected to act as a toxic rather than as a stimulant.
With phanerogams the zinc question is not only concerned with the effect of the metal upon germination, but also with its effect upon the later growth of the green plants, and on the physiological functions involving the construction of substances at the expense of mineral elements and the carbon dioxide of the air. Javillier holds that the indications are that zinc would prove to be profitable if applied to crops as a “complementary” manure.
4. Direct action of zinc salts on leaves.
[Dandeno (1900)] applied zinc sulphate in drops to the leaves of Ampelopsis, and found that the solution was not all absorbed by the leaf, but that a slight dark ring of a yellow colour was produced, and he was induced to think that some local stimulation was produced if the salt was presented in sufficient dilution.
[Klopsch (1908)] discussed the effect on plant growth of zinc derived from industries producing zinc fumes. Zinc oxide from the fumes is deposited on the leaves, and Klopsch stated that the rain and dew containing dissolved zinc compounds find entrance to the tissues by way of the stomates and work injury to the plants. Against this, however, it must be remembered that these same fumes also contain other substances which are admittedly harmful to plant life, and so the deleterious effect may be partly or even chiefly due to these substances rather than to the zinc. Yet it is probable that at least some of the depreciation is due to the zinc. [Treboux (1903)] tested the effect of zinc sulphate on shoots of Elodea canadensis. If the shoots were placed in n/100,000 (= ·000016%) zinc sulphate no reduction of assimilation (as observed by counting the number of oxygen bubbles emitted per minute) took place, and replacement in water apparently had no effect either way. When however the shoots were placed in (1) water, (2) ·00008% zinc sulphate, (3) fresh ·00008% zinc sulphate, (4) water again, it was found that while the first solution of zinc sulphate had apparently no effect on assimilation, yet during the second immersion a gradual reduction in assimilation set in, which reduction was continued after the return to pure water, so that the toxic action of the zinc sulphate upon the shoots was clearly demonstrated.
III. Effect of Zinc on Certain of the Lower Plants.
Among the fungi, one species stands out in special prominence on account of the great amount of work that has been done on it with regard to its reactions to zinc salts. Aspergillus niger = Sterigmatocystis nigra van Tgh was used as a test plant by [Raulin (1869)], who evidently considered that zinc was an essential primary constituent of the food solutions of the fungi, ·07 parts zinc sulphate being added to each 1500 parts of water. In his experiments he tested (1) ordinary nutritive solution, (2) nutritive solution with various salts added, as zinc sulphate, (3) nutritive solution and salts (as 2) and also powdered porcelain. (2) gave a crop of Aspergillus about 3·1–3·5 times better than (1), while (3) was even better still. Sulphate of iron also proved stimulating in its action, but Raulin stated that zinc cannot replace iron, as both are essential.
[Ono (1900)] determined the relation between the weight of the mould crop in grams and the quantity of sugar used up in the presence of varying amounts of zinc sulphate. The amount of sugar used was always greater in the crops with ·0037–·0297% zinc sulphate by weight than in the control crops, indicating a stimulation caused by zinc.
[Richter (1901)] carried out rather similar experiments. When grown in solutions without and with 1/700,000 gram molecule zinc sulphate the dry weights of the mould were practically the same for the first two days, then the dry weight of the zinc crop shot ahead for a day or two, a depression setting in on the fifth day. Without zinc a less increase took place, and a similar drop was noticeable about the sixth day. The conclusion drawn is that the stimulation due to the zinc occurs chiefly in the first few days and also that the rise in the sugar consumed is more rapid at first with the moulds treated with zinc. Concentrations above 1/600 are harmful, but in weaker solutions zinc is a definite stimulant.
[Coupin (1903)] re-investigated some of Raulin’s work under more antiseptic conditions in order to see what substances were really needed by the mould and whether certain elements declared essential were really so. He concluded that iron and zinc are of no use in the nutrition of Sterigmatocystis nigra, but that the zinc retards the development of mycelium when food is abundant, killing it if it is badly nourished. This denial of stimulation was controverted by [Javillier (1907)] who re-tested Raulin’s solution with extreme care, growing Sterigmatocystis in