[Treboux (1903)] demonstrated the harmful action of solutions of copper salts on leaves by means of experiments on shoots of Elodea canadensis. The activity of photo-synthesis was measured by the rate of emission of bubbles of oxygen. On placing the shoots first in water, then in N/1,000,000 copper sulphate (·0000159%), there was a reduction from 20 to 15 or 16 bubbles in 5 minutes. On replacing in water there was an increase to 18, but not to 20, indicating a permanent injury. With N/10,000,000 copper sulphate there was little or no reduction in the number of bubbles. This experiment had an interesting side issue in that it was noticed that not only the concentration, but also the quantity of fluid was concerned in the toxic action, indicating that both the proportion and the actual amount of poison available play their part. For instance, with a shoot 10 cm. long in 100 c.c. solution the plants were only slightly affected by ·000015% copper sulphate, but in 500 c.c. solution the shoots were killed after some days in ·0000015% copper sulphate, a concentration only one-tenth as great.
While it is evident that copper sprays have a definite action upon green leaves, whether favourable or unfavourable, the question arises as to the means whereby the copper obtains access to the plant in order to take effect. [Dandeno] found that solutions of copper sulphate were absorbed by the leaves of Ampelopsis, forming a brown ring. Generally speaking inorganic salts in solution are absorbed through both surfaces of the leaves, whether the leaves are detached or not, provided the surrounding atmospheric conditions are favourable, the absorption being usually more ready through the lower surface. Dilute solutions applied in drops stimulate the leaf tissue in a ring, whereas if the solutions are concentrated the entire area covered by the drop is affected. Too concentrated solutions of copper sulphate applied to leaves caused scorching, but if this was avoided while the solution was still strong enough to cause a darkening of green colour after a time, Dandeno considered that the action was probably of the nature of a stimulus to growth, and produced a better development of chlorophyll and protoplasm in the region where the tissues appeared dark to the naked eye, a conclusion which tallies very closely with that of Frank and Krüger.
[Amos (1907–8)] experimented to see whether the application of Bordeaux mixture affected the assimilation of carbon dioxide by the leaves of plants, and whether any stimulation was produced. Brown and Escombe’s methods and apparatus were used and the summarised results indicate that the application of Bordeaux mixture to the leaves of plants diminishes the assimilation of carbon dioxide by those leaves for a time. The effect gradually passes off, whatever the age of the leaves may be. The suggestion is made that the stomata are blocked by the Bordeaux mixture, so that less air diffuses into the intercellular spaces and less carbon dioxide comes into contact with the absorptive surfaces. If this hypothesis is correct, the physiological slackening of assimilation is not due to the toxic action of the copper in the Bordeaux mixture, but to a mechanical hindrance due to blocking of the stomata.
III. Effect of Copper on Certain of the Lower Plants.
On turning to the lower plants, especially to some species of fungi, one notices a striking contrast in their behaviour to that of the higher plants. Some species of fungi have the power of living and flourishing in the presence of relatively large quantities of copper compounds, or even of copper or bronze in the solid state. [Dubois (1890)] found that concentrated solutions of copper sulphate, neutralised by ammonia, which were used for the immersion of gelatine plates used in photography, showed white flocculent masses resembling the mycelium of Penicillium and Aspergillus, which grew rapidly and fructified in Raulin’s solution, but which remained as mycelium in cupric solutions. The mould proved capable of transforming copper sulphate into malachite in the presence of a piece of bronze, but it was found that the presence of the latter was not essential for the conversion into basic carbonate. The same result was obtained if the culture liquid was put in contact with a body which prevented it from becoming acid, fragments of marble acting in this way. Copper sulphate solution in the presence of the mould produced a green deposit on the marble, while without the fungus the solution simply evaporated leaving a blue stain of copper sulphate.
[Trabut (1895)] found that on treating smutty wheat with a 2% solution of copper sulphate he obtained a mass of flocculent white mycelium, whose surface was soon covered with aerial branches bearing pale rose-coloured spores, and he gave the provisional name of Penicillium cupricum to the species. On preparing nutritive solutions by steeping a handful of wheat in water for 24 hours, and then adding various amounts of copper sulphate to them, Penicillium was found to vegetate quite well until the amount of copper sulphate reached 91⁄2 grams in 100 c.c., after which the seedings with spores did not develope at all. [De Seynes] tested this Penicillium more exhaustively with different culture media under various conditions and decided that Trabut was right in only assigning the name P. cupricum provisionally, as the mould reverts to the form P. glaucum when seeded in a natural medium, indicating that P. cupricum has not an autonomous existence, but is P. glaucum which modifies the colour of its conidia under the influence of copper sulphate, in the same way that it often modifies them in other media. It is noticeable that the mycelium arising from the germination of conidia of P. cupricum in a normal medium has a very poor capacity for producing reproductive organs, but this diminished activity is attributed not to a special deleterious action of the copper sulphate but to the impulse given to the vegetative functions, at the expense of the reproductive, when the spores are seeded in a richer medium than the solutions of copper sulphate which serve as the soil for P. cupricum.
[Ono] found that Aspergillus and Penicillium are retarded in growth in the higher concentrations of copper sulphate, but that they are stimulated by weaker strengths. The range of stimulating concentrations is given as from ·0015%–·012%, the biggest crop being obtained with both moulds in the strongest of these solutions. [Hattori] gives the optimum as being considerably lower for the two fungi mentioned, Penicillium being at its best in a solution of ·008% and Aspergillus in ·004%. [A. Richter (1901)] opposes this absolutely so far as Aspergillus niger is concerned. In his experiments copper appears invariably as a depressant, all concentrations from 1/150 to 1/150,000,000 giving growth below the normal, no stimulative action ever being observed. Zinc however proved to be a definite stimulant and in a mixture of copper and zinc salts in appropriate concentrations the toxic effect of the copper was completely paralysed by the stimulating action of the zinc, 1/200,000 zinc salt paralysing or overcoming the copper salt at 1/1125.
[Ono] states that the optimal quantity of such poisons as copper salts is lower for algae than for fungi, copper failing to stimulate algae at dilutions which were the most favourable to the growth of fungi. [Bokorny] indicates that silver and copper salts work harm in unusually dilute solutions.
Attempts have been made to utilise the poisonous action of copper on algae in clearing ponds of those plants. [Lindsay (1913)] describes experiments carried on in a reservoir infested with Spirogyra. A quantity of copper sulphate sufficient to make a solution of 1/50,000,000 was found necessary to kill off the Spirogyra, but it is suggested that the solution was probably weaker before it reached the algae, owing to the currents of fresh water. Anaboena needed 1/10,000,000 before it was killed off, while Oscillatoria is less sensitive still, 1/5,000,000 usually representing the mortal dose, though 1/4,000,000 was necessary in some instances. Algae seem to be peculiarly sensitive to the copper sulphate, far more so than the higher plants, as Nuphar lutea, Menyanthes trifoliata, and Polygonum amphibium grew in the water unharmed by the addition of the poisonous substance. For some unexplained reason it seems that “the concentration of copper sulphate necessary to kill off the algae in the laboratory is five to twenty times as great as that needed to destroy the same species in its natural habitat.”