[Miyajima (1897)] showed that the germinating power of such seeds as Vicia Faba, Pisum sativum, and Zea Mays was partly destroyed by a 1% solution of copper[4], Zea Mays being the most resistant and Vicia Faba the least resistant of the three. [Micheels (1904–5)] stated that water distilled in a tinned copper vessel was more favourable for germination than water from a non-tinned vessel. He suggests that this is due to copper being present in the water in a colloidal form in which the particles are exceedingly small and maintain themselves in the liquid by reason of a uniform disengagement of energy in all directions, to which energy the influence on germinating seeds must be attributed, the nature of the suspended substance determining whether the influence be favourable or not. It is questionable, however, whether Micheels was really dealing with a true colloidal solution of copper or with a dilute solution of some copper salt produced by oxidation of the copper vessel from which his distilled water was obtained.

(b) Spores and pollen grains.

[Miani (1901)] brought fresh ideas to bear upon the problem of the action of copper on living plant cells, in that he sought to attribute the toxic or stimulant effects to an oligodynamic action, i.e. spores and pollen grains were grown in hanging drop cultures in pure glass distilled water with the addition of certain salts or traces of certain metals. While the salts are known to be often disadvantageous to germination, [Nägeli] had asserted that the latter often exerted an oligodynamic action. In some cases pure copper was placed for varying times in the water from which the hanging drop cultures were eventually made, or tiny bits of copper were placed in the drop itself. Various kinds of pollen grains were tested, and as a rule, pollen was only taken from one anther in each experiment, though occasionally it was from several anthers of the same flower. It was generally found that the germination of pollen grains or Ustilago spores was not hindered by the use of coppered water or by the presence of small bits of copper in the culture solution. The only cases in which some spores or pollen grains were more or less harmed were those in which the water had stood over copper for more than two weeks, and even so the deleterious effect was chiefly noticeable when the pollen itself was old or derived from flowers in which the anther formation was nearly at an end. As a rule germination was better in the presence of copper, whether in pure water or food solution, the stimulus being indicated both by the greater number of germinated grains and by the regular and rapid growth of the pollen tubes. Miani attributes this favourable action to the mere presence of the copper, corroborating Nägeli’s idea of an oligodynamic action.

3. Does copper stimulate higher plants?

From the foregoing review it is evident that it is the toxic action of copper that is most to the front, so far as the higher plants are concerned, and that little or no evidence of its stimulative action in great dilution has so far been discussed. [Kanda] dealt with this question, with the deliberate intention of obtaining such evidence, if it existed. He worked with Pisum sativum, var. arvense, Pisum arvense, Vicia Faba, var. equine Pers, and Fagopyrum esculentum Mönch, which were grown in glass distilled water, without any food salts, so that the plants were forced to live on the reserves in the seeds, which were carefully graded to ensure uniformity of size. It was found that in water cultures copper sulphate solutions down to ·00000249% (about 1 in 40,160,000) are harmful to peas, and still further down to ·0000000249% (about 1 in 4,016,000,000) the copper salts act as a poison rather than as a stimulant. Against this, however, is the statement that in certain soils copper sulphate acts as a stimulant when it is added in solution. [Jensen] again could obtain no stimulation with copper sulphate.

The Rothamsted experiments go to uphold Kanda’s statements as to the failure of copper sulphate to stimulate plants grown in water cultures. Peas are perhaps slightly more resistant to the greater strengths of copper sulphate than are barley and buckwheat, for while 1/100,000 proves mortal to the latter, peas will struggle on and fruit in 1/50,000, though this strength is very near the limit beyond which no growth can occur ([Fig. 4]). As a general rule, with barley the depression caused by the poison is still evident with 1/5,000,000 and 1/10,000,000, though occasionally these doses act as indifferent doses, no sign of stimulation appearing in any single instance. With peas again, even 1/20,000,000 copper sulphate is poisonous, although to the eye there is little to choose between the control plants and those receiving poison up to a concentration of one part in 21⁄2 million ([Fig. 5]). In the case of buckwheat the matter is still undecided, as in some experiments apparent stimulation is obtained with 1 in 21⁄2 or 1 in 5 million copper sulphate, while in others a consistent depression is evident, even when the dilution is carried considerably below this limit. The reason for the variation with this particular plant is so far unexplained.

Fig. 4. Photograph showing the action of copper sulphate on pea plants in the presence of nutrient salts. (Oct. 3rd–Dec. 20th, 1912.)

1. Control.
2.1/50,000coppersulphate.
3.1/100,000„„
4.1/250,000„„
5.1/500,000„„
6.1/1,000,000„„
7.1/2,500,000„„
8.1/5,000,000„„
9.1/10,000,000„„
10.1/20,000,000„„