I. Presence of Copper in Plants.

Copper has been recognised as a normal constituent of certain plants for at least a century, so much so that in 1816 [Meissner] brought out a paper dealing solely with the copper content of various plant ashes. The ash of Cardamomum minus, of the root of Curcuma longa, and of “Paradieskörner[3],” amongst others, were tested and all yielded copper in very small quantity. Meissner was led to conclude that copper is widespread in the vegetable kingdom, but that it exists in such minute traces that its determination in plants is exceedingly difficult. In 1821 [Phillips] made an interesting observation as to the effect of copper on vegetation. Some oxide of copper was accidentally put near the roots of a young poplar, and soon after the plant began to fail. The lower branches died off first, but the harm gradually spread to the topmost leaves. As a proof that copper had been absorbed by the plant the record tells that the blade of a knife with which a branch was severed was covered with a film of copper where it had been through the branch, and the death of the plant was attributed to the absorbed copper.

After this preliminary breaking of the ground little more seems to have been done for some sixty years, but from about 1880 till the present day the association of copper with the vegetable kingdom has been actively investigated in its many aspects. [Dieulafait (1880)] showed that the quantity of copper present in the vegetation is largely determined by the nature of the soil, which thus affects the ease with which the element can be detected and estimated. Copper was shown to exist in all plants which grow on soils of “primary origin” (“roches de la formation primordiale”), the proportion being sufficient to enable it to be recognised with certainty in one gram of ash, even by means of the ammonia reaction. Samples of white oak from the clay soils, and plants from the dolomitic horizons also gave evidence of copper in one gram of ash, though less was present than in the first case considered, but with plants grown on relatively pure chalk 100 grams of ash had to be examined before copper could be recognised with certainty.

[E. O. von Lippman] found traces of copper in beets, beet leaves, and beet products; [Passerini] estimated as much as ·082% copper in the stem of chickpea plants, though he regarded this figure as too high; [Hattensaur] determined ·266% CuO in the total ash of Molinia cærulea (·006% of total plant, air-dried).

After this Lehmann ([1895], [1896]) carried out more exhaustive studies on the subject of detecting and estimating the copper in various articles of food: wheat, rye, barley, oats, maize, buckwheat, and also in various makes of bread; potatoes, beans, linseed, salads, apricots and pears; cocoa and chocolate. He found that only in those plants which are grown on soil rich in copper does the copper reach any considerable value, a value which lies far above the quantity present in an ordinary soil. Plants from the former soils contained as much as 83–560 mg. Cu in 1 kilog. dry substance, whereas ordinarily the plants only contained from a trace to 20 mg. Apparently the species of the plants concerned seems to be of less importance for their copper content than is the copper content of the soil. The deposition of copper (in wheat, buckwheat and paprika) is chiefly in the stems and leaves, little being conveyed to the fruits and seeds, so that a high content of copper in the soil does not necessarily imply the presence of much copper in the grain and seed. The metal is variously distributed among the tissues, the bark of the wood being the richest of the aerial parts in that substance. The form in which the copper exists in the plant is uncertain and it is suggested that an albuminous copper compound possibly exists.

[Vedrödi (1893)] tackled the problem at about the same time as Lehmann but from a rather different standpoint. He ratifies the statement as to the absorption of copper by plants, and going still further he states that in some cases the percentage of copper found in the seed may be four times as great as that occurring in the soil on which the plants grow, quoting one instance in which the soil contained ·051% CuO and the seed ·26% CuO. It is assumed that copper must play some physiological rôle in the plant, but no explanation of this action is yet forthcoming. Lehmann criticised Vedrödi’s figures of the copper content of certain plant ashes, and the latter replied in a further paper ([1896]) in which he brings most interesting facts to light. The quantity of copper in any species of plant varies with the individuals of that species, even when grown on the same soil, in the same year, and under similar conditions. The copper content of certain plants is put forward as a table, the years 1894 and 1895 being compared, and enormous differences are to be noticed in some cases. A quotation of the table will illustrate this more clearly than any amount of explanation.

Milligrams of copper in 1 kilog. dry matter.

1894 1895
/ \ / \
“Seeds” min. max. min. max.
Winter wheat80710200680
Summer wheat190630190230
Maize60901030
Barley801201070
Oats4019040200
Buckwheat160640150160
“Fisolen” (Beans)160320110150
Linseed120150110150
Peas6010060110
Soy Beans701007080
Lupins8019070290
Mustard seed701306070
Paprika pods7901350230400