Altogether, after looking at the question from many points of view, one is forced to the conclusion that under most typical circumstances copper compounds act as poisons to the higher plants, and that it is only under particular and peculiar conditions and in very great dilutions that any stimulative action on their part can be clearly demonstrated.
CHAPTER IV
EFFECT OF ZINC COMPOUNDS
I. Presence of Zinc in Plants.
The presence of zinc in the ash of certain plants has been recognised for many years, especially in so far as the vegetation of soils containing much zinc is concerned. [Risse], before 1865, stated that most plants when grown on such soils prove to contain greater or less quantities of zinc oxide. He states that the soil at Altenberg, near Aachen, is very rich in zinc, which rises as high as 20% in places. The flora of the soil is very diversified and zinc has been determined qualitatively in most and quantitatively in some of the plants. Viola tricolor and Thlaspi alpestre are most characteristic under such circumstances, both showing such constant habit changes that they resemble new species, while other plants such as Armeria vulgaris and Silene inflata are peculiarly luxuriant. Risse’s figures of the zinc content of these four plants may prove of interest. The figures are based on the dry weights, air dried.
| Thlaspi alpestre, var. calaminaria. | |||||||||
| Root | 6·28% ash, | 0·167% ZnO, | 1·66% ZnO in ash. | ||||||
| Stem | 11·75% | „ | 0·385% | „ | 3·28% | „ | „ | ||
| Leaves | 11·45% | „ | 1·50% | „ | 13·12% | „ | „ | ||
| Flowers | 8·49% | „ | 0·275% | „ | 3·24% | „ | „ | ||
| Viola tricolor. | |||||||||
| Root | 5·59% ash, | 0·085% ZnO, | 1·52% ZnO in ash. | ||||||
| Stem | 10·55% | „ | 0·065% | „ | 0·62% | „ | „ | ||
| Leaves | 9·42% | „ | 0·110% | „ | 1·16% | „ | „ | ||
| Flowers | 7·66% | „ | 0·075% | „ | 0·98% | „ | „ | ||
| Armeria vulgaris. | |||||||||
| Root | 4·74% ash, | 0·17% ZnO, | 3·58% ZnO in ash. | ||||||
| Stem | 5·37% | „ | 0·02% | „ | 0·37% | „ | „ | ||
| Leaves | 9·36% | „ | 0·11% | „ | 1·17% | „ | „ | ||
| Flowers | 6·08% | „ | 0·07% | „ | 1·15% | „ | „ | ||
| Silene inflata. | |||||||||
| Root | 2·71% ash, | 0·02% ZnO, | 0·74% ZnO in ash. | ||||||
| Stem | ⎫ | ||||||||
| Leaves | ⎬ | 11·43% | „ | 0·22% | „ | 1·92% | „ | „ | |
| Flowers | ⎭ | ||||||||
[Freytag (1868)] carried out various experiments on the influence of zinc oxide and its compounds on vegetation, and found that all plants are capable of absorbing zinc oxide by their roots when grown on soils containing such oxide. Generally speaking the zinc is deposited chiefly in the leaves and stems, very little being found in the seeds, such minute traces occurring that he stated that the seeds must be harmless for men and animals. The general content of ZnO in plants is given as ·5–1·0% of ash, except in the abnormal case of plants growing on calamine.
[Lechartier and Bellamy (1877)] demonstrated the presence of zinc in such food substances as wheat, American maize, barley and white haricots, but they failed to find it in maize stems and beetroot, so they cautiously concluded that if it does occur in the latter cases it must be far less in quantity than in the former. [Hattensaur (1891)] analysed the ash of Molinia cærulea and discovered the presence of copper, manganese, zinc and lead, zinc oxide forming ·265% of the total ash, (·006% of the air dried plant).
[Jensch (1894)] observed that the flora on calamine soils was somewhat scanty, the chief plants that came under his notice being Taraxacum officinale, Capsella Bursa-pastoris, Plantago lanceolata, Tussilago Farfara, and Polygonum aviculare, all of which showed certain morphological peculiarities. Generally speaking the growth of these plants on the calamine soils was weak and poor, the stems and leaves being very brittle. Jensch found that the roots were deformed and showed a tendency towards a plate-like superficial spread of root. The leaves of Tussilago were uneven in shape and lacked the white hairs on the under side, the flower stalks were twisted, while the flowers themselves were a deep saturated yellow colour. The stems of Polygonum aviculare were much thickened at the nodes, the leaves weak and rolled in character, while the flowers were long-stalked, the calyces being usually of a purple red colour. The following figures are given for the quantities of zinc carbonate (ZnCO3) in the ash of these two plants:—
| Tussilago Farfara. | ||
| Root | Leaf-stalk | Leaf-blade |
| 2·51%–3·26% | 1·75%–1·63% | 2·90%–2·83% ZnCO3 |
| = 1·629%–2·115% | 1·136%–1·058% | 1·882%–1·836% ZnO. |
| Polygonum aviculare. | ||
| Root | Stem | Leaves |
| 1·77%–1·93% | 2·25%–2·86% | 1·24%–1·49% ZnCO3 |
| = 1·148%–1·252% | 1·46%–1·856% | ·804%–·967% ZnO. |