Inorganic Plant Poisons and Stimulants

Tests with white lupins gave no conclusive results, as for some reason it proved very difficult to get satisfactory plants in water cultures. When they are grown under such conditions the roots always tend to get more or less diseased and covered with slime, probably fungal in nature. In the presence of much boric acid the roots remain in a much healthier condition, which suggests that the acid has in this case a strong antiseptic action, and protects the roots. With high concentrations the lower leaves of the plant are badly affected, just as with peas and barley, turning brown and withering at an early date. Various experiments have been made with yellow lupins, but these again are very difficult to grow well in water cultures, as they are apt to drop their leaves for no apparent reason. Generally speaking, the evidence goes to prove that boric acid is toxic down to a concentration of about 500 parts in 25 million. It is difficult to get a true control with which to make comparisons as the plants without boric acid are encumbered with the slime on their roots, which naturally interferes with normal growth, while the plants in the presence of boric acid have the unfair advantage due to the probable antiseptic action of the boron. The effect of the boron poisoning is again evident in the dying off of the lower leaves, which become flaccid and drooping and finally drop off. The lupins grown with boron are very active in the putting forth of lateral roots, so much so that the cortex of the roots is split along the line of emergence of the laterals, which are very numerous and crowded.

(b) Toxic action of boron compounds in sand cultures.

Agulhon (1910 a) moistened 2 kgm. pure sand with 500 c.c. nutritive solution for each pot, and boron was added at the rate of 0, 0·1, 1, 10, and 50 mg. boric acid per litre of nutritive solution. Twenty wheat seeds were sown in each pot, and after twelve days the healthy plants in the first four pots were 6–8 cm. high, but those with the maximum amount of boron showed yellowish leaves only 3 cm. long. After three months’ growth the plants were harvested, when those with most boron were found to have died after making about 10 cm. growth. The toxic doses in sand proved to be weaker than those in water cultures, probably because evaporation from the surface of the sand caused concentration of the poisonous liquid.

(c) Toxic action of boron compounds in soil experiments.

Long before any experimental work was done with boron in water cultures, the poisonous properties of the substance were recognised with regard to plants growing in soil. Peligot (1876) grew haricots in porous earthenware pots, the plants being watered by rain and by solutions, each containing about 2 grams per litre of such substances as borax, borate of potassium, and boric acid, other pots receiving various fertilisers, as potassium nitrate, sodium nitrate, &c. This quantity of boron completely killed off the plants receiving it, whether it was applied as free or combined boric acid, while the fertilised plants completed their development well. On this account the deleterious action was attributed to the boric acid and not to the sodium or potassium base supplied. Peligot hinted at the improbability of a substance like boron, which is so poisonous to plants, being really innocuous to human beings when it is used as a preservative for foods.

Nakamura (1903) also found that borax is harmful in pot cultures if present in large quantities, 50 mg. borax per kgm. of soil exerting a very injurious influence, while even 10 mg. per kgm. did some damage. Agulhon (1910 c) found that the toxic doses of boric acid in soil cultures approached those in nutritive solutions rather than in sand cultures, a phenomenon that he attributed to the fact that the boric acid was fixed by the soil, probably as insoluble borate of calcium, so that the surface concentration obtained with sand cultures was avoided. He found that the ash of plants grown with excess of boron contained more than the normal amount of boron, while the weight of ash per 100 dry matter was also increased. He concluded that the plant thus suffers an over-mineralisation and in consequence an augmentation of its hold on water, so that the fresh weight of the plant may indicate a more favourable action of the boric acid than does the dry weight. Other investigators (Fliche and Grandeau 1874) had found the same increase in the proportion of ash in chestnut trees grown on too calcareous soil, so Agulhon concluded that one is here dealing with a general reaction of plants to an excess of a useful element.

Other experiments were carried on in the open field, maize being grown on control plots and on plots receiving 2 gm. boron per square metre. At first the latter plants were behind, the dose being too strong. Eventually, however, they pulled up level and the dry weights from the two plots proved to be nearly the same, the fresh weights being identical. Maize is evidently far less sensitive to boron poisoning than are peas and oats, for with these one-half the original amount of boron (= 1 gm. per sq. metre) proved toxic.

Interesting results were obtained (Agulhon 1910 a) by repeated experiments with the same soil containing boron. It was found that sand or soil containing a proportion of boron which is lethal or toxic to a first culture will allow much better growth with a second and subsequent crops. Repeated experiments on the same soil may show the change from a lethal dose to a toxic one, thence to an indifferent and finally to an optimum concentration. Furthermore (Agulhon 1910 b) the very plants may accustom themselves to greater quantities of boron, the increased power of resistance being transmitted. He concluded from his experiments that the progeny of the second generation of maize were able to withstand quantities of boron that were toxic to control plants[13]. Agulhon once again emphasised the fact that for toxic doses of boron the first symptom is the more or less marked disappearance of chlorophyll, though the aerial parts are not affected so soon as the roots.

2. Effect of boron compounds on germination.

One of the first indications that boron compounds affect the germination of seeds was given by Heckel (1875) who found that germination was retarded for 1–3 days by weak solutions of borates (·25 gm. to 20 gm. water), and was stopped altogether by stronger solutions (·60 gm. to 20 gm. water). Archangeli (1885) tested the germination of a variety of seeds of Leguminosae, Gramineae, and of Cannabis, Iberis, Raphanus, Collinsia, and Linum in the presence of boric acid. The seeds were placed in bowls with solutions of ·25, ·5, and 1% boric acid at temperatures ranging from 16°–23° C. The bowls were covered with glass plates to prevent evaporation and consequent increase of concentration, controls in spring water being dealt with under similar conditions. 1% boric acid was found to check germination altogether, and the weaker the concentration the less was the process hindered. Morel soaked seeds of haricots and wheat in various solutions of boric acid, and found that germination was generally hindered or inhibited. The deleterious action diminishes as the strength of the solution or the time of contact diminishes, but solutions of the same concentration do not act equally on all seeds. Boric acid and borax proved to be similar in their action qualitatively.

The deleterious effect of strong doses of boric acid on germination was confirmed by Agulhon (1910 a), the higher quantities (above 10 mg. boric acid per litre) retarding germination of wheat.

3. Does boron stimulate higher plants?

Of recent years a few investigators have thrown out hints as to the stimulant action exerted by boron compounds on plants. Roxas indicated that M/100,000 (M = molecular weight) of boric acid exercised a favourable action on rice. Nakamura (1903) tested the point by means of pot cultures. Peas and spinach plants were grown in soil which received 1 and 5 mg. borax per kgm. With peas the 1 mg. exerted evident stimulant action, as determined by the increase in height of the shoot over that of the control, 5 mg. seeming to be slightly depressant in action. With spinach a stimulation was observed both in weight and height with a dose of 5 mg. borax per kgm.

Agulhon (1910 c and d) took the matter up still more definitely and made many tests of various kinds, in water, sand and pot cultures.

(a) Water cultures.

His water cultures were made under sterile conditions, the seeds when possible being sterilised with corrosive sublimate, the germinating apparatus being also sterilised. With wheat a stimulant action was evident, maximum growth being obtained with between 2·5 and 10 mg. boric acid per litre, though the dry weight increase did not quite keep pace with that of the fresh weight, a fact to which previous reference has been made. The chief improvement is in the root, the stem/root ratio falling to 5, as against 6 in the control series. Visual observation indicated that the roots of plants receiving 5–10 mg. boric acid per litre are longer than the others, though they are less rich in adventitious roots. The increased dry weight due to boron may amount to as much as 30%.

(b) Sand cultures.

Agulhon again observed stimulation in this case. 2 kgm. of sand were moistened with 500 c.c. nutritive solution, varying quantities of boric acid being added in addition. ·1 mg. boric acid per litre of N.S. (·05 mg. per pot) gave an increase of 25% fresh weight, and 7·5% dry weight. The stimulating doses seem to be weaker than in the experiments with liquid media, probably because the evaporation from the sand increases the concentration of the boric acid at the surface. It was also noticed that the increase of weight varied in experiments made at different times. With oats the stimulating influence is greater than with wheat, showing that some plants are more sensitive than others to the influence of boron. With radish 1 mg. boric acid per litre exercised a stimulating effect, the enormous average increase of 61% in fresh weight occurring with this strength, though this only represented an average increase of 9·6% dry weight.

(c) Soil cultures.

Here again the stimulating action was evident with higher concentrations than in sand cultures, and Agulhon obtained good results with strengths that are toxic in sand. The evaporation from earth is not so rapid as from sand, so that the concentration is not increased, and also some of the boric acid is withdrawn from the solution by interaction with the soil, so that the stimulating concentration rises in the scale.

In field experiments Agulhon found that peas were more sensitive to the toxic action of boric acid than is maize. A strength of boric acid (= 1 gm. B per sq. metre) that poisoned peas, gave an increase of 61% fresh weight and 39% dry weight with maize; half the strength proved to be indifferent for peas, the improvement with maize equalling 56% increase fresh and 50% increase dry. Curiously enough, judging by appearances in the first experiment, an unfavourable influence was at work, though in reality a great stimulation was being caused. Colza gave a good increase with similar strengths, but with turnips 1 gm. B per sq. metre only favoured the aerial parts, while ·5 gm. B per sq. metre only increased root development. Agulhon concluded that it is as yet impossible to determine with any precision the exact part that boron plays in the plant economy. He suggests that boron is a “particulier” element characteristic of a certain group of individuals or of life under particular conditions. In his summary he argues that each series of individuals adapted to different environments has doubtless need of particular elements, and that perhaps chemical causes and morphological differences are very closely connected. Boron may be of this “particulier élément” type in the higher plants of the vegetable kingdom, and it may be useful commercially as a manurial agent, the “catalytic manure” of Bertrand and Agulhon.

While the higher concentrations of boric acid proved definitely toxic to both peas and barley in the Rothamsted water cultures, some evidence of stimulation was obtained with the lower strengths. With barley the question of stimulation is still an open one, as below the toxic limit growth seems fairly level in most of the experimental series. The lower limit of toxicity varies from 40–4 parts boric acid per 10,000,000 according to circumstances. Below this critical concentration the boric acid has apparently no action, either depressant or stimulant, unless the stimulation should prove to begin at a dilution of 1/50,000,000, but the evidence on this point is not sufficiently well marked or consistent to be conclusive. This failure to detect stimulation was somewhat unexpected, as when judged by the eye the plants treated with the lower concentrations of boric acid seemed better than the controls, and also exhibited a particularly healthy green colouration.

Peas on the other hand are definitely stimulated with traces of boric acid, concentrations of 1/100,000 and less causing an improvement in growth, while under some experimental conditions even higher amounts of boric acid were beneficial. All the stimulated plants showed the characteristic dark green colour which seems to be associated with the presence of minute traces of boron in the nutritive solution. An interesting morphological feature was the strong development of small side shoots from the base of the plants in the presence of medium amounts of boric acid, from 1 part in 100,000 downwards. This gave rise to a certain bushiness of growth, which was less evident as the concentration of the stimulant decreased. The general outcome of the tests seems to be that boric acid needs to be supplied in relatively great strength to be fatal to pea plants, and that the toxic action gives place to a stimulative one high up in the scale of concentration. As far as experiments have already gone it seems as though the stimulation is not a progressive one, as the effect of 1/100,000 boric acid is as good as that of 1/20,000,000, a flat curve connecting the two. This, however, needs confirmation.

Yellow lupins also give some evidence of stimulation with concentrations of about 1/50,000 boric acid, the improvement being far more strongly marked in some sets of experiments than in others.

III. Effect of Boron Compounds on Certain of the Lower Plants.

Our knowledge of the action of boron on the lower plants is less definite and complete than with regard to the higher plants. Morel (1892) found that boric acid acts as a strong poison to the lower fungi and similar organisms, their development being completely arrested by very weak solutions of the acid. He suggested, on this account, that boric acid might be used in the same way as copper to attack such diseases as mildew, anthracnose, &c., which attack useful plants.

On the other hand Loew (1892) stated that such algae as Spirogyra and Vaucheria showed no harmful influence for many weeks when the culture water contained as much as ·2% (= 1/500) boric acid. This may be supplemented by a recent observation at Rothamsted, in which certain unicellular green algae (unidentified), were found growing at the bottom of a stoppered bottle containing a stock solution of 1/100 boric acid.

Agulhon (1910 a) dealt chiefly with yeasts and certain ferments, and found that yeasts grown in culture solutions are not influenced favourably or unfavourably by relatively large quantities of boric acid up to 1 gram per litre, while all development is checked with 10 grams per litre. The presence of boron affects the action of yeast on glucose and galactose. Galactose alone is not attacked even after 40 days in the presence of ·66% boric acid. When glucose is mixed with the galactose the latter is said to be at first left untouched, but later it disappears very slowly.

Boric acid exercises an antiseptic action on lactic ferments, 5 gm. per litre checking their action sufficiently to enable milk to remain uncoagulated. Lactic acid is still produced even with as much boric acid as 10 gm. per litre. The microbe is not actually killed by the boric acid, but its development is so arrested that reproduction cannot take place. The same phenomenon was observed with yeast. With moulds again, while no stimulation could be obtained with small quantities of boric acid, yet the toxic action does not begin to set in until 5 gm. boric acid per litre are present.

Thus it appears that such lower organisms as yeast, lactic ferment and Aspergillus niger are remarkably indifferent to the action of boric acid, as is shown by the fact that the toxic dose is remarkably high, while stimulation effects cannot be observed even in the presence of the smallest quantities yet tried.

Conclusion.

Boric acid is less harmful to the growth of higher plants than are the compounds of copper, zinc, and arsenic. Evidence exists that below a certain limit of concentration boron exercises a favourable influence upon plant growth, encouraging the formation of stronger roots and shoots. This stimulation is more strongly marked with some species than with others, peas responding more readily than barley to the action of boric acid. Fungi are very indifferent to boron, whether it is present in large or small quantities, and there is evidence to show that certain of the green algae can also withstand large quantities of it.

CHAPTER VII
EFFECT OF MANGANESE COMPOUNDS

I. Presence of Manganese in Plants

The presence of manganese as a constituent of plant tissues has been known for many years, and in view of the close association between iron and manganese it was natural that the early investigators should seek for the latter element. De Saussure (1804) gives one of the earliest references to manganese in plant ash, stating that it occurs in the seeds in less great proportion than in the stems, and also that the leaves of trees contain less in autumn than in spring. At first oxides of iron and manganese were put together as “metallic oxides” and little or no attempt was made to separate them so as to get an idea of their relative abundance. John (1814) gives a number of rough analyses of plants and indicates the presence of manganese in many plants, including Solanum tuberosum, Brassica oleracea viridis L., Conium maculatum, Aesculus (in outer bark), and Arundo Sacchar. No further references presented themselves until 1847, as probably manganese was overlooked and always classed with iron in any analyses made during that time. Kane (1847) found traces of manganese in the ashes of some samples of flax, but none in others, and examinations of the soils on which the plants were grown gave similar results. Mayer and Brazier (1849) confirmed this result. Herapath (1849) analysed the ashes of various culinary vegetables, finding manganese in cauliflowers, swede turnips, beetroot, and in one variety of potato (Forty fold).

Malaguti and Durocher (1858) tried to investigate the matter quantitatively. The oxides of iron, manganese, and aluminium were all classed together, and the mean percentage of the three varied from ·85%–5·06% according to the varieties of plants concerned, Cruciferae possessing least and Leguminosae most. Different mean results with the same plant were obtained from different soils.

Wolff (1871) made other quantitative analyses including Trapa natans (·15% Mn₃O₄), Acorus Calamus (1·52% Mn₃O₄), Alnus incana (trace–·73% Mn₃O₄), Pyrus communis (2·15% Mn₃O₄). Many other plants were mentioned by Wolff as containing manganese.

Campani (1876) found manganese in ash by a method in which it was detected as phosphate of manganese, and he claimed to be the first to discover manganese in wheat ash. Warden (1878) found traces of Mn₃O₄ in the ash of opium from Behar.

Dunnington (1878) detected manganese in the ash of wheat, ·00144 gm. (as Mn₃O₄?) in 300 grams of “Dark Lancaster” variety, equivalent to ·027% of the pure ash. The ash was exhausted with nitric acid, and after separating the iron the ammonium sulphide precipitate was found to contain manganese, and gave by fusion with nitre and sodium phosphate a violet coloured mass. Andreasch (1878) found slight traces of Mn₃O₄ in the flowers of Dianthus caryophyllus, none occurring elsewhere, while in Rosa remontana it appeared in both leaves and flowers.

Maumené (1884) tested many food plants and concluded that some quantity of manganese is frequently present in potato, rice, barley, carrot, lentil, pea, beetroot, asparagus, chicory, most fruits, tea, and also in some fodder plants, as lucerne, oats, and sainfoin. Ricciardi (1889), Hattensaur (1891) also added to the list of plants proved to contain manganese. Guerin (1897) studied the manganese content of woody tissues. Sawdust was treated with distilled water containing 1% caustic potash, expressed, and filtered after two or three days. A brown coloured liquid was obtained, which when treated with a slight excess of hydrochloric acid gave an abundant flocculent precipitate. This precipitate proved to be soluble in pure water, so it was washed with slightly acidulated distilled water, and after further purification was analysed. No trace of iron was obtained, but about ·402% Mn was found. Guerin regarded the precipitate as a “nucleinic” combination, which he supposed to occur generally in wood and to contain the manganese present in the woody tissues of all plants.

Schlagdenhauffen and Reeb (1904) detected manganese in a petrol extract of such cereals as barley, oats, and maize, and since inorganic salts of manganese are not soluble in such liquids as ether or petrol they concluded that the manganese must be present in the plant in organic combination, thereby upholding Guerin’s view. Loew and Seiroku Honda (1904) give a table of Mn₃O₄ in the ashes of certain trees. This is very high in some cases, rising to 11·25% in the ash of beech leaves, 6·73% in birch leaves, and 5·48% in chestnut fruits.

Gössl (1905) gives lists of the distribution of manganese in plants, both Thallophytes and Phanerogams, indicating the presence of much or little of the element. As a rule, he states, marsh and water plants gather up more manganese than do land plants.

The Gymnosperms seem to be particularly rich in their manganese content. Schröder (1878) tested for the element in firs and pines and found the following amounts of Mn₃O₄.

He gave a table of detailed analyses showing the differing proportions of manganese in the different parts of the fir.

Baker and Smith (1910) paid special attention to manganese in their exhaustive work on the Pines of Australia. They state that “in the anatomical investigations of the timber, bark, and leaves of the various species, there was found to be present, in a more or less degree, a naturally brownish-bronze coloured substance, which invariably stained dark brown or almost black with haematoxylin.” This substance on careful investigation proved to be a compound of manganese. The quantity present varies with the species and also with the plant organs. The different species of the genus Callitris show variable percentages of manganese from a maximum of 0·230% in C. gracilis, to a minimum of 0·010% in C. robusta. The percentage of manganese in Australian Coniferae other than Callitris is given by the authors in the following table:

Baker and Smith assume that manganese is essential to the production of the most complete growth of Coniferae. The element is found in these plants even when they grow on soils containing only traces of manganese and it is suggested that possibly the excess or deficiency of manganese in the soil helps to govern the location of certain of the Australian Coniferae. The authors conclude that manganese may be essential to the growth of these plants, and that its association with plant life may be considered to date back to past geological time, as is indicated by plates illustrating fossil woods.

II. Effect of Manganese on the Growth of Higher Plants.

1. Toxic effect.

(a) Toxic action of manganese compounds in the presence of soluble nutrients.

Little work seems to have been done on the action of manganese compounds in water cultures. Knop (1884) just indicated that manganese compounds had no effect on maize, but gave no details. Japanese investigators touched on the matter in the course of their extensive experiments with this element. Asō (1902) found that the greater concentrations of manganese sulphate exercised an injurious influence on barley. Even in solutions with as little as ·002% manganese sulphate (= 1/50,000 MnSO₄) the roots gradually turned brown, the lower leaves following suit. The brown colour was concentrated at certain points of the leaves, and microscopical examination showed that the membranes of the epidermal cells, and in some cases the nuclei, were stained deeply brown. The greatest concentration endured by barley without injury seemed to be about ·01 per 1000 = 1/100,000. The presence of iron in the food solutions seems to counteract the effect of the manganese to some extent by delaying the yellowing of the leaves. Wheat proved very similar to barley in its reactions, though more iron is necessary to give good healthy growth. Asō states that wheat is able to overcome the injurious action of manganese much more readily than is barley. With peas the yellowing of the leaves was delayed, probably on account of a sufficient supply of iron in the reserve stores of the seeds.

Loew and Sawa (1902) found that ·25% = 1/400 MnSO₄ (anhydrous) kills pea plants within five days and that the green colour is gradually affected with more dilute solutions. Barley and soy beans were grown in nutritive solutions with either iron sulphate or manganese sulphate or both (·01% FeSO₄, ·02% MnSO₄, ·01% FeSO₄ + ·02% MnSO₄). At first the growth was increased by the action of two salts together, but eventually the shoots turned yellowish, and assimilation was depressed, so that decreased nutrition led to relaxation in the speed of growth, indicating the toxic action due to the manganese sulphate.

The Rothamsted experiments supported Asō’s work on the action of manganese sulphate on barley, concentrations of the salt above 1/100,000 having a retarding influence on the growth, the roots being coloured brown and the leaves also showing discolouration. At an early stage in growth the lower leaves of the plants receiving the most poison began to be flecked with brown spots, which were at first attributed to an attack of rust. Suspicion was soon aroused, however, and a closer microscopic investigation showed that no disease was present, but that the cells in the affected spots were dead and brown, though they retained their shape. The dead cells at first occurred in small patches, which spread and coalesced until ultimately the whole leaf was involved. Some of the affected leaves were detached and fused with a mixture of sodium carbonate and potassium nitrate. On dissolving up the resulting mass with water a green colouration was obtained, indicating the presence of manganese in the leaves. This shows that the manganese is taken up by the roots, transferred to the leaves and then deposited in them, the lower leaves being the first affected.

The presence of manganese in the nutritive solution retarded the ripening of the grain to some extent, as when the grains from the control plants were hard and ripe, those from plants treated with 1/10,000 MnSO₄ were green, those with 1/100,000 were a mixture of ripe, half-ripe, and green grains, while plants which had received 1/1,000,000 MnSO₄ possessed ripe grains.

Peas give similar results to barley so far as the vegetative growth is concerned, the same retardation with the higher concentrations being observed, while the brown discoloured patches in the lower leaves are much in evidence. All traces of manganese in the leaves disappear when the concentration falls to 1/250,000. On the whole peas are more sensitive to manganese poisoning than is barley, and the higher strengths of manganese prove more deleterious to them.

(b) Toxic action of manganese compounds in sand cultures.

Little work has been done on this aspect of the problem. Prince de Salm Horstmar (1851) grew oats in sand with various combinations of nitrogenous substances and inorganic mineral salts. He stated that until the time of fruit formation manganese does not seem to be essential to the oat unless iron is in excess in the substratum.

(c) Toxic action of manganese compounds in soil cultures.

A large body of work has been done with manganese in soil cultures, but the toxic effect is hardly indicated, possibly because it is less manifest under soil conditions, possibly because the observation of the toxic action has been almost completely overshadowed by the interest in the stimulation observed under the same circumstances. Namba stated that ·5 gm. MnSO₄ added to 8 kgm. Japanese soil exerted a depressing influence on the growth of various plants. The Hills Experiments (1903) indicated some toxic effect. Various soluble and insoluble salts of manganese were added to soil in pots at the rate of 2 cwt. per acre, wheat being sown. On the whole the plants from untreated pots were as good as any with manganese except those that received manganese nitrate or phosphate. Manganese iodide distinctly retarded growth. The plants that grew did well eventually, but development of the ear was greatly or entirely retarded. If the seeds were soaked in the iodide, a concentration of 10% was found to be harmful, 5% allowing normal growth. Similar experiments with barley showed that plants treated with manganese carbonate and sulphate were both inferior to the untreated plants; with iodide less plants were obtained and their development was abnormal. Soaking the seeds in the iodide, even in 10% solution, did not do damage as it did with wheat. The oxides were apparently innocuous, but gave no increase either in corn or straw.

Kelley (1909) found that on soils in Hawaii in which excessive quantities of manganese are present (5·61% Mn₃O₄) pineapples do not flourish, but turn yellow and produce poor fruits, and also that if rather less manganese is present (1·36% Mn₃O₄) the pineapples show the toxic effect by yellowing during the winter months, but they recover completely during the hot summer months. Kelley also observed that the deleterious effect is hardly noticeable during the first twelve months of growth, and that after a time a darkening occurs in the colour of the soil, which he attributes to some change in the constitution of the manganese compounds.

Some interesting observations were made by Guthrie and Cohen (1910) on certain Australian soils. A bowling green that was initially covered with a healthy mat of couch grass developed a number of small patches after about three years growth, on which the grass died off. No reason was apparent for this phenomenon, as the cultural conditions were uniform and to all appearances the soil over the whole area was similar in character. Analyses of soil samples from the dead patches and from the neighbouring healthy parts of the green showed that the chemical composition in both cases was practically the same, except that while no manganese occurred in the soil from the unharmed part, as much as ·254% Mn₂O₃ was found in that from the dead patches. As no other differences were found it was argued that the manganese, present in such large quantities, acted as a toxic agent and killed off the grass. Other instances of manganese poisoning in which wheat and barley were affected are quoted by these authors, the analytical results indicating that possibly barley is able to withstand without injury a greater quantity of manganese compounds in the soil than is wheat.

2. Effect of manganese compounds on germination.

Nazari (1910) rolled wheat grains in a paste of manganese dioxide, iron sesquioxide (both with and without organic matter), and in what he terms “artificial oxydases.” The seeds rolled in the last-named showed the greatest energy in germination, while those with manganese gave an appreciable acceleration. The presence of organic matter decreased the action of manganese. The plants from the manganese seedlings gave an increased yield in both straw and grain, while those treated with sesquioxide of iron showed no gain over the check plants.

The Hills Experiments yielded some information as to the differing effects of various compounds of manganese on germination. With wheat plants in pot experiments manganese oxide (MnO₂) distinctly retarded germination when applied at the rate of 2 cwt. per acre. With barley MnO₂, manganese carbonate and sulphate all retarded germination, while with the iodide 50% of the seeds were entirely prevented from germinating.

3. Does manganese stimulate higher plants?

With manganese the evidence in favour of stimulation is more weighty than with such poisons as copper, zinc and arsenic, and the literature on the subject is correspondingly plentiful.

(a) Stimulation in water cultures.

While Asō (1902) asserted that plants can develope normally in water cultures in the absence of any trace of manganese, he further stated that manganese compounds exercise both an injurious and a stimulant action on plants. With increasing dilution of the compound the deleterious action diminishes, while the stimulant action increases, and a dilution can be reached in which only the favourable influence of the manganese becomes obvious. The addition of ·002% manganese sulphate (= 1/50,000) to culture solutions stimulated radish, barley, wheat and peas. The intensity of the colour reaction of the oxidising enzyme of the manganese plants was found to exceed that of the control plants, at least with regard to those leaves on the manganese plants which had turned a yellowish colour.

Loew and Sawa (1902) obtained an initial increase of growth with barley and soy beans in nutritive solutions + ·01% ferrous sulphate + ·02% manganese sulphate, but this initial stimulation was followed by depression. These authors support Asō’s contention that manganese exerts both an injurious and a stimulative action upon plants, and that the promoting effect is still observable with manganese compounds in high dilution, while the injurious effects disappear under this condition.

The Rothamsted experiments with barley show a decided stimulation with 1/100,000 MnSO₄ and less. Care was taken to utilise sublimed FeCl₃ to avoid error due to the introduction of manganese into the control solution through the agency of this salt. It is interesting to notice that concentrations that are weak enough to stimulate the vegetative growth still show a depressing action in that they retard the ripening of the grain, a fact which supports Loew and Sawa’s contention that manganese exerts both a toxic and a stimulative action at one and the same time, the balance showing itself according to the concentration (Fig. 17). In the later experiments the plants were not allowed to form ears, but similar results were obtained, except that when dealing with the vegetative growth only, a definite stimulus was obtained with a higher concentration than in those experiments in which the plants were allowed to form seed. This may or may not be significant, as it is possible that seasonal variation and individuality of the plants may have played some part. Barley seems to be most extraordinarily sensitive to the action of manganese, as even 1 part in 100,000,000 was found to exercise a beneficial action (Fig. 18). With peas the evidence of stimulus is less well marked. No sign of stimulation is obtained until a greater dilution is reached than is necessary with barley. Even so the resulting curves are not sufficiently conclusive to warrant the definite statement that manganese does act as a stimulant to peas when present in very small quantities (Fig. 19).