Inorganic Plant Poisons and Stimulants – Effect of Manganese Compounds

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.

    In 100 parts ash. In 1000 parts dry matter.
Fir Pine Fir Pine
33·18 13·46 2·76 ·77

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:

Ash of timber of Agathis robusta 0·145% Mn.
Araucaria Cunninghamii 0·054%
Araucaria Bidwilli 0·077%
Actinostrobus pyramidalis 0·077%
Podocarpus elata 0·002%
Dacrydium Franklini 0·129%
Athrotaxis selaginoides 0·019%
Phyllocladus rhomboidalis 0·145%
Air-dried black gum of Agathis robusta 0·0046%
Araucaria Cunninghamii 0·0038%

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. Aso (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. Aso 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 Aso’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 MnSO4possessed 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 Aso (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 Aso’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 utilize 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).

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Fig. 17. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of manganese sulphate and nutrient salts. (Feb. 5th–March 29th, 1909.)

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Fig. 18. Photograph showing the action of manganese sulphate on barley plants grown in the presence of nutrient salts. (Feb. 5th–March 29th, 1909.)

1. Control.
2. 1/10,000 manganese sulphate.
3. 1/100,000
4. 1/1,000,000
5. 1/10,000,000
6. 1/100,000,000

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Fig. 19. Photograph showing the action of manganese sulphate on pea plants in the presence of nutrient salts. (Oct. 2nd–Dec. 20th, 1912.)

1. Control.
2. 1/5,000 manganese sulphate.
3. 1/10,000
4. 1/25,000
5. 1/50,000
6. 1/100,000
7. 1/250,000
8. 1/500,000
9. 1/1,000,000
(b) Stimulation in soil cultures.

Roxas carried out pot experiments with rice in soil to which was added varying proportions of manganese sulphate, with and without the addition of nutrient salts of ammonium, potassium, and calcium. The criterion of stimulation was the length of the growing leaves as measured daily, a strength of M/1000 MnSO4 (M = molecular weight) giving a favourable result.

In the Hills Experiments (1903) an increase of produce was obtained with wheat by manuring with manganese phosphate, chloride, sulphate, or oxide (MnO2), while an increase of straw was gained with nitrate, though this compound decreased the yield of corn. With barley no evidence of stimulation is set forth for any compound, except that the root growth was improved by the addition of manganese iodide, in spite of the general unfavourable action this substance exerted upon germination and growth.

Bertrand (1905) whose work will later be considered in detail, experimented on arable land, adding quantities of manganese sulphate (?) equivalent to about 1·6 gm. Mn to each square metre, growing oats from February to May. Increase of weight was found in the plants growing on the manganese plots, the differences in favour of manganese being

For total crops 22·5%.
grain only 17·4%.
straw only 26·0%.

A certain alteration in the quality of the grain was also noted from the manganese plots, the weight per hectolitre exceeding that from the untreated plot, the % of water and of total nitrogen being somewhat lower than that from the untreated, while the ash and the quantity of manganese present was the same in the grain from both plots. Bertrand suggested that these results might indicate a new line to follow in the study of the causes of the soil fertility.

Strampelli (1907) tested the effect of manganese dioxide, carbonate, and sulphate, and of a manganiferous mineral from the Argentine upon wheat, and found that while all four substances exercised a favourable influence on the vegetation, the best result was obtained with the sulphate. When however other manures were used in conjunction with the manganese compounds the balance of improvement shifted. With nitrogen, applied as nitrate of soda, manganese dioxide proved the most beneficial, with farmyard manure the manganiferous mineral, and with blood the carbonate. It was also found that a manganese compost did not increase production when phosphatic manure was applied as basic slag.

Feilitzen (1907) indicated that the nature of the soil plays its part in determining whether manganese acts as a stimulant or not. His experiments were made in the field on poor moor soil, which carried a little Sphagnum turf and Eriophorum, and which was poor in food salts. The soil was prepared and manured and then the plots were watered with a solution of ·1 gm. MnSO4?.?4H2O per litre at the rate of 10 kgm. sulphate per hectare, six control plots being left untreated. Oats were sown and the soil rolled. During growth no difference was noted between the various plots, and after harvesting the weights of the different crops showed that the manganese had not caused increase of crop in either grain or straw on this poor moor soil.

The great bulk of the work on this problem has been carried out by various Japanese investigators, whose work extends over several years. Loew and Sawa (1902) found that small quantities of manganese sulphate in soil cultures stimulated the growth of rice, pea, and cabbage. They suggested that soils of great natural fertility contain manganese in an easily absorbed condition, and that this forms one of the characteristics of such soils.

Nagaoka (1903) dealt with plots in the rice fields which had not been manured for the three previous years and which were then treated with manure at the rate of 100 kgm. ammonium sulphate, 100 kgm. potassium carbonate and 100 kgm. double superphosphate per hectare. Twelve series were worked in triplicate and received manganese sulphate in varying quantities, equivalent to 0–55 kgm. Mn2O3 per hectare, one set of three being left untreated. The cultivation was normal and the application of manganese was found to influence the yield of rice. 25 kgm. per hectare gave the best result and increased the harvest of grains by one-third; higher doses of Mn2O3 gave no better crop. The percentage of grain relative to the straw was also increased. The increase in both respects was evident all through the series from 10 to 55 kgm. Mn2O3 per hectare. The conclusion was reached that the application of this salt to soils poor in manganese would be a commercial advantage.

The next year (1904) the experiments were extended to observe the after effects of the initial doses of manganese sulphate. The harvest of grain was greatest in those plots that had received 30 kgm. Mn2O3 per hectare, while it was approached very closely by that from the plot with 25 kgm. Mn2O3, which had proved the best in the first year’s experiments. The maximum increase of yield over the unmanured plots in the first year was 37%, while in the second year it dropped to 16·9%.

Aso (1904) also obtained an increase of one-third in produce of grain when 25 kgm. Mn3O4 per hectare (as manganous chloride) was applied to rice. The development of the plants was improved and the treated plants flowered about four days before the untreated ones.

Loew and Honda (1904) grew Cryptomeria japonica in beds, treating the soil with various manures and with iron or manganese sulphate. The latter favoured increase in height, and within 11/2 years the cubic content of the trees had increased to double.

Fukutome (1904) grew flax in pot cultures, each pot containing 8 kgm. soil, to which was added ·4 gm. MnCl2?.?4H2O and ·4 gm. FeSO4?.?7H2O. This mixture had a marked effect on the growth of the flax, but the individual salts in doses of ·4 gm. per 8 kgm. soil had but little effect.

Namba (1908) applied manganese salts to onion plants in pots with a considerable measure of success. Pots containing 8 kgm. loamy soil were manured and received:

  • (1) no manganese,
  • (2) ·1 gm. MnSO4?.?4H2O,
  • (3) ·2 gm. MnSO4?.?4H2O,

the manganese sulphate being applied in high dilution as top dressing. The bulbs and leaves were considerably stimulated by small doses of manganese sulphate, the best results being obtained from (2), which represents a manuring of 22 kgm. MnSO4 per hectare. An increase of the dose lessens the beneficial effect, as the toxic action begins to come into play. The actual figures obtained may prove of interest.

Wt. leaves   Wt. bulbs Total weight Bulbs & roots
& roots  Absolute  Relative leaves
gm. gm. gm. gm.
1. 29·5   8·5 38·0 100·0 ·28
2. 38·0 22·5 60·5 159·2 ·59
3. 35·5 16·5 51·0 134·2 ·46

Uchiyama (1907) carried on a variety of experiments with manganese sulphate on several plants on different soils, both in the field and in pots, and found that the compound exercised a favourable action in most cases when applied in appropriate quantities. In summarising his results he stated that both manganese and iron stimulate the development of plants, different plants varying in their susceptibility to the action. Sometimes a joint application of the two salts is the most beneficial, sometimes an individual application is the better, in which case manganese sulphate is generally better than ferric sulphate in its action. The stimulating action of manganese varies greatly with the character of the soil, and the mode of application also affects results. As a general rule the manganese acts best when applied as a top dressing rather than when added together with the manure. Further the stimulating action differs greatly with the nature and reaction of the manurial mixture. Uchiyama concludes that 20–50 kgm. per hectare of crystallised manganese sulphate is a good general amount to apply.

Takeuchi (1909) corroborates the statements of the various writers that plants differ in their response to the manganese manuring. Pot cultures, in each of which 8 kgm. soil were similarly manured, received ·2 gm. MnSO4?.?4H2O applied as a solution of 1/100 strength, the controls receiving the same amount of water. The manganese increased the green weight of spinach by 41%, while the dry weight of barley, peas and flax rose 5·3%, 19·4%, and 13·9% respectively above that of the untreated. The control plants of flax were behind the manganese plants in growth and flowering, while barley was the least stimulated of all the test-plants. Other observations seemed to show that Leguminosae and Cruciferae are more susceptible to manganese stimulation than are the Gramineae.

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

The information on this point is exceedingly meagre, possibly because of the diversion of general attention to the higher plants in view of the commercial interests involved.

Richards (1897) carried out experiments with various nutritive media with the addition of certain metallic salts, including those of zinc, iron, aluminium and manganese. The fungi tested were Aspergillus niger, Penicillium glaucum andBotrytis cinerea. His general conclusion was that fungi may be stimulated, though it must not be concluded without further investigation that all fungi react in the same degree to the same reagent, but this conclusion is traversed by Loew and Sawa (1902). These writers state that fungi are not stimulated by manganese, and take this as a proof that the improvement in the growth of phanerogams, induced by manganese compounds, is not due to direct stimulation of the protoplasmic activity, but to some other more obscure cause.

IV. Physiological Considerations of Manganese Stimulation.

The physiological cause of the stimulation exerted by manganese compounds has raised much controversy. Loew and Sawa suggested that the action of the sun’s rays upon a normal plant puts a certain check on growth, arising out of the action of certain noxious compounds which they supposed to be produced in the cells under the influence of light. The stimulation of the manganese compounds may be due to a supposed increase in the oxidising powers of the oxidising enzymes, so that destruction of the checking compounds can be accomplished as quickly as they are formed, so enabling growth to continue more rapidly.

Aso (1902) had previously stated that colorimetric tests for oxidising enzymes indicate that the yellowish leaves from plants treated with manganese compounds give reactions of higher intensity than the green leaves from control plants, the difference between the reactions being specially marked in barley, and less so in radish.

Bertrand has devoted much time to the consideration of this and allied problems. In 1897 (a, b, c) he proceeded to investigate the essential nature of manganese in the economy of the plant, his experiments showing its constant presence in a ferment (laccase) obtained from plants. He also extracted from lucerne a substance very poor in manganese, which was somewhat inactive, but which regained or increased its activity on the addition of manganese. Bertrand stated that manganese was apparently not to be replaced by another metal, not even by iron, and that the small quantity of it occurring was no reason for regarding it as a secondary element in the composition of plants. The view was also put forward that in the presence of certain organic substances, such as hydroquinone, pyrogallol or similar bodies, manganese is capable of fixing free oxygen from the air, the volume of oxygen absorbed varying according to the compound of manganese used. Bertrand was led to conceive the oxydases as special combinations of manganese in which the acid radicle, probably protein in nature and variable according to the ferment considered, would have just the necessary affinity to maintain the metal in solution, i.e. the form the most suitable for the part it has to play. The manganese would then be, according to his view, the true active element of oxydase, which functions as the “activator”; the albuminous matter, on the other hand, gives to the ferment those special characters, which show themselves in their behaviour with regard to reagents and physical agents. From this point of view manganese could no longer be considered as a non-essential element, but as a substance of vital necessity to the functions of plant-life. The name “complementary” manure was suggested for compounds of such elements as manganese, which exert a physiological action and which were proposed for use as manures. Later (1905) Bertrand considered that he had still further proved the indispensable nature of manganese. The absence or insufficiency of one essential element arrests or diminishes growth. This applies not only to those substances which are present in the greatest abundance, such as C, P, N, &c., but also to those elements like manganese, boron, and iodine, which only occur in traces. These elements are usually specialised in function, and for them the name “catalytic” elements was suggested, in view of the work they are held to do. As late as 1910 the rôle of manganese in the functioning of the oxidising enzymes was again insisted on. It was concluded that manganese intervenes as a catalytic agent in the material changes of which plants are the seat, and that it participates in an indirect manner in the building up of the tissues and in the production of organic matter.

Conclusion.

Manganese exerts a toxic influence upon the higher plants, if it is presented in high concentration, but, in the absence of great excess of the manganese compounds, the poisoning effect is overshadowed by a definite stimulation. As is the case with boron, manganese stimulates some species more than others, the action on barley being more evident than that on peas. It seems probable that manganese may prove to be an element essential to the economy of plant life, even though the quantity usually found in plants is very small.