[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 and Botrytis 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.
[Asō (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.