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