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]).

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.)

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.
2.1/10,000manganese sulphate.
3.1/100,000„„
4.1/1,000,000„„
5.1/10,000,000„„
6.1/100,000,000„„

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