224 BOTANICAL GAZETTE [MARCH 
magnesium in a given soil as the ratio between these two substances 
in solution in the soil moisture that determines the physiological 
effects. In the case of plants grown on manganiferous soil, the 
relative amounts of lime and magnesia actually absorbed become 
greatly different from those absorbed from normal soils, regardless 
of the amounts of these elements present in the soil. 
From these evidences, we may believe, then, that an important 
effect of manganese is of an indirect nature, being due to its bringing 
about a modification in the osmotic absorption of lime and magnesia, 
and that the toxic effects are chiefly brought about by this modifi- 
cation, rather than as a direct effect of the manganese itself. As 
has been mentioned already, not all species of plants are equally 
sensitive to modifications in the lime-magnesia ratio; and from 
some recent experiments of GiLE (17) it seems that the effects of a 
modification in this ratio are, to some extent, dependent upon the 
concentration of the culture solution. Likewise, different ratios 
are best suited to different species. Therefore, the effect of man- 
ganese may be very different on different soils and with different 
species of plants. With certain plants it is toxic on certain soils 
for the reason that the absorbed lime and magnesia are thrown out 
of their optimum ratio for this plant, while in others it may exert 
a stimulating effect by bringing this ratio more nearly to its 
optimum. 
The small amounts of manganese in natural soils, therefore, 
probably perform a twofold function in plant growth: (1) it acts 
catalytically, increasing the oxidations in the soil and acceler- 
ating the auto-oxidations in plants; and (2) it tends to modify the 
absorption of lime and magnesia, perhaps by partially replacing 
calcium from insoluble combinations, but especially, through . 
direct effect on the osmotic absorption of lime and magnesia, 
increasing the former and decreasing the latter. 
The absorption of phosphoric acid is likewise decreased in the 
presence of manganese. By reference to the preceding table of 
ash analyses it will be seen that frequently the ash of a given specie 
of plant from manganese soil was found to contain not more than 
one-half as much phosphoric acid as from normal soil. The intet- 
ference with the absorption of phosphoric acid would also tend to 
