Boric acid exercises an antiseptic action on lactic ferments, 5 gm. per litre checking their action sufficiently to enable milk to remain uncoagulated. Lactic acid is still produced even with as much boric acid as 10 gm. per litre. The microbe is not actually killed by the boric acid, but its development is so arrested that reproduction cannot take place. The same phenomenon was observed with yeast. With moulds again, while no stimulation could be obtained with small quantities of boric acid, yet the toxic action does not begin to set in until 5 gm. boric acid per litre are present. Thus it appears that such lower organisms as yeast, lactic ferment and _Aspergillus niger_ are remarkably indifferent to the action of boric acid, as is shown by the fact that the toxic dose is remarkably high, while stimulation effects cannot be observed even in the presence of the smallest quantities yet tried. _Conclusion._ Boric acid is less harmful to the growth of higher plants than are the compounds of copper, zinc, and arsenic. Evidence exists that below a certain limit of concentration boron exercises a favourable influence upon plant growth, encouraging the formation of stronger roots and shoots. This stimulation is more strongly marked with some species than with others, peas responding more readily than barley to the action of boric acid. Fungi are very indifferent to boron, whether it is present in large or small quantities, and there is evidence to show that certain of the green algae can also withstand large quantities of it. CHAPTER VII EFFECT OF MANGANESE COMPOUNDS I. PRESENCE OF MANGANESE IN PLANTS