III. Effect of Boron Compounds on Certain of the Lower Plants.
Our knowledge of the action of boron on the lower plants is less definite and complete than with regard to the higher plants. [Morel (1892)] found that boric acid acts as a strong poison to the lower fungi and similar organisms, their development being completely arrested by very weak solutions of the acid. He suggested, on this account, that boric acid might be used in the same way as copper to attack such diseases as mildew, anthracnose, &c., which attack useful plants.
On the other hand [Loew (1892)] stated that such algae as Spirogyra and Vaucheria showed no harmful influence for many weeks when the culture water contained as much as ·2% (= 1/500) boric acid. This may be supplemented by a recent observation at Rothamsted, in which certain unicellular green algae (unidentified), were found growing at the bottom of a stoppered bottle containing a stock solution of 1/100 boric acid.
[Agulhon (1910 a)] dealt chiefly with yeasts and certain ferments, and found that yeasts grown in culture solutions are not influenced favourably or unfavourably by relatively large quantities of boric acid up to 1 gram per litre, while all development is checked with 10 grams per litre. The presence of boron affects the action of yeast on glucose and galactose. Galactose alone is not attacked even after 40 days in the presence of ·66% boric acid. When glucose is mixed with the galactose the latter is said to be at first left untouched, but later it disappears very slowly.
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.