IN the interesting article in your issue for May 6 (p. 5), on “Plants and their Defences,” there is one sentence on which I should like to be allowed to offer a few remarks. It runs thus:—“This fluid [of the stinging-gland of the stinging-nettle] is generally conjectured to be formic acid—a view based on the fact that this acid can be obtained from the nettle-plant by suitable means.” Does this “conjecture” rest on any other basis than the similarity of the effect produced by the sting of the nettle and the bite of the ant? I am inclined to think not. Certainly the fact that formic acid can be obtained from the nettle-plant is not in itself a cogent argument, seeing that it has been shown that this acid is a widely-spread constituent of the cell-sap of living plants. The formic acid theory is also out of harmony with the fact that the fluid contained in the stinging glands of the nettle has frequently, if not always, an alkaline reaction. It seems strange that we have at present no trustworthy observations on so interesting a question. Can none of our physiological chemists come forward and remove it from the region of conjecture?