My dear Sir

Your discovery of the acidity of the Drosera secretion under stimulus is very interesting, and your experiments altogether certainly afford considerable ground for believing that pepsin or some substance analogous to it, and hydrochloric acid are secreted by the leaves.¹ If the acid be really hydrochloric, there would be no difficulty in detecting it in, say, the washings of a dozen leaves. It will be necessary however to avoid the presence of chlorides in the stimulant and I would therefore advise you to use either casein perfectly free from salt or better, fibrin thoroughly macerated & washed in several successive quantities of distilled water.

As the plant seems to make an effort to neutralize carbonate of soda in the presence of food, you could perhaps considerably increase the quantity of acid secreted by continually neutralizing it with carbonate of soda. I fear that no pepsin would be free from hydrochloric acid or chlorides.²

There is no necessity to assume that the materials for the hydrochloric acid are obtained from the bodies of insects: all soils contain chloride of sodium & this salt is never absent from the rain water & even the air of this country. It is quite conceivable that by a kind of osmotic action aided perhaps by chemical affinity, the plant may decompose a solution of chloride of sodium into soda which passes inwards & free hydrochloric acid which exudes from the tentacles. The presence of fragments of pure casein or fibrin or of tentacles in the washings will not interfere with the detection of hydrochloric acid, but I must ask you to be very careful that the washing water does not touch your fingers, & also that all the vessels &c used be carefully washed out with distilled water.

I regret that I have no mucin or vegetable casein³ in my possession neither do I know where to procure any; but, if that will not be too late, I shall be glad to prepare you some when my assistants return on the 1^st. ult.

I do not think that anything can be inferred from the gold experiment.⁴ Gold is precipitated from its chloride by many animal, vegetable & other substances & even by light alone.

I am ready at any time to test the liquid for hydrochloric acid.

Yours sincerely | E. Frankland.

Charles Darwin Esq. F.R.S.