My dear Sir

I had resolved not to trouble you with another letter, but I cannot resist thanking you for your last note. When you have watched the aggregated masses in the cells of the tentacles for some little time, under a high power, you will be able to judge whether any other substance except protoplasm undergoes such movements.¹ Your remark about the acid passing thro’ the cell-membrane of the glands when irritated has led me to think that this probably is the explanation of the process of aggregation; for, as I have attempted to show (supported as my view is by Gorup-Besanez p.362 of my book) that protoplasm is held in solution by a ferment (the same which gives to Drosera the power of digestion) together with an acid; consequently as soon as the acid passes outwards, the protoplasm is precipitated, & then displays its characteristic spontaneous movements.² If this view is correct, it would have to be extended to the roots of Drosera, Euphorbia &c (p. 64— & 141) with this difference, that in these cases the Carbonate of Ammonia is absorbed by the roots & unites with some free acid, causing the precipitation of the protoplasm.³ I am however aware that I ought to have worked more on the roots of plants, & on the action of Carb. of Ammonia on the grains of Chlorophyll but time & strength failed me. You are very good to speak so encouragingly about my observations on the so-called process of aggregation.⁴

I have addressed this note to Breslau, from not knowing how long you will remain at your present residence.⁵

My dear Sir | yours sincerely | Charles Darwin