To Francis Darwin   [before 5 June 1879]1

My dear F.

I begin to think that I shall prove that tip of radicle is its brain, as far as geotropism is concerned.—2 I touched 4 tips with lunar caustic3 so as blacken only for 12 to 23 of m.m.— They grew in length during 24 hrs 9 mm.. Now only 1 of the 4 became at all curved in the 24 h.— There were 4 other radicles in your jar, to which nothing had been done, & of these 3 after 24hrs. pointed vertically downwards & one for some unknown reason was not acted on.— I showed George4 the jar & the contrast between the 4 which had been touched with L. Caustic & 4 which had not been touched, he thought most striking.

C. D.

I must try & retry many more radicles.—

1
The date is established by the relationship between this letter and the letter to W. T. Thiselton-Dyer, 5 June 1879.
2
CD was studying the movement of radicles (embryonic roots) and had already done many experiments relating to the sensitivity of the apex to touch (see, for example, Correspondence vol. 26, letter to W. T. Thiselton-Dyer, 9 May [1878]).
3
Lunar caustic is silver nitrate (AgNO3); it was used in medicine as a cauterising agent.

Manuscript Alterations and Comments

1.1 its] above del ‘the’
1.1 as] over ‘as’
1.3 during] above del ‘in’
1.7 with L. Caustic] interl

Please cite as “DCP-LETT-12084,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-12084