Ɛpsilon

To Francis Darwin   2 July [1879]1

My dear F.

That was a splendid idea of yours in your letter of June 30th about touching upper & lower surface of apex of radicles, extended horizontally in earth. It shows beautifully that caustic at least on one side, does not interfere with the bending—2 I have no shade of doubt that the apex is a a kind of brain for certain movements, like the gland of Drosera for inflection—or the hairs on Dionæa—ie a specialised centre for receiving certain irritations3

C. D.

Whilst you remember, make notes about the horizontally extended roots, with caustic above & below.— On your return we will make trial with cutting off 1 mm. on horizontal & vertical radicles.—

July 2d.—

1
The year is established by the relationship between this letter and the letter from Francis Darwin to Emma Darwin, 30 June 1879.
2
See letter from Francis Darwin to Emma Darwin, 30 June 1879 and n. 4. CD and Francis had been experimenting with lunar caustic (silver nitrate) applied to the tips of radicles (embryonic roots); Francis, who was working in the laboratory of Julius Sachs at Würzburg, had informed CD of Sachs’s objections to the use of caustic (see letter from Francis Darwin, [before 26 June 1879] and n. 3). The results Francis described in his letter to Emma of 30 June 1879 seemed to vindicate their experimental protocol.
3
CD had observed inflection in the leaves of Drosera rotundifolia (common or round-leaved sundew) and Dionaea muscipula (Venus fly trap); see Insectivorous plants. He concluded that the motor impulse was transmitted from the leaf gland that had been touched to the surrounding glands in a manner comparable to a reflex action in animals; he further noted that in the fly trap, there existed an electrical current that was disturbed by touch in the same manner as in the contraction of muscle in animals (ibid., pp. 276–7, 318).

Manuscript Alterations and Comments

3.1 horizontally] interl

Please cite as “DCP-LETT-12132,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-12132