# To J. S. Burdon Sanderson   9 September [1873]1

Down, | Beckenham, Kent.

Tuesday Sep. 9.

My dear Dr Sanderson

I will send up early tomorrow two plants with 5 goodish leaves, which you will know by their being tied to sticks.2 Please remember that the slightest touch, even by a hair, of the 3 filaments on each lobe, makes the leaf close; & it will not open for 24 hours.

You had better put $\frac{1}{4}$ of inch of water into the saucers of the pots. The plants have been kept too cool in order to retard them. You had better keep them rather warm, (i.e. temp. of warm greenhouse) for a day, & in a good light.

I am extremely glad you have undertaken this subject. If you get a positive result, I shd think you ought to publish it separately, & I cd quote it;3 or I shd be most glad to introduce any note by you into my account.

I have no idea whether it is troublesome to try with the thermo-electric pile, any change of temp. when the leaf closes.4 I cd detect none with a common thermom. But if there is any change of temp., I shd expect it wd occur some 8 to 12 or 24 hrs after the leaf has been given a big smashed fly, & when it is copiously secreting its acid digestive fluid.

I forgot to say that as far as I can make out, the inferior surface of the leaf is always in a state of tension, & that the contraction is confined to the upper surface;5 so that when this contraction ceases, or suddenly fails (as by immersion in boiling water) the leaf opens again, or more widely than is natural to it.—

Whenever you have quite finished I will send for the plants in their basket. My son Frank is staying at 6 Queen Anne St, & comes home on Sat. afternoon,6 but you will not have finished by that time

yours very sincerely | Ch. Darwin

P.S. I have repeated my exper. on digestion in Drosera with complete success. By giving leaves a very little weak hydro-chloric acid, I can make them digest albumen i.e. white of egg quicker than they can do naturally—7

I most heartily thank you for all your kindness. I have been pretty bad lately & must work very little.

## Footnotes

The year is established by the relationship between this letter and the letter to J. S. Burdon Sanderson, 27 August 1873.
CD had asked Burdon Sanderson to investigate the electric currents in Dionaea muscipula (Venus fly trap; see letters to J. S. Burdon Sanderson, 15 August 1873 and 27 August 1873).
Sanderson did not report experiments with a thermopile, but used a galvanometer (see Burdon Sanderson 1874, p. 128).
In fact, CD had mentioned tension and contraction of the upper surface of the leaf of Dionaea in his letter of 15 August 1873.
Francis Darwin lived with CD’s brother, Erasmus Alvey Darwin, while a medical student (F. Darwin 1920, p. 68 n. 1). According to Emma Darwin’s diary (DAR 242), Francis arrived at Down on Saturday 13 September 1873. As part of his medical training, Francis studied histology under Edward Emanuel Klein at the Brown Animal Sanatory Institute, where Burdon Sanderson was director.

## Summary

Pleased JSBS has decided to work on Drosera; sends plants. Does not know whether thermo-electric pile could detect temperature change when leaves close.

CD’s experiment with very weak hydrochloric acid repeated with success: the plants digest albumen more quickly.