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Darwin Correspondence Project

To J. D. Hooker   25 March [1878]1

Down, | Beckenham, Kent. | ☞ Railway Station | Orpington. S.E.R.

March 25th

My dear Hooker

I think we have proved that the sleep of plants is to lessen injury to leaves from radiation.—2 This has interested me much & has cost us great labour, as it has been a problem since the time of Linnæus.3 But we have killed or badly injured a multitude of plants. (N.B Oxalis carnosa4 was most valuable, but last night was killed.)

There is one very important point about which we have failed from want of specimens,—namely with those few plants which turn the under side of leaves outwards or to the zenith—that is to ascertain whether the under side is hardier than the upper side.

Luckily I have 2 pots of young Clovers; but I have injured badly all the Cassias & all my plants of Arachis hypogea.5 If you think our work useful could you spare us 3 or 4 plants of Cassia— the common greenhouse species would be the best for us?—6 And have you any plants of Arachis above ground? Unfortunately there is no time to lose, as there may be few more frosts. Do you think I could buy Cassias at Veitch?7 If you can send us Cassias or Arachis they must be well packed up & directed

Orpington Station

“To be forwarded immediately”.

May God & you forgive me for being so troublesome | Yrs affectly | C. Darwin

P.S. I have just found one specimen of Cassia not experimented on, but one is hardly enough.


The year is established by the reference to Oxalis carnosa (see n. 4, below).
CD discussed his theory that sleep in leaves was a protection against ‘radiation’ or loss of heat in Movement in plants, pp. 284–6. For his early interest in the subject, see Correspondence vol. 5, letter to J. D. Hooker, 5 June [1855]; for his initial observations, see Correspondence vol. 21, letter to J. D. Hooker, 31 October 1873.
Carl von Linné (Carolus Linnaeus) had supervised a dissertation on the sleep of plants by Peter Petersson Bremer (Linnaeus 1755; later republished in Amoenitates academicae (Linnaeus 1749–90, 4: 333–50)). Hooker had supplied CD with the reference to the book (see Correspondence vol. 21, letter from J. D. Hooker, 30 August 1873).
CD was sent Oxalis carnosa (fleshy sorrel) from the Royal Botanic Gardens, Kew, on 14 February 1878 (see letter to J. D. Hooker, 15 February 1878 and n. 2).
CD had received Arachis hypogaea (peanut) from Kew in July 1877 (see Correspondence vol. 25, letter to W. T. Thiselton-Dyer, 9 July [1877], n. 2; he discussed the effects on leaves of exposure to freezing temperatures in Movement in plants, p. 289.
CD discussed sleep in Cassia in Movement in plants, pp. 369–73. He described Cassia floribunda (a synonym of Senna floribunda) as ‘a common greenhouse bush’ (ibid., p. 369 n.).
CD often purchased plants from the nursery firm Veitch & Sons.


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Linnaeus, Carolus (Carl von Linné). 1749–90. Amoenitates academicae: seu dissertationes variæ physicæ, medicæ, botanicæ antehac seorsim editæ nunc collectæ et auctæ cum tabulis aeneis. 10 vols. Stockholm: Laurentius Salvius.

Linnaeus, Carolus (Carl von Linné). 1755. Somnus plantarum. Doctoral dissertation of Peter Petersson Bremer under the supervision of Linnaeus. Uppsala: n.p.

Movement in plants: The power of movement in plants. By Charles Darwin. Assisted by Francis Darwin. London: John Murray. 1880.


CD and Frank think they have proved that function of plant sleep is to protect leaves from injury by chilling radiation. Requests plants for experiment to determine whether underside of leaf is hardier than upper.

Letter details

Letter no.
Charles Robert Darwin
Joseph Dalton Hooker
Sent from
Source of text
DAR 95: 469–70
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 11443,” accessed on 20 September 2021,