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Darwin Correspondence Project

To Nature   [before 15 September 1881]1


Leaves Injured at Night by Free Radiation

Fritz Müller, in a letter to me from Sta. Catharina in Brazil, dated August 9, supports the view which I have advanced with respect to leaves placing themselves in a vertical position at night, during their so-called sleep, in order to escape being chilled and injured by radiation into the open sky.2 He says: “We have had last week some rather cold nights (2° to 3° C. at sunrise), and these have given me a new confirmation of your view on the meaning of the nyctitropic movements of plants. Near my house there are some Pandanus trees, about a dozen years old; the youngest terminal leaves stand upright, whereas the older ones are bent down so as to expose their upper surfaces to the sky. These young leaves, though of course the most tender, are still as fresh and green as before; on the contrary, the older ones have suffered from the cold, and have become quite yellowish. Again, the leaves of Oxalis sepium were observed by me to sleep in a very imperfect manner during the summer, even after the most sunny days; but now, in winter, every leaflet hangs down in a perpendicular position during the whole night.” It is a new fact to me that leaves should sleep in a more or less perfect manner at different seasons of the year.3

Charles Darwin


The date is established by the date of publication of this letter in Nature.
In Movement in plants, p. 560, CD had concluded that the purpose of nyctitropic (sleep) movement was to protect the leaf blade from damage caused by radiant heat loss from leaves. This damage occurs on cold calm nights when the plant surfaces become colder than the air above them. Pandanus is the genus of screw pine or pandan palm; its branches produce crowns of sword-shaped leaves. For Oxalis sepium, see the letter from Fritz Müller, 9 August 1881 and n. 2.


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


Quotes from a Fritz Müller letter of 9 Aug supporting CD’s views that leaves position themselves at night so as to minimise heat loss by radiation. It is a new fact to CD that leaves take different positions at different seasons.

Letter details

Letter no.
Charles Robert Darwin
Sent from
Source of text
Nature, 15 September 1881, p. 459

Please cite as

Darwin Correspondence Project, “Letter no. 13338,” accessed on 27 March 2023,