Leaves Injured at Night by Free Radiation^). 



(From a letter to Ch. Darwin). 



Fritz Müller, in a letter to me from St. Catharina in Brazil, dated August g, 

 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. 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. 



I) Nature 1881. Bd. XXIV. p. 4.59. 



