Movements of Plauts ^f). 



(From a letter to Charles Darwin.) 



Fritz Müller, in a letter from St. Catharina, Brazil, dated January 9, has given 

 me some remarkable facts about the movements of plants. He has observed 

 striking instances of allied plants, which place their leaves verticall}^ at night, by 

 widely different movements ; and this is of interest as supporting the conclusion 

 at which my son Francis and I arrived, namety, that leaves go to sleep in order 

 to escape the füll effect of radiation. In the great family of the Graminea? the 

 species in one genus alone, namely Strephium, are known to sleep, and this the)- 

 do by the leaves moving vertically upwards; but Fritz Müller finds in a species 

 of Olyra, a genus which in Endlicher's "Genera Plantarum" immediatel)^ precedes 

 Strephium, that the leaves bend vertically down at night. 



Two species of Phyllanthus (Euphorbiacese) grow as weeds near Fritz Müller's 

 house; in one of them with erect branches the leaves bend so as to stand ver- 

 tically up at night. In the other species with horizontal branches, the leaves 

 move vertically down at night, rotating on their axes, in the same manner as do 

 those of the Leguminous genus Cassia. Owing to this rotation, combined with 

 the sinking movement, the upper surfaces of the opposite leaflets are brought 

 into contact in a dependent position beneath the main pctiole ; and they are thus 

 excellently protected from radiation, in the manner dcscribed b}^ us. On the foUo- 

 wing morning the leaflets rotate in an opposite direction, whilst rising so as to 

 resume the diurnal horizontal position with their upper surface exposed to the 

 light. Now in some rare cases Fritz Müller has observed the extraordinär}^ fact 

 that three or four, or even almost all the leaflets on one side of a leaf of this 

 PhyUanthus rise in the morning from their nocturnal vertically dependent position 

 into a horizontal one, without rotating, and on the wrong side of the main petiolc. 

 These leaflets thus project horizontally with their upper svirfaces dirccted towards 

 the sky, but partly shaded by the leaflets proper to this side. I have never be- 

 fore heard of a plant appearing to make a mistake in its movements: and the 

 mistake in this instance is a great one, for the leaflets move 90*^ in a direction 

 opposite to the proper one. Fritz Müller adds that the tips of the horizontal bran- 



1) Nature 1880/81. Bd. XXIII. p. 409. 



2) Siehe auch Ges. Schriften S. 874. 



