The movements of Leaves^). 



(From a letter to Charles Darwin.) 



Fritz Müller has sent mo some additional observations on the movements 

 of leaves, when exposed to a bright light. Such movements seem to be as well 

 developed and as diversified under the bright sun of Brazil, as are the well- 

 known sleep or nyctitropic movements of plants in all parts of the world. This 

 result has interested me much, as I long doubted whcther paraheliotropic move- 

 ments were common enough to deserve to be separately designated. It is a 

 remarkable fact that in certain species these movements closely resemble the sleep 

 movements of allied forms. Thus the leaflets of one of the Brazilian Cassiae 

 assume when exposed to sunshine nearly the same position as those of the not 

 distantly allied Hsematox3don when asleep, as shown in Fig. 153 of "The Mo- 

 vements of Plants". Whereas the leaflets of this Cassia sleep by moving down 

 and rotating on their axcs, in the same peculiar manner as in so many other 

 species of the genus. Again, with an unnamed species of Phyllanthus, the leaves 

 move forwards at night, so that their midribs then stand nearly parallel to the 

 horizontal branches from which they spring; but when they are exposed to bright 

 sunshine they rise up vertically, and their upper surfaces come into contact, as 

 they are opposite. Now this is the position which the leaves of another species, 

 namely Phyllanthus compressiis, assume when they go to sleep at night. Fritz 

 Müller States that the paraheliotropic movements of the leaves of a Mucuna, a 

 large twining Papilionaceous plant, are stränge and inexplicable ; the leaflets sleep 

 by hanging vertically down, but under bright sunshine the petiole rises vertically 

 up, and the terminal leaflet rotates by means of its pulvinus through an angle of 

 180", and thus its upper surface Stands on the same side with the lower surfaces 

 of the lateral leaflets. Fritz Müller adds, "I do not understand the meaning of 

 this rotation of the terminal leaflet. as even without such a movement it would be 

 apparentiy equally well protected against the rays of the sun. The leaflets, also 

 on many of the leaves on the same plant assume various other stränge positions". 

 With one species of Desmodium, presentl}^ to be mentioned as sleeping in a 

 remarkable manner, the leaflets rise up vertically when exposed to bright sun- 

 shine, and the upper surfaces of the lateral leaflets are thus brought into contact. 



I) Nature i88o/8i. Bd. XXIII. p. 603. 



