Movements of Plants. 86 ^ 



ches of this Phyllanthus curl downwards at night, and thus the youngest leaves 

 are still better protected from radiation. 



The leaves of some plants, when brightl}^ illuminated, direct their edges to- 

 wards the light; and this remarkable movement I have caUed paraheliotropism. 

 Fritz Müller informs me that the leaflets of the Phyllanthus just referred to, 

 as well as those of some Brazilian Cassiae, "take an almost perfectly vertical 

 Position, when at noon, on a summer day, the sun is nearly in the zenith. To- 

 day the leaflets, though continuing to be fully exposed to the sun, now at 3 p. m. 

 have already returned to a nearly horizontal position". F. Müller doubts whether 

 so strongly marked a case of paraheliotropism would ever be observed under the 

 duller skies of England; and this doubt is probably correct, for the leaflets of 

 Cassia neglecta, on plants raised from seed formerly sent me by him, moved in 

 this manner, but so slightly that I thought it prudent not to give the case. With 

 several species of Hedychium, a widely different paraheliotropic movement occurs, 

 which may be compared with that of the leaflets of Oxalis and Averrhoa; for 

 "the lateral halves of the leaves, when exposed to bright sunshine, bend down- 

 wards, so that they meet beneath the leaf". 



Down, Beckenham, February 22. Charles Darwin. 



