CH. VIll] PARAHELIOTROPISM. 207 



called "diurnal sleep" but is now known as parahelio- 

 tropism. Oxalis acetosella, in which the leaves in bright 

 sun assume the same vertically dependent position that they 

 take at night, is a familiar example. Averrhoa bilimM 

 (one of the Oxalidse) also drops its leaflets in sunshine 

 as it does at night. The leaves of Averrhoa, as described 

 in exp. 261, exhibit remarkable autonomous movements", 



FiQ. 41. Exp. 257. 



in which the leaflets drop rapidly through 15° — 20°, 



then rise slowly to their original position, repeating the 



movement once in 15 minutes or so. When sunshine 



strikes the plant the leaflets fall until they make an angle 



of 70° or 80° below the horizon, but this is not in a single 



> Lynch in Linnean Soc. Journal, xvi. p. 231 ; Power of Movement in 

 Plants, p. 330. 



