CHAP. VII. SUMMARY ON SLEEP OF LEAVES. 401 



carpaea, the petioles of some leaves sank down as 

 much as 57 at night; with Arachis they sank 39, 

 and then stood at right angles to the stem. Gene- 

 rally, when the rising or sinking of several petioles on 

 the same plant was measured, the amount differed 

 greatly. This is largely determined by the age of the 

 leaf : for instance, the petiole of a moderately old leaf 

 of Desmodium gyrans rose only 46, whilst the young 

 ones rose up vertically ; that of a young leaf of Cassia 

 floribunda rose 41, whilst that of an older leaf rose 

 only 12. It is a more singular fact that the age of 

 the plant sometimes influences greatly the amount of 

 movement ; thus with some young seedlings of a Bau- 

 hinia the petioles rose at night 30 and 34, whereas 

 those on these same plants, when grown to a height 

 of 2 or 3 feet, hardly moved at all. The position of 

 the leaves on the plant as determined by the light, 

 seems also to influence the amount of movement 

 of the petiole; for no other cause was apparent 

 why the petioles of some leaves of Melilotus officinalis 

 rose as much as 59, and others only 7 and 9 at 

 night. 



In the case of many plants, the petioles move at 

 night in one direction and the leaflets in a directly 

 opposite one. Thus, in three genera of Phaseolese the 

 leaflets moved vertically downwards at night, and the 

 petioles rose in two of them, whilst in the third they 

 sank. Species in the same genus often differ widely 

 in the movements of their petioles. Even on the same 

 plant of Lupinus pubescens some of the petioles rose 30, 

 others only 6, and others sank 4 at night. The 

 leaflets of Cassia Barclayana moved so little at night 

 that they could not be said to sleep, yet the petioles 

 of some young leaves rose as much as 34. These 

 several facts apparently indicate that the movements 



