CHAP. VII. 



SLEEP OF LEAVES. 



371 



perpendicularly from the end of the petiole. The second pair 

 of leaflets likewise moves a little backwards, but less than the 

 terminal pair; and the third pair moves vertically downwards, 

 or even a little forwards. Thus all the leaflets, in those species 

 which bear only 3 or 4 pairs, tend to form a single packet, with 

 their upper surfaces in contact, and their lower surfaces turned 

 outwards. Lastly, the main petiole rises at night, but with 

 leaves of different ages to very different degrees, namely, some 

 rose through an angle of only 12°, and others as much as 41°. 



Cassia calUantha. — The leaves bear a large number of leaflets, 

 which move at night in nearly the same manner as just 

 described; but the petioles apparently do not rise, and one 

 which was carefully observed certainly fell 3°. 



Cassia pubescens. — The chief difference in the nyctitropio 



Fig. 155. 



Cbnta puiescens: A, upper part of plant during the day ; B, same plant 

 at night. Figures leduced fi'om photographs. 



movements of this species, compared with those of the former 

 species, consists in the leaflets not rotating nearly so much; 



