400 MODIFIED CIRCUMNUTATION. CHAP. VII. 



than the upper surfaces ; but this depends merely on 

 each leaflet becoming folded at night so as to be able 

 to sink vertically downwards. The torsion or rotation 

 of leaves and leaflets, which occurs in so many cases, 

 apparently always serves to bring their upper surfaces 

 into close approximation with one another, or with 

 other parts of the plant, for their mutual protection. 

 We see this best in such cases as those of Arachis, 

 Mimosa albida, and Marsilea, in which all the leaflets 

 form together at night a single vertical packet. If 

 with Mimosa pudica the opposite leaflets had merely 

 moved upwards, their upper surfaces would have come 

 into contact and been well protected ; but as it is, 

 they all successively move towards the apex of the 

 leaf ; and thus not only their upper surfaces are pro- 

 tected, but the successive pairs become imbricated and 

 mutually protect one another as well as the petioles. 

 This imbrication of the leaflets of sleeping plants is a 

 common phenomenon. 



The nyctitropic movement of the blade is gene- 

 rally effected by the curvature of the uppermost part 

 of the petiole, which has often been modified into a 

 pulvinus ; or the whole petiole, when short, may be 

 thus modified. But the blade itself sometimes curves 

 or moves, of which fact Bauhinia offers a striking 

 instance, as the two halves rise up and come into 

 close contact at night. Or the blade and the upper 

 part of the petiole may both move. Moreover, the 

 petiole as a whole commonly either rises or sinks at 

 night. This movement is sometimes large : thus the 

 petioles of Cassia pubescens stand only a little above 

 the horizon during the day, and at night rise up 

 almost, or quite, perpendicularly. The petioles of the 

 younger leaves of Desmodium gyrans also rise up ver- 

 tically at night. On the other hand, with Amphi- 



