402 MODIFIED CI11CUMNUTATION. CHAP. VII. 



of the petioles are not performed for any special pur- 

 pose; though a conclusion of this kind is generally 

 rash. When the leaflets sink vertically down at night 

 and the petioles rise, as often occurs, it is certain that 

 the upward movement of the latter does not aid the 

 leaflets in placing themselves in their proper posi- 

 tion at night, for they have to move through a 

 greater angular space than would otherwise have been 

 necessary. 



Notwithstanding what has just been said, it may be 

 strongly suspected that in some cases the rising of 

 the petioles, w r hen considerable, does beneficially serve 

 the plant by greatly reducing the surface exposed to 

 radiation at night. If the reader will compare the 

 two drawings (Fig. 155, p. 371) of Cassia pubescens, 

 copied from photographs, he will see that the dia- 

 meter of the plant at night is about one-third of 

 what it is by day, and therefore the surface exposed 

 to radiation is nearly nine times less. A similar 

 conclusion may be deduced from the drawings (Fig. 

 149, p. 358) of a branch awake and asleep of Des- 

 modium gyrans. So it was in a very striking manner 

 with young plants of Bauhinia, and with Oxalis 

 Ortegesii. 



We are led to an analogous conclusion with respect 

 to the movements of the secondary petioles of certain 

 pinnate leaves. The pinnae of Mimosa pudica con- 

 verge at night; and thus the imbricated and closed 

 leaflets on each separate pinna are all brought close 

 together into a single bundle, and mutually protect 

 one another, with a somewhat smaller surface exposed 

 to radiation. With Albizzia lopliantha the pinnae close 

 together in the same manner. Although the pinnae 

 of Acacia Farnesiana do not converge much, they 

 sink downwards. Those of Neptunia oleracea likewise 



