364 BOTANY [Chap. XI 



Letter 687 should be prevented by these storms to assume their usual 

 nocturnal position, just when nocturnal radiation was most to 

 be feared. It is rather strange, also, that Pkaseolus vulgaris 

 should not sleep during the early part of the summer, when 

 the leaves are most likely to be injured during cold nights. 

 On the contrary, it would not do any harm to many sub- 

 tropical plants, that their leaves must be well illuminated 

 during the day in order that they may assume at night a 

 vertical position ; for, in our climate at least, cold nights are 

 always preceded by sunny days. 



Of nearly allied plants sleeping very differently I can give 

 you some more instances. In the genus Olyra (at least, in 

 the one species observed by me) the leaves bend down verti- 

 cally at night ; now, in Endlicher's Genera plantarum this 

 genus immediately precedes StrepJiium, the leaves of which 

 you saw rising vertically. 



In one of two species of PJiyllantJius, growing as weeds 

 near my house, the leaves of the erect branches bend upwards 

 at night, while in the second species, with horizontal branches, 

 they sleep like those of PJiyllantJius Niruri or of Cassia. In 

 this second species the tips of the branches also are curled 

 downwards at night, by which movement the youngest leaves 

 are yet better protected. From their vertical nyctitropic 

 position the leaves of this PJiyllantJius might return to 

 horizontally, traversing 90 , in two ways, either to their own 

 or to the opposite side of the branch ; on the latter way no 

 rotation would be required, while on the former each leaf must 

 rotate on its own axis in order that its upper surface may be 

 turned upwards. Thus the way to the wrong side appears to 

 be even less troublesome. And indeed, in some rare cases I 

 have seen three, four or even almost all the leaves of one side 

 of a branch horizontally expanded on the opposite side, with 

 their upper surfaces closely appressed to the lower surfaces of 

 the leaves of that side. 



This PJiyllantJius agrees with Cassia not only in its manner 

 of sleeping, but also by its leaves being paraheliotropic. 1 Like 

 those of some Cassice its leaves take an almost perfectly 

 vertical position, when at noon, on a summer day, the sun is 



1 Paraheliotropism is the movement by which some leaves temporarily 

 direct their edges to the source of light. See Movements of Plants, 

 p. 445- 



