442 MODHIED CIECUMNUTATION. Chap. Vm 



tlian can the nyctitropic movements of cotyledons 

 and leaves. In the latter case they place themselves 

 so that their upper surfaces may radiate at night 

 as little as possible into open space, with the upper 

 surfaces of the opposite leaflets often in contact. These 

 movements, which are sometimes extremely complex, 

 are regulated, though not directly caused, by the alter- 

 nations of light and darkness. In the case of diahelio- 

 tropism, cotyledons and leaves place themselves so 

 that their upper surfaces may be exposed to the light, 

 and this movement is regulated, though not directly 

 caused, by the direction whence the light proceeds. In 

 both cases the movement consists of circumnutation 

 modified by innate or constitutional causes, in the 

 same manner as with climbing plants, the circumnu- 

 tation of which is increased in amplitude and rendered 

 more circular, or again with very young cotyledons 

 and leaves which are thus brought down into' a hori- 

 zontal position by epinasty. 



We have hitherto referred only to those leaves and 

 cotyledons which occupy a permanently horizontal 

 position ; but many stand more or less obliquely, and 

 some few upright. The cause of these differences of 

 position is not known ; but in accordance with Wiesner's 

 views, hereafter to be given, it is probable that some 

 leaves and cotyledons would suffer, if they were fully 

 illuminated by standing at right angles to the light. 



We have seen in the second and fourth chapters 

 that those cotyledons and leaves which do not alter 

 their positions at night sufficiently to be said to sleep, 

 commonly rise a little in the evening and fall again 

 on the next morning, so that they stand during the 

 night at a rather higher inclination than during the 

 middle of the day. It is incredible that a rising' 

 movement of 2° or 3°, or even of 10° or 20°, can be of 



