560 SUMMARY AND CHAP. Xll 



tating movements those modified through external 

 agencies. The so-called sleep or nyctitropic move- 

 ments of leaves are determined by the daily alterna- 

 tions of light and darkness. It is not the darkness 

 which excites them to move, but the difference in the 

 amount of light which they receive during the day 

 and night ; for with several species, if the leaves have 

 not been brightly illuminated during the day, they 

 do not sleep at night. They inherit, however, some 

 tendency to move at the proper periods, indepen- 

 dently of any change in the amount of light. The 

 movements are in some cases extraordinarily complex, 

 but as a full summary has been given in the chapter 

 devoted to this subject, we will here say but little on 

 this head. Leaves and cotyledons assume their noc- 

 turnal position by two means, by the aid of pulvini and 

 without such aid. In the former case the movement 

 continues as long as the leaf or cotyledon remains in 

 full health ; whilst in the latter case it continues only 

 whilst the part is growing. Cotyledons appear to 

 sleep in a larger proportional number of species than 

 do leaves. In some species, the leaves sleep and not 

 the cotyledons ; in others, the cotyledons and not the 

 leaves ; or both may sleep, and yet assume widely 

 different positions at night. 



Although the nyctitropic movements of leaves and 

 cotyledons are wonderfully diversified, and sometimes 

 differ much in the species of the same genus, yet the 

 blade is always placed in such a position at night, that 

 its upper surface is exposed as little as possible to full 

 radiation. We cannot doubt that this is the object 

 gained by these movements ; and it has been proved 

 that leaves exposed to a clear sky, with their blades 

 compelled to remain horizontal, suffered much more 

 from the cold than others which were allowed to assume 



