CHAP. VI. SLEEP MOVEMENTS. 281 



between the sleep of animals and that of plants,* 

 whether of leaves or flowers. It seems, therefore, 

 advisable to give a distinct name to the so-called 

 sleep-movements of plants. These have also generally 

 been confounded, under the term " periodic," with the 

 slight daily rise and fall of leaves, as described in the 

 fourth chapter ; and this makes it all the more desir- 

 able to give some distinct name to sleep-movements. 

 Nyctitropism and nyctitropic, i.e. night-turning, may 

 be applied both to leaves and flowers, and will be 

 occasionally used by us ; but it would be best to con- 

 fine the term to leaves. The leaves of some few plants 

 move either upwards or downwards when the sun shines 

 intensely on them, and this movement has sometimes 

 been called diurnal sleep ; but we believe it to be of 

 an essentially different nature from the nocturnal 

 movement, and it will be briefly considered in a 

 future chapter. 



The sleep or nyctitropisin of leaves is a large 

 subject, and we think that the most convenient plan 

 will be first to give a brief account of the position 

 which leaves assume at night, and of the advantages 

 apparently thus gained. Afterwards the more re- 

 markable cases will be described in detail, with 

 respect to cotyledons in the present chapter, and to 

 leaves in the next chapter. Finally, it will be shown 

 that these movements result from circumnutatiou, 

 much modified and regulated by the alternations of 

 day and night, or light and darkness ; but that they 

 are also to a certain extent inherited. 



Leaves, when they go to sleep, move either upwards 

 or downwards, or in the case of the leaflets of com- 



* Ch. Royer must, however, bo Nat.' (5th series), But. voL iz 



exempted ; see * Annalcs des So. 18G8, p. 378. 



