Chap. YII. MODIFIED CIECUMNUTATION. 413 



and cotyledons, is merely a modification of their ordi- 

 nary circumnutating movement, regulated in its period 

 and amplitude by the alternations of light and dark- 

 ness. The object gained is the protection of the upper 

 surfaces of the leaves from radiation at night, (iftea 

 combined with the mutual protection of the several 

 parts by their close approximation. In such -cases as 

 those of the leaflets of Cassia — of the terminal leaflets 

 of Melilotus — of all the leaflets of Arachis, Marsilea, 

 &c. — we have ordinary circumnutation modified to the 

 extreme extent known to us in any of the several great 

 classes of modified circumnutation. On this view of 

 the origin of nyctitropism we can understand how it 

 is that a few plants, widely distributed throughout the 

 Vascular series, have been able to acquire the habit of 

 placing the blades of their leaves vertically at night, 

 that is, of sleeping, — a fact otherwise inexplicable. 



The leaves of some plants move during the day in 

 a manner, which has improperly been called diurnal 

 sleep ; for when the sun shines brightly on them, they 

 direct their edges towards it. To such cases we shall 

 recur in the following chapter on Heliotropism. It 

 has been shown that the leaflets of one form of 

 Porlieria hygrometrica keep closed during the day, as 

 long as the plant is scantily supplied with water, in 

 the same manner as when asleep ; and this apparently 

 serves to check evaporation. There is only one other 

 analogous case known to us, namely, that of certain 

 Graminese, which fold inwards the sides of their narrow 

 leaves, when these are exposed to the sun and to a 

 dry atmosphere, as described by Duval-Jouve.* We 

 have also observed the same phenomenon in Elymus 

 arenareus. 



* - Annal. des Sn. Nat. (Bot.).' 1875, torn. i. pp. 32e -329. 



