DARWIN ON THE MOVEMENTS OF PLANTS. 505 



Re is the right, and Lc the left one, stand directly opposite one 

 another, and the first true leaf (7^") projects at right angles to them. 

 At night (see II and III) the right cotyledon {Re) is greatly raised, 

 but is not otherwise .changed in position. The left cotyledon {Lc) 

 is likewise raised, but it is also twisted so that its blade, instead of 

 exactly facing the opposite one, now stands at nearly right angles to 

 it. This nocturnal twisting movement is effected by the twisting of 

 the whole length of the petiole. At the same time the true leaf {F) 

 rises up vertically, or even inclines inward. It also twists a little, 

 so that the upper surface of its blade fronts the upper surface of the 

 twisted left cotyledon. The whole case is remarkable, as with the 

 cotyledons of no other plant have we seen any nocturnal movement 

 except vertically upward or downward." 



The various ways in which the leaves of plants are protected from 

 loss of heat by radiation at night are shown by diagrams and pictures 

 from which we select some of the most striking. A good deal of space 

 is given to an account of the circumnutation and nyctitropic move- 

 ments of the Oxalidce. In most of the species of oxalis the three leaf- 

 lets sink vertically down at night. But, as their sub-petioles are short, 

 the blades could not assume this position from want of space, unless 

 they were in some manner rendered narrower, and this is effected by 

 their becoming more or less folded (Fig. 6), so that their lower sur- 

 faces are brought near together (see B), as if the object were their 

 protection rather than that of the upper surface. This would form a 

 marked exception to the rule, that the object of sleep is protection of 

 the upper surfaces from radiation, if it had not been found that, in 

 species where the sub-petioles are longer, the leaflets sink without 

 folding together. By thus crowding together at night, a much smaller 

 surface is exposed than during the day. 



i! 



A. B. 



Fig. 7. Medicago marina : A, leaves during the day ; B, leaves asleep at night. 



" The drawing of Medicago marina, awake and asleep (Fig. 7), 

 answers almost as well for Gytisus fragrayis, which rose at night on 

 one occasion 23 and on another 33. The three leaflets also bend up- 

 ward, and at the same time approach each other so that the base of 

 the central leaflet overlaps the bases of the two lateral leaflets. They 



