CHAP. VII. SUMMAEY ON SLEEP OF LEAVES. 399 



always asume at night a vertical, or nearly vertical 

 position, it is a point of complete indifference whether 

 the apex, or the base, or one of the lateral edges, is 

 directed to the zenith. It is a rule of wide generality, 

 than whenever there is any difference in the degree of 

 exposure to radiation between the upper and the lower 

 surfaces of leaves and leaflets, it is the upper which is 

 the least exposed, as may be seen in Lotus, Cytisus, 

 Trifolium, and other genera. In several species of 

 Lupinus the leaflets do not, and apparently from 

 their structure cannot, place themselves vertically at 

 night, and consequently their upper surfaces, though 

 highly inclined, are more exposed than the lower ; and 

 here we have an exception to our rule. But in other 

 species of this genus the leaflets succeed in placing 

 themselves vertically ; this, however, is effected by a 

 very unusual movement, namely, by the leaflets on 

 the opposite sides of the same leaf moving in opposite 

 directions. 



It is again a very common rule that when leaflets 

 come into close contact with one another, they do so 

 by their upper surfaces, which are thus best protected. 

 In some cases this may be the direct result of their 

 rising vertically ; but it is obviously for the pro- 

 tection of the upper surfaces that the leaflets of 

 Cassia rotate in so wonderful a manner whilst sinking 

 downwards ; and that the terminal leaflet of Melilotus 

 rotates and moves to one side until it meets the lateral 

 leaflet on the same side. When opposite leaves or 

 leaflets sink vertically down without any twisting, 

 their lower surfaces approach each other and some- 

 times come into contact; but this is the direct and 

 inevitable result of their position. With many species 

 of Oxalis the lower surfaces of the adjoining leaflets 

 are pressed together, and are thus better protected 

 18 



