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Show 398 1\IODIFIED CIRCUMNUTATION. CHAP. VII. must extend the same conclusion to a largo number of sleeping plants; for the most complicate<l and the simplest nyctit.ropic movements are connected together by the finest gradations. But owing to tho causes spe· cified in the beginning of this chapter, it is impossible in some few cases to determine whether or not certain movements should be called. nyctitropic. Generally, the position which the leaves occupy at night indicates with sufficient clearness, that the benefit thus derived, is the protection of their upper surfaces from radiation into the open sky, and in many cases the mutual protection of all the parts from cold by their being brought into close approximation. It should be remembered that it was proved in the last chapter, that leaves compelled to remain extended horizontally at nio·ht suffered much more from radiation than those b ' which were allowed to assume their normal vertical ·position. The fact of the leaves of several plants not sleeping unless they have been well illuminated during the .day, made us for a time doubt whet~e~ the pr?· tection of their upper surfaces from radmtwn was m all cases the final cause of their well-pronounced nyctitropi~ movements. But we have no reason to suppose that the illumination from the op~n sky, during even the most clouded day, is insufficwnt fur this purpose ; and we should bear in mind that leaves ·which are shaded from beino- seated. low down on the plant, and which sometimes bdo not sleep, are likewise protected at night from full radiation. N everthele~s, we do not wish to deny that there may exist cases m which leaves change their position considerably at night, without their deriving any benefit from such movements. t Although with sleeping plants the blades almos CHAP. VII. SUMl\IARY ON SLEEP OF LEAVES. 399 always assume 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 iu 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 tho direct result of their rising vertically; but it is obviously for the protection 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, t~eir lower surfaces approach each other and sometimes 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 |