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 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, their 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 [page 400] than the upper surfaces; but this depends merely on each leaflet becoming folded at night so as to be able to sink vertically downwards. The torsion or rotation of leaves and leaflets, which occurs in so many cases, apparently always serves to bring their upper surfaces into close approximation with one another, or with other parts of the plant, for their mutual protection. We see this best in such cases as those of Arachis, Mimosa albida, and Marsilea, in which all the leaflets form together at night a single vertical packet. If with Mimosa pudica the opposite leaflets had merely moved upwards, their upper surfaces would have come into contact and been well protected; but as it is, they all successively move towards the apex of the leaf; and thus not only their upper surfaces are protected, but the successive pairs become imbricated and mutually protect one another as well as the petioles. This imbrication of the leaflets of sleeping plants is a common phenomenon.
The nyctitropic movement of the blade is generally effected by the curvature of the uppermost part of the petiole, which has often been modified into a pulvinus; or the whole petiole, when short, may be thus modified. But the blade itself sometimes curves or moves, of which fact Bauhinia offers a striking instance, as the two halves rise up and come into close contact at night. Or the blade and the upper part of the petiole may both move. Moreover, the petiole as a whole commonly either rises or sinks at night. This movement is sometimes large: thus the petioles of Cassia pubescens stand only a little above the horizon during the day, and at night rise up almost, or quite, perpendicularly. The petioles of the younger leaves of Desmodium gyrans also rise up vertically at night. On the other hand, with Amphi- [page 401] carpaea, the petioles of some leaves sank down as much as 57o at night; with Arachis they sank 39o, and then stood at right angles to the stem. Generally, when the rising or sinking of several petioles on the same plant was measured, the amount differed greatly. This is largely determined by the age of the leaf: for instance, the petiole of a moderately old leaf of Desmodium gyrans rose only 46o, whilst the young ones rose up vertically; that of a young leaf of Cassia floribunda rose 41o, whilst that of an older leaf rose only 12o. It is a more singular fact that the age of the plant sometimes influences greatly the amount of movement; thus with some young seedlings of a Bauhinia the petioles rose at night 30o and 34o, whereas those on these same plants, when grown to a height of 2 or 3 feet, hardly moved at all. The position of the leaves on the plant as determined by the light, seems also to influence the amount of movement of the petiole; for no other cause was apparent why the petioles of some leaves of Melilotus officinalis rose as much as 59o, and others only 7o and 9o at night.
In the case of many plants, the petioles move at night in one direction and the leaflets in a directly opposite one. Thus, in three genera of Phaseoleae the leaflets moved vertically downwards at night, and the petioles rose in two of them, whilst in the third they sank.