(24.) These trees also present in their leaves the same difference that is to be observed in those of all the rest: the underside, which looks towards the ground, is of a green, grassy colour, and has a smooth surface;[2442] while the veins, the callous skin, and the articulations, lie upon the upper face, the veins making incisions in the parts beneath, like those to be seen upon the human hand. The leaf of the olive is whiter above, and not so smooth; the same is the case, too, with that of the ivy. The leaves of all trees turn[2443] every day towards the sun, the object being that the under side may be warmed by its heat. The upper surface of them all has a down upon it, in however small quantity it may be; in some countries this down is used as a kind of wool.[2444] CHAP. 37.—THE CARE BESTOWED ON THE LEAVES OF THE PALM, AND THE USES TO WHICH THEY ARE APPLIED. We have already said[2445] that in the East strong ropes are made of the leaves of the palm, and that they are improved by lying in the water. Among ourselves, too, the leaves of the palm are generally plucked immediately after harvest, the best being those that have no divisions in them. These leaves are left to dry under cover for four days, after which they are spread out in the sun, and left out in the open air all night, till they have become quite white and dry: after this they are split before they are put to any use. CHAP. 38.—REMARKABLE FACTS CONNECTED WITH LEAVES. The broadest leaves are those of the fig, the vine, and the plane; while those of the myrtle, the pomegranate, and the olive are narrow. The leaf of the pine and the cedar is fine and resembles hair, while that of the holly and one variety of the holm oak[2446] is prickly—indeed, in the juniper, we find a thorn in place of a leaf. The leaf of the cypress and the tamarisk[2447] is fleshy, and that of the alder is remarkable for its thickness.[2448] In the reed, the willow, and the palm,[2449] the leaf is long, and in the latter tree it is double as well: that of the pear is rounded, and it is pointed in the apple.[2450] In the ivy the leaf is angular, and in the plane divided.[2451] In the pitch-tree[2452] and the fir the leaf is indented like the teeth of a comb; while in the robur it is sinuous on the whole of the outer margin: in the bramble it has a spiny surface. In some plants the leaf has the property of stinging, the nettle for instance; while in the pine,[2453] the pitch-tree, the fir, the larch, the cedar, and the holly, it is prickly. In the olive and the holm-oak it has a short stalk, in the vine a long one: in the poplar the stalk of the leaf is always quivering,[2454] and the leaves of this tree are the only ones that make a crackling noise[2455] when coming in contact with another.