CHAP. 36.—LEAVES WHICH TURN ROUND EVERY YEAR.
We find a most remarkable and, indeed, a marvellous peculiarity2441 existing in the elm, the lime, the olive, the white poplar, and the willow; for immediately after the summer solstice the leaves of these trees turn completely round; indeed, we have no sign which indicates with greater certainty that that period has past.
(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 turn2443 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