Some of them follow the sun's apparent course, that is, from east to west,--and always twine around the stem which supports them in the direction of left to right. Such is the case with the common black briony, so common in our woods and groves. Others invariably twine _contrary_ to the sun, or from right to left; as is the case with the convolvulus, or large white bindweed. This singular tendency, be it observed, is always constant in each individual of the species, and if you endeavour to train one of these plants in a different direction, you will infallibly kill it. The convolvulus will not grow from left to right, and the black briony will not grow from right to left. _Crede experto._ The convolvulus, or white bindweed (_Convolvulus sepium_, or _Calystegia sepium_), is one of the most elegant, though one of the commonest climbing plants which festoon our willows, or creep over our grassy banks, or wind in and about our hedges. Its large white bells, which the country people unpoetically call "old men's nightcaps," are remarkable for their purity of hue and exquisite beauty of outline; and the leaves, which are heart-shaped, equally claim our admiration. Like the pink field-convolvulus (_Convolvulus arvensis_), or the rosy-hued seaside bindweed (_Calystegia soldanella_), it is very tenacious of life, and if it once secures a footing, is eradicated with difficulty. Hence it is dearer to the poet and artist than to the farmer and gardener, each of whom pursues it with a determined hostility. The Convolvulaceæ form a distinct family or order, containing forty-five genera, and upwards of seven hundred species. They are found in temperate and tropical countries; and include the dodder, sweet potato, scammony, spomœa, and the jalap plant. METAMORPHOSIS.--A PHYSICO-PHILOSOPHICAL MEDITATION.[86] If we are to understand by the term _metamorphosis_ simply "a change," it is evident that everybody undergoes metamorphosis, is changed or transformed; nothing _is_, all _becomes_. The water which flows on for ever, but never twice washes the same pebbly bed, will afford us an apt image of this perpetual "to become."