Ivy grows two completely different kinds of leaf on the same plant
Down on the ground and up a wall, ivy produces lobed leaves on slender stems that grip with tiny aerial roots. Once a shoot reaches full sun high on a tree or cliff, it switches to thick, rootless, self-supporting stems with plain heart-shaped leaves, and only those adult shoots flower.
Hedera is a genus of roughly 12 to 15 evergreen woody climbers in the family Araliaceae, native from western Europe and northwest Africa across Asia to Japan and Taiwan. Left on flat ground it creeps at 5 to 20 cm high, but given trees, quarry faces or masonry it can climb to at least 30 m. Species differ in leaf shape, hairs and flower size, and even in chromosome counts, which run from a basic 48 up to 192 in octoploid forms.
Ivy matters to wildlife because it offers food when little else does. Its greenish-yellow flowers open in autumn and early winter and are packed with nectar; the ivy bee Colletes hederae times its whole life cycle to them, and common wasps visit eagerly. Berries ripen from late winter into spring and feed thrushes, blackcaps and woodpigeons, which spread the seeds. The small dusty wave moth feeds on nothing else, and the dense tangle shelters overwintering insects, nesting birds and small mammals.
Its reputation is mixed. The UK's Woodland Trust says the charge that ivy strangles trees is unfounded, though it can compete for light, water and nutrients and make old trees prone to windthrow. In the milder parts of western and southern North America, several species have become serious invaders of woodland and riverside habitats. The berries are moderately toxic to people, and the foliage contains falcarinol, which can cause contact dermatitis. The genus probably arose in East Asia, with the oldest fossil found in Korea from the Oligocene, and reached Europe in the Miocene.
People have given ivy plenty of meanings. Romans believed an ivy wreath prevented drunkenness, and Bacchus wore one. British taverns advertised with ivy bushes, which is why many pubs are still called The Ivy. Medieval Christians saw its evergreen clinging to dead trees as a sign of eternal life, and ivy-covered old buildings gave America's Ivy League its name. The American Ivy Society registers more than 400 cultivars.
Source: Hedera
