If you look closely at ivy on an old oak, you might notice two quite different leaf shapes. The lower climbing shoots have the familiar lobed, star shaped leaves. Higher up, near the light, the leaves suddenly turn broader, smoother and more oval. It can look like two different climbers jostling for space. In fact it is one plant changing form with age.
Botanically this is Hedera helix, known in French as Le Lierre and in Dutch as Klimop. The English name comes from Old English ifig, probably linked to words meaning to grasp or cling, which it certainly does. Helix in Latin simply means spiral or winding, a good description of its growth around trunks.
This shape shifting has a scientific name: heteroblasty. Ivy begins life in a juvenile form, creeping and clinging with lobed leaves. Once it reaches steady support and decent light, it shifts into its adult, fertile phase, producing round green berries and those plain looking leaves. Birds rely on the berries in late winter, when almost nothing else is available, which makes ivy more helpful than its reputation suggests.
Ivy is not a parasite and does not draw food from the oak. It uses the bark as scaffolding, nothing more. A healthy oak usually takes no harm at all. In the shaded lower forest, the juvenile ivy gathers what little light it can. High on an old trunk, in a patch of sun, it produces flowers and fruits, hence the change of shape.
So next time you see two kinds of leaves on what looks like a single tangle, you will know that it is just ivy growing up, and showing us one of the clever botanical tricks that plants use to survive in the layered light of the Forêt de Soignes.