Ivy

Today’s species is very common and probably overlooked by most of us as a slightly irritating, uninteresting plant that seems to grow everywhere. But there's more to ivy than meets the unobservant eye. 
Ivy is the only species of the Araliaceae family found naturalised in the British Isles and two subspecies exist: Hedera helix ssp helix which is shown here and is the commoner of the two; and H. h. ssp hibernica which is known as atlantic ivy. The latter, as the name suggests, is the prevalent subspecies in Ireland and in the west and south-west of Britain. It differs in having slightly larger leaves which are less deeply palmate with yellowish hairs (as opposed to whitish) on the underside.

Ivy does a weird thing: the leaves of young, non-flowering stems are different to those on the mature flowering shoots. The latter 'lose' their lobes, becoming simple oval or elliptic and fooling the unwary into thinking it must be a different species. 
Ivy flowers late in the year, from September to December, with heavily scented and nectared flowers with five yellow-green petals in umbels at the topmost part of the plant where there is plenty of light. This makes it a star attraction for many insects at a time when few other food plants are available. 
Ivy is also important as it provides year-round cover for nesting and roosting birds and insects on account of its evergreen nature.
In this country it is generally thought that ivy doesn't harm a tree if it

climbs up and around it. This is at odds with one's impression when finding an 'ivy tod'. These are trees which are so smothered in ivy that it appears that there never was anything but ivy growing. They occur when the scaffold tree dies, leaving a self-supporting ivy plant masquerading as a tree in its own right.
There is a huge amount of folklore about ivy and it has one of the longer entries in Richard Mabey's fabulous Flora Britannica.