Not to be confused with: Poison Ivy, a non-native climber/small shrub which is highly toxic (and which does not, as far as your Author is aware, grow in the UK). It can cause severe and damaging blisters when touched.
Lookee-Likees : several other Ivies, most of which are non-native and escaped from gardens.
No relation to : Ivy Broomrape, Ground-Ivy,
Ivy-Leaved Crowfoot or Ivy-Leaved Bellflower [plants with similar names]
The sap of Ivy, if it comes into contact with the skin, can cause dermatitis, sometimes with severe blistering and inflammation.
The flowers of Ivy make a useful contribution with providing out-of-season nectar to butterflies in late autumn. This is especially important in really wet and cool years when flowers are not in great abundance, such as the record-breaking wet year of 2012 (which was drought-ridden until April, then it almost never stopped raining, flooding huge areas time and time again, month by month).
Although it has 'roots' which readily attach themselves to substrates such as fences and trees, the 'roots' do not draw sustenance from the tree it climbs: it is not parasitic. And, contrary to common thinking, it will not strangle trees and prevent them from growing; far from it, it avoids going anywhere near the smaller branches where the leaves of the tree harvest sunlight.
Falcarinols are naturally occurring acetylenic alcohols (note the two highly energetic triple bonds, extremely unusual in the natural world) present in (English) Ivy. They are strongly fungicidal, a natural defence from invading organisms. In their fungicidal role, they often function as phytoalexins, the mammalian equivalent being anti-bodies. In Ivy, they are both allergenic and irritant to humans. Another
polyyne) is also present:
11-dehydrofalcarinol. Falcarinol (aka Panaxynol) is present in Ivy and in Hemlock Water-Dropwort (Oenanthe crocata) and causes contact dermatitis in certain individuals when the plants are handled.
Falcarinols also occur in the roots of carrots, their concentration increasing with prolonged storage. In carrots they can cause allergenic contact dermatitis, an occupational hazard for carrot pickers. Because of the highly reactive triple-bonds, polyacetylenes such as these are very toxic.
The flowers of Ground-Elder contain the highest concentration of falcarindiol, up to 9% by weight. Falcarindiol has been shown to possess anti-cancer activity. Some other umbellifers also contain compounds containing triple-bonded carbon atoms.
polyacetylenes, or more correctly, polyynes, are responsible for the contact dermatitis that may result from touching and handling Ivy, or any other plant containing them, such as the roots of
Water Hemlock (Oenanthe crocata)
Hederagenin is a saponin and pre-cursor to many of the compounds present in Ivy, some of which are shown below. It is not a main component of Ivy. Bayogenin is almost identical except that an extra 'OH' is added on the carbon atom just above the left-most 'HO-' group.