IVY is an extraordinary plant. I was in a mixed-species wood the other day and I started to notice how selective it is about the species of tree it chooses to climb. In this wood its favourite was ash and its least favourite was holly.
The reason for this is pretty obvious. Ash is the last native Irish tree to get its leaves in the spring and one of the first to lose them in the autumn. Plants in a wood are engaged in a ceaseless battle for light so by selecting out ash trees to climb the evergreen ivy gets the maximum number of weeks in the year for uninterrupted photosynthesis.
Holly, on the other hand, is also evergreen and tends to have very dense foliage which doesn’t suit the ivy at all. In between these two extremes there seem to be grades of preference based on a mathematical equation that balances foliage density with the number of weeks in the year that the leaves are on the tree.
But how does the ivy do this? How does it know with such precision which trunk to climb up? It could be just trial and error. It could be that the ivy climbs up the nearest available vertical object and that the plants that are lucky enough to get it right survive and flourish while the rest don’t.
But I think there’s more to it. I think that over millions of years ivy has evolved some mechanism for selecting the correct tree.
One of the reasons I believe this is comes from observing the behaviour of ivy on telephone and ESB poles. Very occasionally you’ll see a pole totally smothered in ivy. This is natural because a pole is the ideal ‘tree’ for ivy to climb because it never develops any leaves.
But the vast majority of poles, even when they’re in the middle if ivy-rich hedges, have no ivy at all. ESB and telephone poles have not been around for millions of years and I believe that, with the odd exception, ivy does not ‘recognise’ them for the good thing they are.
There was some honeysuckle in the wood too. It’s similar to ivy in that it has become a climber instead. This is a good decision because being a tree involves investing a huge amount of resources in growing a trunk strong enough to support your leaves and raise them up to that all-important light.
Honeysuckle is not as vigorous or as fast growing as ivy and in the competitive world of this wood it seems to have been relegated to the second best climbing trees. I found none of it clinging to ash trees but some vines struggling to make a living on smaller hawthorn and crab apple trees, though honeysuckle has the capacity to grow as tall as ivy.
But one thing that puzzles me about ivy is why it has two kinds of leaves. Juvenile leaves have five lobes and are dark green. But fertile flowering stems in full sun have totally different leaves —- long and pale green with no lobes.
There are other plants that do this too —- most eucalyptus species, for example. And holly leaves above three metres are generally without spines.
I can understand why the holly does this —- the spines are a defence against browsing animals and, in the absence of giraffes, this is not needed above three metres. But I don’t understand what evolutionary advantage ivy gets from having two types of leaf.
* dick.warner@examiner.ie
a d v e r t i s e m e n t
This appeared in the printed version of the Irish Examiner Monday, January 14, 2008
