Science Interviews


Tue, 22nd Dec 2015

Mistletoe: love and parasites

Alex Summers, Cambridge University Botanic Gardens.

Listen Now    Download as mp3 from the show Cracking the science of Christmas

If you're planning to steal a kiss under the mistletoe this Christmas take note: thisMistletoe plant may be beautiful but its life cycle (depending on your world view) is not such a romantic process. The parasite hijacks other plants and animals to reproduce, as Ginny Smith found out from Alex Summers at Cambridge University Botanic Gardens.

Alex - So at this time of the year, if you look into the gardens, you'll see adorning the trees.  Hawthorn, apples, or any of those Rosacea genus, you'll definitely see mistletoe in those trees.

Ginny - So we're looking for deciduous trees that shouldn't have any leaves on but actually have big balls of greenery effectively?

Alex - Yes.  This time of year the mistletoe really does stand out.

Ginny - Okay, so let's go have a look then shall we?

We're going a bit further into the Botanic Gardens now.  I'm walking past a beautiful pond full of ducks on one side and a lovely rockery on the other side, and I've just spotted a tree in the distance there that looks like it might have some mistletoe on it.

Alex - So that is, indeed, mistletoe.

Ginny - How does mistletoe spread?  How does it get so far up into those trees?

Alex - So mistletoe is bird dispersed.  The berries are eaten by, particularly, thrushes and the main outer part of the berry is removed but the sticky substance that's around the seed remains.  When the bird defecates, the seed is left on a branch and that sticky substance allows it to stick to the branch.  The seed then produces three appendages that make it look like a spaceship and these allow it to attach to the branch.  The plants that you see in the tree will probably have started as one initial founder plant, and then mistle thrushes and other thrush species generally and will have turned up and moved the plant throughout the tree.  This plant is doing something that's very different to most of the one's that we're used to.  So from the tree, it will be getting a number of things through a special attachment, called a haustorium: mainly water and possibly some other nutrients, irons and salts.

Ginny - What are the benefits to mistletoe of being a parasite rather than being a normal shrub?

Alex - All plants are competing for one major thing, which is light, but one way to get around that is to elevate yourself on another plant above the canopy.  But not only can you get the light more easily but, if you plug into that plant, you can also steal water and nutrients that you would have had to work to get if you planted yourself into the ground.

Ginny - Does it damage the tree that it grown on?

Alex - Absolutely.  The plant is taking water and nutrients from the tree and, therefore, the tree is losing out.  However, one or two mistletoe plants are not going to do major damage to a tree.

Ginny - Shall we go a bit closer and have a look at some mistletoe?

Alex - Yes, let's do that.

Ginny - Okay, so there's a tree here which has got quite a nice crop of mistletoe low enough down for us to get a proper look at it and you can see those beautiful pairs of leaves and the berries, just how I'd expect...

Alex - If I cut this little bit off here, and we can actually see here this dichotomous branching and that creates that very traditional mound like structure within the trees.  At the branching point there, we can see this is a female, so it's got three berries at each point.

Ginny - I've never noticed quite how perfectly it splits in two, and then two again, and how that gives you that perfect sphere.

Alex - Yes, it's a very interesting branching habit and, in fact, it's not very much repeated throughout the plant kingdom


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