Parasitic Plants

Mistletoe

Parasitic plants can get a bad name for themselves, in fact some of the most detested 'parasites' aren't parasites at all.

Take, for instance, ivy. Ivy often has a bad name because people thing that it's parasitic on trees and can kill them. Ivy isn't parasitic as it can photosynthesize and has it's feeding roots, that provide nutrients and water, in the ground. However, ivy does throw out 'roots' that provide the superglue-like adhesive that allow it to stick to the tree - or house! - to get it to a height where it can change from juvenile to adult. So it can be easy to make this mistake.

True parasites are either Holoparasites or Hemiparasites: 

• Hemiparasites contain little or no chlorophyll and therefore cannot photosynthesise. They reply completely on the host plant for nutrients, fixed carbon and water.  

• Hemiparasites can photosynthesise and therefore normally just rely on the host for water and nutrients.

They attach to their host either at the host's stem or root.

The parasitic mistletoe is a classic hemiparasite that attaches via the stem. In the UK mistletoes are now being encouraged due to worries that they may disappear from our landscape in the near future. They are commonly grown on trees in orchards, like the example below that was photographed at The Courts Gardens in Holt that is run by the National Trust. As mistletoe doesn't require too many resources, the tree isn't badly affected by the presence of the mistletoe and will do just fine.

Here's another example of mistletoe, this time in The Netherlands. Mistletoe seems much more common there than here - and is obviously abundant in the photo above. I have visited this part of The Netherlands over the past 28 years and always remember the balls of mistletoe in the area, which is testament to the ability of the trees to cope with this sort of parasite.













To the left is the hemiparasite, the purple toothwort. As you can see it has no leaves or other green parts and therefore has no chlorophyll to photosynthesise with and relies on the perennials that it parasitises, including trees, via their roots to survive.














Another example of a hemiparasite is the common broomrape. However, which this parasite attaches to the root of host plants, it parasitises plants similar in size to itself. While purple toothwort attaches to perennials such as trees, the common broomrape attaches itself to the roots of clover.
One can only imagine how many attachments it requires to nearby clover plants to throw up these impressive flower spikes.










It's easy to see why parasitism is an attractive way of live, in fact there are around 4000 species of parasitic plants identified to date. However if the parasite puts too much of a drain on the host, then when the host dies, it may die too. To combat this, root parasites can attach to the roots of many plants and will only be affected when wider environmental aspects come into play.