Sundews – the midge hunters

Flanders Moss NNR and Blawhorn Moss NNR

Many people know about the bog specialist plant the sundew and how it supplements its diet by catching flies.  It is pretty cool for a plant to eat animals but have you ever wondered just how does it do it? Well, Charles Darwin wondered and his son also and they decided to investigate. With other researchers filling some of the gaps it makes a fascinating story.

A sundew’s leaf is covered in gland bearing tentacles (or trichomes).  Darwin found that each leaf has an average of 192 tentacles per leaf. There are short ones in the middle of the leaf and longer ones at the edges. These tentacles and the glands on the end of them are the business end of the plant, what the plant does its insect hunting with. The tentacles can move in different directions by changes in cell growth at the swollen base where they meet the leaf. The glands have a number of jobs: they detect prey, they produce the sticky trapping fluid, they secrete the digestive enzymes that dissolve the insect and they also absorb the dissolved bits of insect that the plant can use.

Photo by Peggy Edwards

Darwin wanted to find out the process by which the actually plants caught their prey so he tried placing tiny pieces of meat on to leaves. He found that the tentacles moved towards a piece of meat, the nearest ones first and then the more distant ones. They covered the prey and pressed it close to the leaf. If a prey lands on the edge of the leaf then the tentacles bend the prey towards the centre of the leaf. Tentacles that are stimulated will start moving in a few minutes but the process of the whole leaf’s tentacles moving to the prey can take up to 1 1/2 hours. Glands that are in contact with the prey start secreting more enzymes but you can’t fool the plant to take something that is not food. If the material has nitrogen in it e.g. meat, then the tentacles will hold the prey to the leaf for between 1-7 days for the dissolving process but if the object is inorganic the plant can detect this and releases it in a much shorter period.

Photo by Peggy Edwards

And these tentacles don’t just work alone. Darwin found that a tentacle that has had its gland removed can’t detect prey but will still move towards prey if tentacles next to it are stimulated. Later research shows that there is actually a small electrical impulse that travels from one tentacle to another so coordinating the whole prey catching process on the leaf.

It appears that these amazing plants don’t have to eat insects but those that do grow much bigger and more importantly produce far more seeds if they do. A test of sundews on Rannoch Moor showed that up to 50% of the nitrogen in the plant had originally come from insects.

We usually think of plants being passive and just sitting there so I love the idea of a plant actively attracting in its prey and then moving in a coordinated way to trap and eat it. It is a bit like a mini-triffid! And I love even more the fact that 50% of the prey of the sundews on Flanders Moss is made up of midges. All power to them!

Reference – Sexton and Longrigg – Plant Report 2011-2012 – Forth Naturalist and Historian Vol. 35, 2012, p 13-15