Butterwort: a botanical Jekyll and Hyde

Carnivorous butterwort Pinguicula vulgaris plants grow amongst the ruins of an old lead mine at Westgate in Weardale. This is a plant with a Jekyll and Hyde personality. On the one hand there's that rosette of sickly yellow leaves, covered in sticky mucilage, that trap and digest small insects (click here to see how they do it). Most of its prey is composed of ants and small flies.

On the other hand, it produces these charming flowers that need to be pollinated by flying insects, principally butterflies and bees. This is a plant that either eats insects or exploits them.

Butterwort blooms have a forest of short hairs in the throat of the flower. There has been a lot of speculation about their role but a plausible explanation is that they act as a barrier to insects that are too small to be effective pollinators; the hairs would steal nectar, but would offer no impediment to the long tongue of a bee or butterfly.

Butterwort isn't very common in Weardale but is more plentiful in Teesdale. Many years ago I visited a site on a small plateau on the edge of the moors where water constainly trickled over it, creating a mire full of interesting bog plants. As I looked over the edge of the plateau I saw a haze of these little mauve flowers just above the ground - there must have been many hundreds of plants in bloom. Must go back, to see if they are still there.

Carnivorous Plants 1: Butterwort

If every there was a Jekyll-and-Hyde of the plant kingdom, this is surely it. At this time of year common butterwort Pinguicula vulgaris produces delightful little violet flowers that attract pollinating insects that ensure that it sets seed, while at the same time catching and digesting small insects on that rosette of sickly yellowish-green leaves down at soil level.

These plants thrive on the permanently boggy conditions on the old lead mine ore-washing floors at Middlehope Burn in Weardale. There's very little nitrogen in the ground but the small insects that they digest make up for that. The surface of the leaf is covered in glistening, stalked glands producing mucilage that glues the insect down, then tiny glands embedded in the leaf surface release diestive enzymes that convert it into nutritious soup. The rolled edges of the leaves keep the pool of digestive enzymes in place. To see the capture and digestive glands of another, more exotic butterwort species in more detail, click here. The glands release protein-digesting enzymes that curdle milk and butterwort leaves were once used in the first steps in making butter - hopefully making sure that there were no flies stuck to the leaf first!

Butterwort keeps its welcoming flowers and deadly digestive equipment well separated via a tall flower stalk...

... and the precious nectar, there to attract small bee pollinators equipped with a suitably long proboscis, lies inside that nectar spur that you can see at the back of the flower - protected by a forest of fine hairs that prevent small, non-pollinating insects from crawling in and stealing it.

Killer Teasels?

Over a century ago there was considerable debate as to whether the teasel Dipsacus fullonum was a carnivorous plant. It’s not difficult to see why – the bases of the leaves clasp the stem to form a large cup that fills with rainwater and this soon fills with drowned insects. The criteria for a plant being truly carnivorous are that it must lure its prey, trap it, kill it, digest the victim and absorb the nutrients that it releases. Teasel fails on the fourth of these criteria and probably the fifth too – it doesn’t secrete any enzymes of its own to digest the prey and there’s no clear evidence that it absorbs any nutrients from the soup of rotting flies in its drowning pools. But it can certainly lure and trap its prey, in much the same way as a pitcher plant. This bumblebee was presumably attracted by the water in the leaf bases and as soon as it landed it slipped into the water. From that moment on it was doomed. It struggled to climb out but its wet feet slipped on the shiny, smooth surface of the stem and leaf bases. I thought about rescuing it but within less than half a minute it had fallen back in, become totally saturated and had drowned. The teasel leaf bases soon fill with dead insects and must also have a rich population of bacteria and fungi that can digest them, so maybe it’s time for another look at the eating habits of this plant, to check again at whether it derives any nutritional benefits from its decaying captures. Birds certainly do – I’ve sometimes seem blue tits and great tits raiding this gruesome larder.