It's not about Food, It's about Sex

How do insect-eating plants avoid eating the insects they need to take pollen from one flower to another?

For a regular, non-insect-eating plant, there is an optimum length for your flower stalk. If it's too short your flowers won't be as easily seen by passing insects and if it's too long it's likely to be munch off by herbivores.

For a insect-eating (carnivorous) plant like the Sundew (Drosera), you have the added problem that if the flower is too close the sticky leaves your potential pollinator meets an untimely death.

According to EurekAlert!, a research report to be published in the October issue of Annals of Botany, suggests there is more to it than this.

Bruce Anderson from the University of Stellenbosch studied two different sundews. Drosera cistiflora with a long flower stalk above its ground-hugging leaves, and the more upright Drosera pauciflora with a shorter flowering stalk.

Anderson tested the theory that if a longer stalk is 'safer' for the insect (i.e. less chance it get stuck to leaf and digested...) there should be more potential pollinators stuck on the leaves of Drosera pauciflora.

After observing pollination and trapping by 500 plants of each species he concluded the length of the flowering stalk made no difference at all to the risk of the plant digesting pollinators. In fact only one plant actually caught a pollinator in its leaves.

The pollinators are generally larger insects, in this case Monkey Beetles, over 5 mm long, whereas the insect that became food were mostly smaller, less than 2 mm long. This system means the plant can eat and reproduce quite happily, even with a short flowering stalk.

This sent Anderson back to the general hypothesis about flower stalk length, and whether longer stalks attract more pollinators.

To test this he cut the flowers from Drosera pauciflora and set them out in test tubes at various heights. This time the result was dramatic. Taller flowers had ten times the number of insect visitors than short flowers.

So it seems the reasons for many carnivorous plants having long flowering stalks is the same as for other plants - to make sure your adverstising material (the colourful petals and other bits and pieces) are flying high in the air. As Anderson puts it, "It's not about food, it's about sex".

Images: Two sundew species from Australia - one with ground-hugging leaves that sends up a long flower stalk, the other with its sticky leaves up amongst the flowers.