Before I became a beekeeper, I can’t remember ever learning about extra-floral nectaries. No-one goes to an extra-floral nectary show, or walks down the aisle clutching a bouquet of exquisite extra-floral nectaries. The world goes by without most of us ever thinking about them. Yet they are very useful to insects such as ants and bees.
So what the heck is a extra-floral nectary anyway? Well, it’s a patch of glandular tissue which secretes sugar but is not part of the plant’s flower, which is why it’s ‘extra-floral’, i.e. not floral. These glands have been observed in at least 2000 different species of plants, including broad bean, cherry, cherry laurel and plum. Below is a picture I drew showing where they are located on these four examples.
Since these nectaries aren’t near the sex parts of a flower, they obviously have no connection with pollination. So what are they for?
There are a couple of different theories about the function of extra-floral nectaries. One is that the nectaries may act as ‘sap valves’ to regulate sap pressure within the plant. The thought is that if the sap in the phloem tubes (which transport sap around the plant) become too concentrated with nutrients, the plant releases nectar from its extra floral nectaries to reduce osmotic pressure.
Another theory, which seems to be more commonly accepted, is that the nectaries are a defensive mechanism to reward ants, which will then stop other animals from eating the plant. In The Honey Bee Around and About, Celia Davis mentions an experiment which found that when broad beans had a proportion of their leaves removed to simulate damage by herbivores, the plant produced a lot more extra-floral nectaries within one week.
Extra-floral nectaries: the unobtrusive, unshowy nectar pools of the insect world.
- Beekeeping study notes (Modules 1, 2 & 3), J.D. & B.D. Yates (2013)
- The Honey Bee Around and About, Celia F.Davis (2009)