Why a Flower Gives Away Free Sugar

Nectar is sugar and water. Nothing else much.

Sugar is not easy to come by. A plant builds it out of thin air, slowly, all day, using sunlight, and every gram of it is work. Sugar is what a plant has instead of money.

So look at what a flower does with it. It puts a small pool of that sugar at the bottom of a cup, points a set of bright petals at the sky, and lets almost anything that turns up drink it for nothing.

That should strike you as strange. Plants are not generous. Nothing in a garden is doing anything for free.

It is not a gift, it is a fare

A plant cannot walk.

That is the whole problem, and everything else follows from it. To make a seed, pollen from this flower has to reach a different flower of the same kind — which may be at the other end of the garden, or three fields away. The plant cannot take it. It cannot throw it that far. It has to be carried.

So it hires a courier, and nectar is the fare.

An insect does not care about the plant at all. It comes for the sugar, and it leaves with pollen stuck to it because of where the sugar was kept. Both sides get something and neither side is being kind. It is a deal.

The other way of paying, and it is worse

You can tell how good a deal that is by looking at plants that refuse to pay it.

Grasses make no nectar. Neither do most of our big trees — oak, birch, hazel. They do not hire anybody. They put their pollen into the wind and let it go.

The wind is free. But the wind has no address, and no idea where it is going. So the pollen that goes into it lands almost entirely on roads and roofs and ponds and other people's noses, and the amount that has to be released to make that work is enormous. A single small catkin can let go of five million grains of pollen, and the overwhelming majority of them will never reach another flower as long as they exist.

That is the real comparison, and it is the answer to the question this article started with.

There are two ways for a plant to pay for post. It can pay in sugar and hire an insect that knows where the next flower is. Or it can pay in waste, and make millions of pollen grains so that a few thousand happen to land somewhere useful.

Nectar is not the plant being generous. Nectar is the cheaper bill.

The trick that makes the fare work

Handing out payment is only half of it. The plant also has to make sure the courier actually does the job — and it cannot check, and it cannot complain afterwards.

So it does something better than checking. It arranges the flower so that the job cannot be avoided.

The nectar goes at the bottom, in the narrow closed end of the flower. The pollen goes at the doorway, right where anything coming in has to squeeze past. There is no route to the payment that does not go through the work.

That is not decoration. The length of the tube, the position of the pollen, the width of the opening — a flower's shape is a machine for making sure the fare buys something.

And some insects cheat

Which, of course, is an invitation.

Watch a patch of long tube-shaped flowers for a while — comfrey, runner beans, foxgloves — and sooner or later you will see a bumblebee land on the outside of a flower, near the bottom, where the petals join the green part. It sits there. It does not go in. Then it flies off.

What it did was bite a hole through the side wall and drink the nectar through it.

It has been given a name, which tells you how often it happens: nectar robbing. The bee gets the full payment. The flower gets nothing at all, because the hole is below the pollen and the bee never went near it.

It is usually a matter of tongues. Some bumblebees have long tongues and can reach down a deep flower the honest way. Some have short tongues and cannot — and a short-tongued bee at a deep flower has a choice between going hungry and going in through the side.

There is a second twist, and it is the one that really shows the size of the problem. Once one bee has made a hole, other insects use it. Honeybees that could never have bitten through the wall themselves will happily drink from a hole somebody else made. One cheat opens a door for everybody.

Does it actually hurt the plant?

Here is where the honest answer is more interesting than a tidy one: it depends, and nobody has finished counting.

Sometimes it clearly costs the plant. Nectar drunk through a side hole is nectar that is no longer there for an honest visitor, so fewer proper deliveries get made, and the flower sets fewer seeds.

But sometimes it does not, and there is a strange reason why. If the nearby flowers have already been drained, an honest bee has to fly further to find a full one. Flying further means carrying pollen further — and pollen carried a long way is more useful to a plant than pollen dropped on its next-door neighbour.

So the robbers can end up doing the plant a favour by accident, while stealing from it. Which of those two effects wins depends on the plant, the insects, and the year.

None of this was agreed by anybody. No flower ever decided on a price, and no bee ever agreed to carry anything. What happened is that flowers which paid too little got no visitors, flowers which paid too much ran out of sugar, and bees which never found a way to the nectar went hungry — and what is left, after a very long time of that, is a deal that mostly holds, with a few holes bitten in the side of it.

Next summer, have a look at the base of a foxglove. The story is right there on the outside of the flower.