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A year ago, after the bushfires, when everything that wasn’t burnt was scorched or looking like it had heat stroke, I planted a big pot of salvia for a quick burst of flowers for whatever insects had survived. It became a gathering point for native bees and honey bees alike, and every time I looked at it I felt I’d done something good. This summer it reflowered, and it continues to flower: an enormous coronet of ‘Hot Lips’ salvia with its circus flowers of pink and white; smaller, more compact, deep purple salvia beneath. Earlier this morning there was a dragonfly on its stalks, and just now two blue-banded bees (Amegilla cingulata), with their familiar buzz, careering around, dipping in and out. One is carrying a big yellow bundle of pollen on one of its back legs.
Not for the first time I wondered about nectar and pollen. Does the flower just keep exuding nectar, or does it run out? Why do the bees choose one flower over another? Why do they sometimes pop into one then pop out again immediately? And why are the blue-banded bees sometimes in the flower and sometimes under the flower, below the base of the petals?
Many hours later I have some answers. But first, I had to get some definitions.
Nectar.
Nectar is a sweet, nutritious secretion produced by a flower’s nectaries. It is mainly sugars (fructose, glucose and sucrose), but may contain traces of other elements, such as amino acids, salts and essential oils. Its composition varies enormously, depending on the plant species, soil and air conditions. Fascinatingly, the connection between a plant and its pollinator may be built in to the nectar:
All these substances often impart a particular taste and odour that may be essential for maintaining certain pollinator groups.[1]
Nectar is secreted from the nectaries in a distinctive pattern for each species, maybe in response to or just in tune with the different pollinators’ needs. The sugar levels may change as nectar is taken, or not. One study of nectar production in salvia showed varying levels of nectar production through the day, depending on the type of salvia, with average production ranging from lows of less than 0.5 µl to highs of 1.75 µl per flower between 9 am and 2 pm. The researchers found that most of the flowers stopped producing nectar after 2 pm. Removal of nectar, either by the researchers or by bees, did not stop the flower from producing nectar.
Nectaries.
The position of the nectaries is not fixed within the flower.
To ensure that ideally only legitimate pollinators can access the reward (and in that way successfully transfer pollen), flowers are often “built” around the nectary or the nectar.[3]
However, nectaries are usually found at the base of the stamens, so the pollinator comes into contact with the pollen as it goes into the flower.
Pollen, and other parts of the flower.
At this point I had to go back to flower terminology. Pollen grains contain the male gametes of plants. They are found on the anther, which is at the top of the stamen. When pollen is transferred to the stigma, it (hopefully) germinates. A pollen tube grows from the stigma down the style to fuse with the female nucleus in the ovary. The style and stamen are those fine upright parts of a flower, typically visible in the middle of the petals. So the importance of attracting pollinators lies in the fact that pollen may be being produced in one flower at a time when its stigmas are not receptive. The pollen carried by the pollinator to another plant’s flowers may find a more receptive stigma, leading to germination.
Putting it all together.
So nectar attracts bees (and other pollinators) in the hope that they will pick up some pollen and carry it around, leading to the survival of the species. Nectar is often exuded in small amounts to attract many different pollinators throughout the day, improving the chances of spreading the pollen around.
And those blue-banded bees sucking at the base of the flower?
Some insects, known generally as nectar robbers, bypass the sexual organs of the flowers to obtain nectar, often by penetrating the outside of the flower rather than entering it. In this way, nectar robbers ‘steal’ the nectar reward without facilitating pollination.[4]
Ooh. Nectar robbers!
[1] https://academic.oup.com/aob/article/94/2/269/174092
[2] http://sixseven.org/NectarMonitoring.pdf
[3] https://www.botany.one/2018/07/on-nectaries-and-floral-architecture/
