Bumble bee
Pollination is the movement of male sex cells (pollen) produced in the anther to the stigma. Pollination can be carried out by wind, or by animals such as birds or insects. This site focuses on pollination by insects, which pollinate many species of flowering plants.
When a pollen grain lands on a stigma of a flower of the same species, it grows a small tube down to the female sex cell (ovule) in the ovary and the genetic information from the male and female parents is combined to make a seed. The whole reason for sexual reproduction is to produce variation that allows the next generation to cope with changes in the environment, so pollination is usually most successful when pollen comes from the flowers of another individual of the same species, and not from the flower that contains the stigma or even from another flower on the same plant.
Flower shape and insect visitors
If you spend any time at all in a garden, you will notice that flowers can have very different shapes. The shape of a flower is important because it limits how an insect can get to different parts of the flower.
Some flowers are arranged so all parts are easily accessible. These “open-access” flowers are visited by a wide range of insects. Because insects can get at the pollen and/or nectar from pretty much any angle, the flower has little control over where pollen is rubbed onto an insect or where on the next flower the pollen rubs off. A flower that is shaped so an insect can only get at pollen or nectar in a certain way restricts where pollen is placed, both on the insect an on the next flower. These “restricted-access” flowers may not get the variety of insect visitors that open-access flowers do, but the pollen is more accurately targeted to the stigma of the next plant, so the plant does not need as many visitors for successful pollination.
Insect features
Halteres – these drumstick-like bits replace the second pair of wings in Diptera (flies)
All insects have the same basic body plan, with changes in body shape for different lifestyles. Pollinators come in a range of shapes and sizes. There is a rough link between insect size and flower size, with larger insects visiting larger flowers, since small flowers do not provide a large enough landing platform. Some flowers get round this by grouping their flowers. The Umbelliferae (carrot family) group their flowers in umbels (think ‘like an umbrella’ and you have the shape in your mind). Larger insects use the whole umbel as a landing platform. Some flies don’t even have to land, they simply lower their mouthparts (which look like a drinking straw) into the flower and drink nectar while they hover. Flower shape and insect shape can be very closely linked – especially for some restricted-access flowers.
Effective Pollinators
The most effective pollinators are insects that move large amounts of pollen from the anthers to stigmas of many different flowers of the same species.
Three things affect how well this happens:
- Pollinator size and shape
- Pollinator behaviour
- Flower arrangement and timing.
These are closely connected, since flowers and their pollinators have co-evolved. Only some interesting examples are given below, but read Barth (Insects and Flowers – the Biology of a Partnership) for lots more interesting examples.
Pollinator size and shape
Large insects will transfer more pollen simply because there is more body surface for pollen to stick to, but if an insect is too large, the flower cannot physically support the insect. Some plants get around this problem by grouping their flowers, so the insect can treat the group as a landing platform.
Pollen from insect-pollinated plants often has bits that stick out and this helps the pollen get trapped between hairs on the insect’s body. Honeybees and bumblebees groom themselves and move the collected pollen into ‘pollen baskets’. Female native bees also collect pollen in the same way, but because they do not pack the pollen down as well, it is more easily dislodged when they visit another flower. The difference in the amount of pollen transferred by exotic and native bees is still being investigated.
Tongue length is important, because some flowers hide their nectar deep inside the flower. Only insects with long tongues can get to this nectar. Other insects may collect only pollen, but since they are using the pollen as a food source, less of it will be transferred between flowers.
Pollinator behaviour
The honeybee, Apis mellifera, is flower constant, which means that on any foraging trip, it focuses on only one kind of flower. Pollen is transferred only between flowers of the same species and this is one of the features that make honeybees so popular for commercial pollination of crops.
Sometimes insects will ‘rob’ a flower of its nectar by cutting a hole in the side of the flower for easy access to the nectaries. Bumblebees, Bombus spp., will do this with kowhai (Sophora microphylla) or banana passionfruit (Passiflora mollissima) flowers. The hole made by the bumblebee will often be revisited by the bumblebee and also by other insects. An interesting observation is that sometimes, bumblebees will visit a flower that they have robbed for nectar in the ‘usual’ way. Why they do this is not fully understood – perhaps there is less nectar at that time, or they are simply collecting pollen for the hive.
Flower arrangement and timing
Simple ‘open-access’ flowers can be visited by almost any insect. The sheer number of visitors means some pollen will end up on a receptive stigma, but the flower has no control over how an insect moves within the flower or where the insect flies to next. ‘Restricted-access’ flowers are shaped so insects can only enter them in a way that results in pollen being placed where it will rub off onto the stigma of the next flower. Pollen is more accurately targeted, but only if the next flower visited belongs to a different plant of the same species.
Some plants manage to time production of flowers with anthers and stigmas that function at different times. For example, foxgloves have their flowers arranged in a spike, with the oldest flowers at the bottom. The anthers mature first and the stigma later. Bumblebees will start feeding in the lower flowers, depositing any pollen on their bodies onto the stigmas of these flowers. As the bumblebees move up the flower spike, they get dusted with pollen from the anthers in the younger flowers. Once the bumblebee reaches unopened flowers that do not produce nectar, they will move onto the next flower spike, again transferring pollen between plants.
Stucture of a 'typical' flower
A typical flower is about as common as a typical person – you have probably not met one yet, but it is useful in helping us understand basic flower structure. So, what do the different bits do?
Stucture of a 'typical' flower
Anther – produce the pollen.
Filament – hold the anther up in the flower in such a way that pollen can be brushed onto visiting insects.
Nectary – produce a sugary liquid that is like aviation fuel for flying insects and are usually located so an insect has to squeeze past the anthers and stigma to get at the nectar.
Ovary – is where the pollen tube grows down to – it may hold one or more ovules.
Ovule – the egg awaiting fertilization by the genetic information traveling down the pollen tube.
Petals – advertise that food is available – some even have guide marks that insects can see which act almost like a landing strip!
Sepal – can be green or coloured depending on whether they protect the flower bud only or also help with the advertising that the petals do.
Stigma – is the female bit where pollen ends up – a variety of mechanisms exist that will only allow pollen from another plant of the same species to go any further.
Style – holds up the stigma and is the path down which the pollen grain grows a tube.
Pollen and Nectar
Pollen not only carries genetic information from the male to the female parent plant, it also needs to be attractive as food to insects so they go looking for it. Insects are only interested in pollen as food. Pollen is rich in protein, the building blocks of growing cells and also contains fats, starches and vitamins – everything a growing insect needs! Most insects eat the pollen while they are visiting the flower, but honeybees (Apis mellifera) and bumblebees (Bombus spp.) take it back to the hive as food for their larvae. The collection of large pollen loads is one of the reasons that bees and bumblebees are such effective pollinators.
Nectar is a sugar-rich liquid produced by flowers and used by insects as ‘aviation fuel’, but it is of limited use for insect growth since it contains only small amounts of protein. It is harder for plants to get nitrogen than carbon, so using carbon-rich nectar rather than nitrogen-rich pollen as a reward makes sense. If nectar is available and the visiting insect is only interested in an energy snack, pollen is less likely to be eaten, meaning less can be produced to achieve pollination, placing less strain on valuable nitrogen resources.
How much nectar is offered as a reward depends on a trade-off. Nectar production uses energy and resources. If too much is produced, insects may not visit enough flowers to cross-pollinate. If not enough it produced, insects will not waste energy visiting. Bees measure distance in terms of energy used, so to be attractive to bees, flowers need to provide enough nectar to ‘fuel’ the flight of the bee while it collects pollen.