Primroses or the adventures in cross fertilisation

The point of flowers is to ensure that plants can exchange genetic material. This is a task that many plants take seriously, providing nectar and pollen to ensure that their pollinator will carry out its task. Although some plants do fall back on self-fertilisation if pollinators haven't been able to do the job, they still utilise some of the methods below to increase the chance that a pollinator will provide the service of pollination.

The colouring of flowers, the purpose of which is to attract the right pollinators, is one way that plants can advertise the availability of their tasty goods. Flowers towards the blue spectrum are required for insects, as they are not as sensitive to red as we are and, for example, cannot tell the difference between red and black. They are, however, sensitive to the ultra-violet part of the spectrum and many flowers appear different under UV, often displaying pathways to the pollen.

Birds can see red, which is a great way to enlist their services. They cannot, however, smell very well. Therefore if a flower is red and scentless, then it is likely to be pollinated by birds. Scent is another method employed by flowers to tell the insects where they are. Flower shape is yet another way to ensure that only the right pollinator can get at the pollen and nectar, a way that ensures that the pollinator will also take the pollen to another flower of the same species.

So, we can see that plants use various methods to ensure that their pollinators not only know where they are, but carry out the task of pollinating. However, some plants want to also ensure that they are not self-pollinated. That is, they want their pollen to only fertilise a flower on a different plant and not flowers of the plant the pollen came from.

Lots of plants can recognise their own pollen from the chemical composition on the pollen grains, making the pollen sterile to the female part of the flower. Other plants ensure that their male and female parts mature at different times so that all of the pollen will be gone from the flower by the time the female parts are mature and receptive. As we shall see below, the method of the primrose is a little different.

As in the photograph above, we can see that there are two different types of flowers on primrose plants. The flower on the left is called the pin-eyed flower and has the stigma at the mouth of the flower and the anthers deep in the throat of the flower. To the right, the thrum-eyed type, which has its sexual anatomy the other way around. The insect collecting the nectar, which is in the throat of the flower, from the pin-eyed flower to the left, will collect pollen on it's head. This pollen can only be passed on to a flower of the thrum-eyed type. Equally, as the insect collects nectar from the thrum-eyed flower, pollen is collected on the insect's body. This pollen can only be passed on to the stigma of a pin-eyed flower, which is brushed on to it as the insect delves deep to retrieve the nectar. As each plant will only have pin-eyed or thrum-eyed flowers, this enforces cross-fertilisation.

References
Attenborough, D. (1995), London, BBC Worldwide.