Mechanisms that prevent self-pollination

Stella is with her grandad James in the community garden. James is really excited about all the vegetables growing in the garden. I can’t wait for all the vegetables we’ll have soon, Stella. Look how many flowers there are on the pumpkin plant! But why are some flowers long and thin, and the other ones have some kind of bulb at the bottom?

Are they from two different plants? No, both types of flowers belong to the same pumpkin plant. The difference between them is that some of them are male flowers and the other ones are female. What do you mean? We say a flower is male or female, depending on what kind of reproductive organs it has.

If it has male reproductive organs, stamens, and produces pollen, it’s a male flower. If it has female reproductive organs, carpels, which receive pollen, it’s a female flower. Species of plants need both types of reproductive organs to reproduce: pollen from the stamen must reach the carpel in order to fertilise it. When pollination occurs between flowers of separate plants, we call it cross-pollination. Some flowers, however, have both the stamen and the carpel.

These are called hermaphrodite flowers. In hermaphrodite flowers there is a risk of pollen being transferred from the stamen to the carpel of the same flower, a process called self-pollination. New plants can be produced through self-pollination, it is quite common in orchids. However plants produced through self-pollination might be more vulnerable to disease and to changes in the environment, and might not survive very long. This is why many other plant species have developed mechanisms that prevent self-pollination.

One way in which hermaphrodite flowers, for example bananas and fig trees, avoid self-pollination, is by having stamens and carpels mature at different times. Pollen won’t be released at the same time as the carpel is ready to be fertilised. This mechanism is called dichogamy. Plants in which dichogamy doesn’t occur, like apple trees, usually have another mechanism: self-incompatibility. Self-incompatibility makes sure that even if pollen falls on the carpel of the same flower, the pollen will fail to fertilise the carpel.

The pumpkin plant that Stella and her grandad were looking at is self-incompatible, but it also has an additional mechanism. It has carpels and stamens separately in different flowers! This makes it impossible for pollen to accidentally drop onto the carpel in the same flower and self-pollinate it. This mechanism is called monoecy. Do you want to check other plants in the garden?

Maybe we can spot some more of these mechanisms. Sure! I had no idea plants were so interesting...