Wednesday, 24 September 2014

Why are strawberry seeds on the outside?

Why do strawberries have seeds on the outside, when apples have seeds on the inside? I wonder that every time I see a strawberry. Lets have a look and see what's going on. 

Below is a strawberry flower. In the centre is a large green blob covered in things that look like tiny green hairs. The hair-type things are the stamens and there are *lots* of them.

The carpels are the female part of the flower, on which pollen is deposited by insects. At the base of each carpel is an ovary containing an egg cell. The egg cell will be fertilized by the sperm that comes from the pollen. Together the fused pollen and sperm cells will develop into a seed. 

That means that for each of these tiny green hairs we will get one seed. So that explains why there are such a lot of tiny seeds. But why do they end up on the outside of the strawberry?

Well now we have to use our imaginations a bit. The critical question, as we recall, is why are apple seeds on the inside of the apple when strawberry seeds are on the outside? 

Lets think of the tasty part of the fruit as a big blob of tasty playdoh. Even in the newly opened flower, the very beginnings of this tasty playdoh are already present. In the strawberry's case, it seems to me that the ovaries just happen to be on the outside of the miniscule blob of playdoh, and so that's where the seeds develop. I assume, conversely that the apple ovaries are inside the blob of tiny blob of playdoh that is already present in the flower, and so when the apple develops, the seeds will be on the inside. Apples contain far fewer seeds, because their flowers have far fewer stamens.

These are my assumptions about the system. Tomorrow we will cut open a strawberry flower to test whether my assumptions are right.

So what happens when the flower starts to develop into a strawberry? First, the fused egg and sperm cells develop into a seed. At the same time the tasty fruit tissue develops and becomes larger. The photo below shows a tiny strawberry with its new green seeds and green strawberry tissue gradually developing. As the seeds mature, the fruit tissue enlarges and takes on its mature shape. 

A few days later, the fruit tissue is becoming larger and less green, while the seeds stay the same size and gradually get further apart. 

Finally the fruit tissue matures and takes on the rich red colour, telling us all that it is ready to eat. The seeds are sill visible on the surface, but are now much less prominent and much smaller in proportion to the mature fruit. 

So now we know. Strawberry seeds develop on the outside of the fruit, because the ovaries are on the outside of the rudimentary fruit tissue, even when the flower opens. In apples, conversely, the ovaries are right inside the fruit tissue, even at the time when the flower opens. 

Next time we look at a flower on a strawberry plant or apple tree, we can take careful note of the number of stamens. We know that we will be seeing just the same number of seeds on the fruit later in the year. We know exactly where they will be, and why. 

Just in case you were wondering, several strawberries were eaten during the making of this blog post.