Thursday, 25 September 2014

Why are strawberry seeds on the outside of the fruit: part 2.

Yesterday we formed a hypothesis about why strawberry seeds are on the outside of the fruit, when apple seeds are on the inside. 

Today we are going to have a look inside the strawberry flower and see if the anatomy backs up our theory. 

Below is a cross-section of a strawberry flower. The sexual organs are in the centre, with the female organs held on a tall column of tissue. The male organs are arranged in a ring around the female organs, and four of them are visible in this photo. The petals and sepals can also be seen flanking the male organs. 

The tall column in the centre is the part that will expand massively to make the strawberry fruit that we are so used to eating. The little green organs that sit all over the column are the many individual female organs. Each one of these tiny female organs has an ovary at the base and can produce one seed in the ovary.

The image below shows a close-up of this area of the flower. We can now clearly see each female organ, with a bean-shaped ovary at the base, a tall style reaching up, and the stigma at the top, where pollen will be deposited. It is very obvious from this photograph that that the ovary does indeed sit on the outside edge of the tissue that will later develop into the bright red strawberry fruit tissue. 

The photograph below shows a cross-section of the same small green strawberry that we looked at yesterday. As we can see, the fruit tissue has already increased hugely in size, and the seeds are already fairly mature, and sitting on the outside of the fruit. 

The photograph below show the fully mature ripe fruit in cross section. By this stage, the fruit tissue has grown even more, and the seeds are still visible on the outside edge. Small white lines of tissue are visible reaching out to each seed, and i assume that these are lines of vascular tissue taking food and water to the seed. This is just an assumption and I have not verified it.

So there we are. The inside of the strawberry flower seems to confirm our hypothesis that seeds grow on the outside of strawberries, because the ovaries sit on the outside of the nascent fruit tissue in even in the flower.