r/Pomes • u/TrevorCidermaker • 4h ago
Pear blossom fertilisation
Now the perry pear trees have blossomed I thought it would be worth taking a look at the journey the pollen takes from the stigma to fertilise the ovule.
After being brushed off the bumblebee coat and trapped by the sticky fluid on the stigma, it rehydrates using the stigmatic fluids. The inner wall (intine) of the pollen swells and pushes through a thin spot in the outer wall (exine) allowing the vegetative cell to form the pollen tube. While it starts as just one cell in a microscopic pollen grain the pollen tube can grow up to several centimetres long to reach the plant's ovary, making it one of the longest single cells in the plant kingdom. Inside this single, stretching cell, there is a moving stream of cytoplasm that carries three structures down toward the ovule. The vegetative nucleus which controls the growth and metabolism of the tube. And two sperm cells. These are distinct cells and carried inside the vegetative cell.
Because it is a single cell growing only at its forward point, it relies entirely on the precise tip-growth mechanics mediated by pectin dynamics to elongate without breaking.
As the tube extends blockades called callose plugs form behind it to keep the contents pushing forward. The pollen tube enters the ovary locule and makes a sharp turn over the obturator, an outgrowth of the funiculus tissue near the ovule. The obturator provides both physical direction and chemical signals to help the pollen tube successfully locate and penetrate the micropyle or opening of the ovule.
Once guided to the micropyle, specialized cells inside the female ovule called synergids take over the final short-range attraction of the pollen tube. The tube then penetrates a synergid cell which then degenerates and bursts open to release the two sperm nuclei, setting up the double fertilisation. The fertilised ovule then rapid grows and the surrounding ovary and hypanthium as seen here.