Monday, September 19, 2011
For a flowering plant to reproduce, the male cells (pollen) and the female cells (ovule) have to join together. This is called fertilization. When a grain of pollen lands on the stigma (the sticky part of the carpel, see my last post) it forms into a pollen tube (the tube that transports male cells to the ovule at the bottom of the pistil). The pollen tube goes down into the ovary and enters an ovule through a tiny hole, the microphyle.
A pollen grain contains two male nuclei. These go down the pollen tube and join the contents of the ovule. One forms a zygote, which is the first cell of the new organism. The other creates a layer of tissue called endosperm, These become a seed. The ovary becomes a fruit. The plant no longer needs the rest of its parts and it dies.
Some plants do not need to be pollinated. The bee orchid, for example, attracts a certain type of bee by looking and smelling like a female bee, but if no bees come, its stamens will bend over and transfer pollen to the stigma.