Sexual reproduction in plants

It’s spring, and the garden is full of insects. They’re flying from flower to flower, helping plants become more plants, helping plants reproduce. How? Let’s take a closer look! Here are two flowers, one is white and the other one is red.

On the inside, the white flower has female reproductive organs, while the red one has male reproductive organs. A bee lands on the red flower to drink nectar. While trying to reach the nectar, the bee unintentionally brushes against the male reproductive organs and collects pollen grains. The bee then flies over to the white flower, to drink some more nectar. The pollen grain the bee brought from the red flower, falls onto the stigma, which is the tip of the female reproductive organ.

The pollen grain contains a male reproductive cell. This male cell needs to reach the ovule that is located at the base of the flower. The ovule contains the female reproductive cell - the egg, and two polar nuclei. Both the egg, and the polar nuclei contain just one set of genes. The egg is destined to become an embryo, while the two polar nuclei are destined to become food for the growing embryo.

But how do the male reproductive cells reach the ovule? Pollen grain absorbs substances from the stigma, and a little tube starts growing out of the pollen grain. This tube is called a pollen tube. The pollen tube grows down through a long, slender stalk, called the style, towards the base of the flower. The pollen tube carries the male reproductive cell along with it.

As the pollen tube grows, the male reproductive cell divides into two sperm cells. When the pollen tube reaches an ovule, it punctures a hole in the ovule. One sperm cell merges with the egg cell, to become a fertilised cell, a zygote. This cell divides, grows and becomes an embryo. The other sperm cell combines with two polar nuclei and forms the endosperm, which will serve as a food source for the embryo.

Together the embryo and the endosperm form a seed. When that seed then falls onto the ground, if the conditions are suitable, the seed will develop a shoot, roots, stems, leaves… it will become a whole new plant! This type of reproduction, when a male and a female reproductive cell combine to make a new plant, is called sexual reproduction. A new plant that grows as a result of sexual reproduction will look a little like its parent plants, but it will also have some new characteristics. In this case, the parent plants had red and white flowers, but the flowers of the new plant are pink.

Insects, such as bees, aren’t the only ones that help plants reproduce in this way. Pollen can be carried between flowers by birds or by the wind. Even humans can transfer pollen, for example, by using a gentle paintbrush!