Have a look at the underside of a fern leaf.

Ferns have leaves with breathing pores on the underside. They also have little light coloured pads on the undersurface of the leaves.

These are pads called Sori.

Sori protect the cell cases from the weather.

Inside the cases the protoplasm divides into many tiny cells and each cell becomes a spore.

As the weather becomes warmer the case dries and ripens.

The spores spurt from the case and are carried away in the air to form new plants.