Mature ferns produce fertile fronds with brown patches called sori on their edges or undersides. Sori are made up of clusters of spore-bearing capsules that split open, releasing large quantities of spores. If these land in a damp place they germinate, developing into tiny heart-shaped plants called prothalli. Within these plants, sexual reproduction takes place.
Fern prothalli produce male and female sex organs on their undersides. When it rains, sperm released from the male organs are carried by water towards the female organs. Fertilisation occurs when the sperm fuse with egg cells in the female organs. The fertilised eggs grow into leafy fern plants.
Lycophytes have a similar reproductive cycle.
Some ferns, notably spleenworts (Asplenium), hybridise. Hybrids are formed when prothalli of two different species germinate alongside one another. They must be physically close enough for sperm from one plant to join with and fertilise the egg cells of the other plant. The fertilised egg develops into a normal-looking plant, but usually produces malformed spores. More than 40 different hybrid fern combinations have been recorded in New Zealand.
Reproduction without sex
Many ferns and lycophytes also have non-sexual or vegetative means of reproduction. The most familiar is the hen and chickens fern, mouki (Asplenium bulbiferum). It produces little copies of itself (‘chickens’) on the upper surface of its fronds. These are easily detached from the parent frond (‘hen’), and take root when they fall to the ground.
Some ferns and lycophytes produce buds on their stems from which new plants grow. The tuber sword-fern (Nephrolepis cordifolia) produces starch-filled tubers along its creeping stem. When the parent fern dies, or if the tubers are detached, the tubers sprout leaves and roots and become a new plant. Crown ferns (Blechnum discolor) produce runners that take root and produce a new plant where they touch the soil. The alpine clubmoss (Huperzia australiana) sometimes has buds near the top of its shoots that sprout roots and a stem when they fall to the ground.