Life Cycle of an Indoor Fern
Although ferns belong to the group referred to as higher plants, they do not formor seed. Their life cycle demonstrates a clear alternation of generations. As non-scientists we are most familiar with one part of this cycle, namely the mature fern plant.
This is the correct name for the actual fern plant, ie. the spore-forming generation. The minutedevelop on the undersides of the fronds in capsules. They are formed through division which means they have only half a set of chromosomes (haploid). Ripe are catapulted out of the capsules and distributed by the wind. Once they settle in ideal conditions, they begin to germinate and are now called gametophytes.
Thedevelops from a germinating . It is small and flat, with lobes (usually heart-shaped), and, apart from one section below the heart-shaped notch, consists of only one layer of cells which also have only half a set of chromosomes. This structure is green, however, and able to photosynthesize just like the leaves of a mature plant. Root-like, hair-shaped growths (rhizoids) anchor the pro-thallium firmly in the ground.
The sexual organs develop on the underside of the, thus representing the second stage of sexual generation. The female cells (archegonia) form in the multi-cellular region beneath the heart-shaped notch. Their lower section, containing the ova, is sunk into the tissue of the , the upper part rears up like a neck. The male cells (antheridia) form between the rootlike growths (rhizoids). They will later release mobile sperm (spermatozoa) which swim towards the female cells and fertilize the ova.
For all of this to take place, the pro-thallium must become wet through the action of rain or dewdrops. The fusion of a sperm and an ovum, each of which carries half a set of chromosomes, creates a single-celled zygote which now contains a full (diploid) set of chromosomes. The zygote begins to grow into a mass of cells called an embryo and a new fern plant develops directly from this. As soon as the plant is large enough to survive on its own, the prothallium withers and dies.
Spores, sporangia, sori
The shape of the and the sporangia (spore capsules) differs according to species. The sporangia are grouped together in characteristic clusters called sori. During their development, they are usually protected by growths on the leaves (veils, indusia), by hairs or by scales. If they are shielded by rolled-up leaf edges, they are referred to as pseudo-indusia (false veils). These occur in maidenhair fern (Adiantum) or in .
The sori may be completely circular, as in. Among , they bear shield-shaped veils. Nephrolepis possess kidney-shaped sod. Davallia form urn-shaped indusia. Interesting patterns are created by long lines of sod. In they run along the narrow feathers very close to the central ribs; in or Pellaea along the edges of the leaves. In the case of species, which are sometimes called necklace ferns, they are arranged in stripes set diagonally towards the central rib. In Platycerium the sporangia, which take the form of brown or black markings, cover entire areas of the undersides of the leaves. Depending on the species, these can be found on the tips of leaf lobes or beneath inward curving leaves.