THE SECRET LIFE OF FERNS : Animation description

Spores and spore cases: 
The large fern plants that are so easily seen outdoors have cells with two sets of chromosomes (diploid condition) and form sporangia (a 'spore case', which contain the spores) on their leaves. In most ferns, these are on the underside of ordinary photosynthetic leaves. However, in some ferns there are two types of fronds: smaller, 'fertile', sporangium-bearing fronds, and larger, 'sterile' fronds that are photosynthetic.

Reduction division or meiosis: 
In many ferns, spore formation occurs at certain times of the year, eg, early spring. In others, it may occur all year round. In all instances, spores are formed by reduction division (meiosis). This takes place within special cells (sporocytes) within the spore case. Meiosis halves the chromosome number to produce spore cells with one set of chromosomes (haploid condition). As a result of meiosis, the chromosomes in the spore cells are likely to contain new combinations of genetic material. Surrounded by thick protective walls, the spore cells are able to withstand periods of exposure to the elements and thus can be widely dispersed with no deleterious effects.

Spore release:
Release is the result of the loss of water from a band of cells, the annulus, which encircles the spore case. Water molecules adhere to one another and to the walls of the band only when it is in liquid phase. This force is strong enough to pull the delicate outer wall of the band inwards, shortening it much like an accordion. However, when too much water is lost, the remaining water molecules are unable to hold together and water goes from the liquid to the gas phase. In this state (water vapour) water is unable to pull on the adjacent walls and they are instantly released and free to return to their original position. In a micro-second, the band returns to its original length and the sporangium closes so fast that the spores are thrown out!

Appearance and structure: 
The spore germinates to form a tiny gamete plant, called the gametophyte. This plant has no roots, leaves or stem but is a simple sheet of cells, one or two cells in thickness. Small filaments (rhizoids) attach it to the substrate. Such a delicate plant requires protection from exposure to full sunlight as it is susceptible to drying out. Gametophytes are therefore most common in shady, sheltered areas.

Reproductive organs: 
Both types of sex organs are surrounded by a sterile jacket of cells and are located on the under surface of the gametophyte. The male organ (antheridium) contains numerous sperm cells and the female organ (archegonium) contains one egg cell. Although animals which are diploid, form gametes by meiosis, the fern gametophyte is already haploid and so gametes are formed by mitosis.

Release of sperm and opening of the egg case (archegonium) take place in the film of water between the undersurface of the gametophyte and the soil. Sperm have many flagella and swim towards the source of a sperm attractant, released by the egg cases. Many fern gametophytes are bisexual (hermaphroditic), but self fertilisation is discouraged by having the maturation of sperm and egg cases at different times. In some ferns, bisexual gamete plants enter into a female phase and induce neighbouring plants to form sperm sacs thereby encouraging cross fertilisation. The act of fertilisation introduces another opportunity for recombination of genetic material when two sets of chromosomes are combined in the fertilised egg cell.

The fertilised egg is held within the egg case and nourished by the gamete plant until it has formed an embryo spore plant with root, shoot, and first leaf. The spore plant then grows and enlarges while the gamete plant withers and dies.

The following books include a discussion of the fern life cycle:

• Gifford, E.M. and Foster, A. S., Morphology and Evolution of Vascular Plants, 3rd ed., Freeman Publishers, 1989
• Moore, R., Clark, W. D., and Vodopich, D. S., Botany, 2nd ed., WCB/McGraw-Hill Publishers, 1998.
• Raven, P., Evert, R. and Eichhorn, S., Biology of Plants, 6th ed., Freeman-Worth Publishers, 1999.
• Roost, T.L., Barbour, M.G., Stocking, C. R., and Murphy, T.M., Plant Biology, Wadsworth Publishing 1998