THE SECRET LIFE OF MOSSES : Animation description

Three animations of increasing complexity are available on a VHS video tape or DVD covering the reproductive life cycle of a typical moss: 
1. Simple. Short, no terminology and easy to understand. (Running Time, 8m: 07s) 
2. Intermediate. Full length, with summaries and simple terminology. (Running Time, 13m: 15s) 
3. Advanced. Full length with scientific terminology. (Running Time, 11m: 59s)

All mosses (indeed, all plants) have what is known as an 'alternation of generations'. This means that there are two plants, one that produces gametes and one that produces spores. 
Moss Life Cycle

Appearance and structure: 
The large, familiar 'moss' is a haploid gamete-producing plant (gametophyte). Although we refer to stem and leaf in most mosses (and liverworts), they are not considered to be equivalent to organs of the same name in other plants because conducting (vascular) tissue is absent. Many have a creeping stem bearing upright branches. These have filaments of cells at their base, called rhizoids, that help hold them upright. The leaves are very simple and are usually one cell in thickness. The cells of both stem and leaf are photosynthetic. However, there are some mosses that do have complex leaves and a primitive form of conducting tissue.

Reproductive structures: 
Most mosses utilise a cup-shaped configuration of the top-most leaves to hold drops of water over the sex organs. The moss gametophyte is haploid and sperm and eggs are produced by mitosis. The sex organs are at the tips of the main branches or at the tips of side branches. Sex organs consist of a jacket of sterile cells that surround numerous male fertile cells (antheridium), and a single egg (archegonium).

The release of sperm by the male reproductive structure (antheridium) and the opening of the egg case (archegonium) are triggered by a film of water. Sperm are very animal-like, containing two flagella and they swim towards the source of a sperm attractant, released by the egg cases. Many mosses are bisexual (hermaphroditic), but self-fertilisation is discouraged by separating the sexes on different stem tips. The act of fertilisation introduces the opportunity for recombination of genetic material when two sets of chromosomes are combined in the fertilised egg cell

The fertilised egg "zygote" is held within the egg case. It develops into an embryo which inserts an absorbing organ called the foot into the female stem tip. The other end of the embryo grows up and above the female stem to form a thin stalk (seta).

Appearance and structure: 
The embryo grows into a sporophyte consisting of the foot and seta and a capsule that forms at the tip of the seta. This remains attached to the female stem and does not look like a plant, as there are no leaves or stem. This tiny sporophyte is the only diploid form in the life cycle. It is green and photosynthetic for a brief period and may even contain stomata with guard cells

The spore case: 
The moss capsule is a spore case (sporangium). It contains special cells called sporocytes. These undergo meiosis to form spores. 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.

Spore release: 
The lid (operculum) falls away exposing an opening to the spore case that is surrounded by fine teeth-like appendages, the peristome. The ring of outer teeth is hygroscopic, moving outward when the humidity is low, and moving inward when the humidity is high. The in and out movement of the outer teeth helps push spores out of the sporangium. Surrounded by a thick, protective wall, the spore cells are able to withstand periods of exposure to the elements and to be widely dispersed without damage.

Under favourable conditions, a spore cell divides and ruptures the spore wall, forming a filamentous chain of branching cells, the protonema. Eventually, one or more buds form, within which radial symmetry is established and a three dimensional growth form, with stem and leaves emerges.

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

• 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