Of Moss and Men: an Introduction to Bryophytes

Mosses (Bryophyta) have to be one of the most common and least known groups of organisms in the world. Actually, they are second only to the flowering plants (Magnoliophyta). But what is a moss? I mean, what truly makes it unique from other organisms. In order to appreciate the intricacies and beauty that is a moss you have to shrink, smaller than dog, smaller then a cat, smaller then a mouse, you need to get right down to past ant size. I guess the best size to be would the size Wayne Szalinski got his kids down to in the classic 1989 movie, Honey I Shrunk the Kids.

The group shares many characteristics with flowering plants as well as green algae (Chlorophyta), and they are actually thought to be the first plants to make it to land. However the major characteristic that sets them apart from the flowering plants we are familiar with is their genetic make up. The vegetative structures (leaves, stem, rhizoids, ect.) of mosses are made up of one set of chromosomes (1n). The greenish moss you see every day is referred to as the gametophyte. The reproductive body consists of two sets of chromosomes (2n) and is referred to as the sporophyte. This is opposed to two sets of chromosomes for the vegetative structures of vascular plants and one set for their reproductive structures.

Also, unlike flowering plants, mosses have no roots or vascular system (xylem and phloem), but some have hydroids and leptoids, which are similar to xylem and phloem. What they do have is a set of rhizoids that anchor the plant to the ground, and their water conduction is mainly done on the external surface of the moss leaf. Since the moss leaf is usually only one cell layer thick, water conduction is not that much of a problem. Mosses are nature’s tribute to capillary action. The thing that makes the mosses absolutely astounding is their poikilohydric ability (“poikilo”– changing; “hyrdic” – water). They can dry up, and as soon as there is water available, they just spring back to life.

However, unlike their ancestors the algae, they have a more complex reproductive process. In algae and mosses, the zygote is unicellular and surrounded by multicellular layer. The difference is that in algae, the zygote doesn’t develop further until it leaves the parent plant. In mosses, the zygote develops in the multicellular archegonium before dispersing from the mother. Another factor that separates the mosses from the algae is that mosses have biflagellate sperm. That is their sperm have two flagella like tails. Like their aquatic ancestors, their sperm still need a continuous film of water to travel through to reach the egg.

There are two other taxonomic groups that that are closely related to the mosses. These groups are the liverworts (Marchantiophyta) and the hornworts (Anthocerotophyta). When first looking at a moss, you can see there are two main types. There are those that stand upright which are known as acrocarps and those that grow along the substrate which are known as pleurocarps. Mosses can grow in many different arrangements relative to these two types. These include cushions, small turfs, tall turfs, smooth mats, rough mats, wefts, and dendriods. The diversity of forms in which they come is only matched by the diversity of habitats in which they occupy. The only place on earth that doesn’t have mosses is the ocean, and the substrates on which they grow rage from soil to sand to rocks and trees.

So, in short, mosses are common plants that are generally overlooked due to their small stature. They occupy a wide variety of habitats and come in all sorts of shapes and forms. They could be considered to be the most successful terrestrial plant due to their distribution, their small yet intricate design and their ability to come back from the “dead.” In one word…they are truly “moss-ome.”

Santino Lauricella