Sometimes I wonder if maybe "moss" isn't one of the more confusing words in the English language. Moss, like many things with pleasant connotations, has innocently lent its name and magic to a confusing array of quite unrelated things. Thus we have Spanish moss, Reindeer moss, Scotch moss and Irish moss, none of which is a true moss. There are moss roses and moss pinks in our own gardens. Mossycup oaks grace our forests and parks. There are moss agates for the rockhound, and moss campion and mossy stonecrop for the rock climber. There is a fish called mossbunker, and there are mossback turtles. We borrow the name for that perennial favourite,
"moss green" on colour paint charts, and we foul it to describe that "mossmouth" morning-after penalty for a late night's indiscretions. Will the real moss please stand up?
Fortunately, botanists do have the situation under control, even if most of the rest of us don't. There really is a natural group of seedless plants, accurately called mosses, although liverworts tend to get a little mixed up in here too.
But before defining just what a "true moss" is, let's first find out what it isn't by unmasking some of its pretenders, most of which are misnomers, pure and simple. Spanish Moss, often seen drooping in grayish veils from the trees of decaying southern plantations, grows parasitically on trees in the southern United States and tropical America. Although it hangs like moss from branches, it produces flowers and seeds (unlike real moss) and is related to the pineapple. Reindeer Moss (also called Caribou Moss) is a finely branched lichen, usually in shades of white, growing on the vast tundra and providing a supplementary diet for the caribou, as well as an emergency diet for lost hunters and explorers. Lichens are a strange amalgam of fungus and algae in one organism, lacking both the leafy structure and the fresh green colour of moss. Irish Moss is a purplish seaweed known as carrageen; it is bleached and used commercially in medicine and as a filler for some foods such as ice cream to keep it from melting. And a mossback turtle is only the proud owner of green algae, not moss, growing on its shell.
What then are the
true mosses? Where do they fit into the scheme of living things? What are the characteristics of moss that set it apart from other plant life?
Well, to answer these questions let's go for a leisurely walk in the woods. It is early spring and a pair of crows noisily announce our approach. The trees are alive with smaller migrants. The leaves we shuffled through last fall have been packed by winter snows and tucked beneath new growth by the spring rains. Trees and bushes have lost no time in screening nature's quiet processes with new foliage, and plant life has everywhere claimed the forest floor. Blue violets peep from along our path and the last of the white trilliums are turning pink.
A thick copse of bushes surrounds a low spot where we find what is left of a fallen tree half buried in earth and water from the spring run-off. It is the deepest part of the woods and the sun will not penetrate here much from now on. Bracket fungi along the side of the log keep company with a few of the early mushrooms. The rotting bark itself is covered with a thick coat of miniature plant life, and this is what now attracts our attention. Besides some goldthread and twin-flower plants there are some white, branched, and some red-tipped lichens. The rest of the log is covered by a mat containing several kinds of mosses and liverworts, almost indistinguishable from each other.
Most people will recognize the lichens by their woody, non-leafy appearance, but may be confused by the strangeness of the other two, and with good reason. Mosses and their close relatives the liverworts form a group know as BRYOPHYTES (from the Greek
bryon meaning moss, and phyton meaning plant), which taxonomically falls between the green algae on the one hand, and horsetails, club mosses and ferns on the other. In other words, mosses and liverworts were the first land plants believed directly evolved from the green algae, but no fossil evidence has ever been discovered to help pigeonhole them in an evolutionary context. Uncertainty and controversy surround their genealogy. Most authorities agree, however, that mosses are not necessarily ancestors of the vascular plants (plants with ducts for conveying sap) but are more likely a separate evolutionary form of plant life.
But back to our rotting log. Liverworts can usually be distinguished visually from mosses by their flattened, straggly, ribbon-like growth, in contrast to the shrubbier, more plant-like mosses. But let's set these relatives aside and leave the true mosses in center stage.
Mosses have "stems" and "leaves," but no flowers or seeds. They don't even have roots to supply the plant with nourishment. Instead fine brown filaments called rhizoids are used for anchoring the plant. With a few exceptions, mosses have no central core or channel in the stem for conducting moisture and nutrients throughout the plant - a major difference from the ferns and other higher vascular plants. They thus depend largely on the atmosphere and rainwater fortified by nutrients washed from the forest canopy overhead.
Moss leaves are usually only a single cell in thickness and greedily absorb water from the environment. Some species have the stems clothed in a thick brown (sometimes white) woolly coat called tomentum, which is an extension of the rhizoid system. In ideal conditions of humidity, this coat is a swollen gelatinous mass feeding water to the stem and leaves externally. Perhaps because of this ready absorption of substances from the atmosphere, mosses are affected more than other plants by pollution.
Mosses are not scarce. The Checklist of the Mosses of Ontario by Ireland and Cain lists 464 verified species and varieties for Ontario alone. And mosses are not hard to find; every woodland harbours a handful of true mosses in its cool shadows. Many more are at home in bogs and along riverbanks where they flourish in profusion.
Some species grow in water while others are able to endure long periods of drought on bare rock, or on brick or stone walls. Some grow only on rotting logs and stumps. Others are epiphytes on the bark of living trees or hang ribbon-like from their branches. (Epiphytes are non-parasitic plants that grow on another plant, but get their nourishment from the air.) Certain mosses need habitats rich in calcium carbonate, such as rich fens, and still others thrive only on the acidic granite of the north. And, believe it or not, some mosses can actually build new rock! Growing along the edge of a lime-laden stream, the moss extracts and becomes encrusted with the alkaline sediment. Over time, the encrustation becomes larger and new limestone forms.
Despite its rich biology, to most people, unfortunately, moss is just moss. It is something green that carpets the forest floor and is nice to walk on - even better to sit or lie upon.
But moss is not always pure green by any means. Although usually some
shade of green, living moss is often tempered with yellow, brown or blue. (So, too, come to think of it, is the "moss green" of the colour paint charts, as many of us have discovered to our chagrin.) White Moss, for instance, though bright green when wet, is almost white when dry. Others are such a dark green they're virtually black, showing faintly green only when moistened by rain or dew. One of our common mosses is called Silvery Bryum. As the name suggests and a hand-lens will show, the tiny plants appear skillfully crafted in silver. The stem or stalk of most mosses (called setae) ranges from green through to red and yellow. Being translucent, the stems pick up the light and assume a metallic sheen; in profusion, they form red, yellow or purplish patches covering the ground.
How often we hear someone say: "I know where there is lots of moss, but it's just ordinary moss." Ordinary moss? To me, there's hardly any such thing. Superficially, many mosses look alike to the novice because of their habit of growing in masses. We do not see the moss for the mass!
That "ordinary moss" could prove to be any one of dozens of species when examined closely. Frequently, a small handful of what appears at first glance to be one kind of moss will yield four or five species. Furthermore, a moss that is "ordinary" in one habitat may be unknown in another. And sometimes a few stray plants of a less common moss may be found isolated in the midst of a more plentiful species. So now perhaps you can see why I greet each clump of "ordinary" moss with enthusiasm. I never know what I'm going to find.
Bryophytes generally are photosynthetic, having chlorophyll to transform sunlight into energy. Being mainly shade-loving plants, they have developed some truly un-ordinary economies in this respect. One species, the Luminous Moss, grows principally in the entrances to dark caves or crevices. It is a tiny plant, part of which gathers the faint light rays and reflects them back, giving the plant an eerie glow and its common name. This characteristic is believed to have inspired legends of cave elves and goblins.
Another extra-ordinary thing about mosses is the way some species are spread across immense geographic distances. And to make things worse, they may look different wherever you find them. For example, some mosses leave a clear record of their annual growth rate, and this varies widely with conditions of latitude and altitude. In southern Ontario the Common Haircap Moss may add from 8 to 12 cm of new growth each year, proportionally less with diminished rainfall and shorter growing season. But some specimens of this species from the Yukon took eight years to grow 2 centimeters!
Sometimes above timber line, the plants will be so tiny that total length is less than 3 cm; the leaves, much shorter and broader, are almost oval in shape, making it seem to the novice like a different species.
A tendency to reddish coloration occurs in some mosses from boreal and alpine habitats where prolonged exposure to ultra-violet rays induces a reddish protective "sunburn." Thus we find that in many of the northern mosses each plant is tinted with a delicate pink or orange blush. The ultraviolet rays also inhibit growth causing some mosses to become uncharacteristic low, flat cushions, while others may be crowded into a deep solid turf. The naturalist who has really learned mosses can be quite confused in the arctic. There you may find that an otherwise familiar species usually found in lush loose carpets, sometimes grows only in squat button-cushions on the tundra rock.