Common Name: Broom moss, Wind-blown moss, Fork moss – Moss may well have been one of the earliest words in the Germanic languages. The word mēos in Old English stems from the Old High German mos which means swamp. In Latin it is muscus, which is related to dampness or wetness. The boggy habitat of most mosses is the basis for the etymology.
Scientific Name: Dicranum scoparium – The generic name is Latin for ‘two heads’ which may refer to the structure of the sporophyte. Scopa is Latin for ‘broom’, the specific name intimates either that the moss looks like a broom or that the leaves appear to have been swept to one side by a broom.
Potpourri: There are many types of moss – about 15,000 species in some 600 genera; most of these are in the order Bryales, which are called the ‘true mosses.’ Mosses are found on every continent including Antarctica; they form an integral part of the arctic tundra. The only exclusion areas for mosses are marine habitats due to the salinity of the water. Many mosses have evolved to occupy a unique niche. For example the mosses of the Splachnaceae family require large amounts of nitrogen and accordingly live on dung and decaying carcasses; one habitat is Point Barrow, Alaska where the native Inupiat once dismembered whales for blubber. However, the most prolific mosses occupy the shady recesses of mesic (well watered) forest regions. The key to understanding the nature of mosses relative to other plants is the manner in which they acquire nutrients in the environment. Mosses are assigned to the Division Bryophyta, which is comprised of the land plants (embryophytes) that are non-vascular; the liverworts and the hornworts in addition to the mosses. The mosses are distinguished from the liverworts and hornworts (sometimes called horned liverworts as they are similar) by their physiology; moss leaves are arrayed with radial symmetry about a central axis whereas liverwort leaves are lobed or branched (not radial) and are dorsiventral (the upper or dorsal side is different from the ventral or lower side). The vascular plants, or tracheophytes, are the more advanced land plants that comprise most of the familiar flower bearing angiosperms and coniferous gymnosperms. Vascular means that they have ducts that convey fluids and salts up from the roots (through the xylem) and sugars from the leaves back to the roots or to growth regions (through the phloem). They can get quite large and complex but they require and extensive resource base of soil for water and minerals. Mosses do not have the complex plumbing of the vascular plants but one of two alternative non-vascular methods for water transport. One group has root-like structures called rhizoids that move water from the soil to a central strand in the leaf. The other group has no rhizoids and must absorb all nutrients from the leaf surface. In either case, the moss habitat is determined by the need for water which is more prevalent in shaded areas to which they have adapted as an evolutionary consequence of epigeal growth. The non-vascular distinction is more subtle than the presence or absence of vessels, since some mosses do have root-like rhizoids. The difference is that vascular plants have vessels made from lignin, the connective tissue of the cellulose, the essence of the woody plant; bryophytes have no lignin.
The proliferation of mosses in forests is a matter of the habitat and preference of ecology. They predominate in areas with a high water table which are adequately shaded to protect against the desiccation attendant to the radiant heat of direct sunshine. The hackneyed mnemonic that associates the presence of moss growing on tree boles as an indication of the northern cardinal point in an otherwise labyrinthine forest is not universal. Moss grows where there is enough moisture. While this is more likely on the north side of trees which are less prone to being dried out by the sun it can equally occur on the downhill side of an upslope. In the northern hemisphere the Sun trends to the south so the north side would be shadier; however, in the southern hemisphere, moss would grow on the south side with syllogistic reason. And it may not be moss at all; the tightly adherent green growth on a tree is more likely to be the ubiquitous green alga of the genus Pleurococcus. Mistaken identity is responsible for the etymology of a number of other moss-like growths that aren’t mosses. Reindeer moss (Cladonia rangiferina) is a lichen which is a combination of an alga and a fungus, Spanish moss (Tillandsia usneoides) is an epiphytic (absorbing nutrients from the air) angiosperm in the bromeliad family and Club moss (Lycopodium clavatum) is a primitive evergreen vascular plant. Irish moss is Atlantic seaweed that is used to clarify beer as an integral part of the brewing process.
Bryophytes (and ferns) have a much more profound and probably primordial distinction that differentiates them from the seed plants: spores are an integral part of the lifecycle, which is dominated by the haploid, or ‘n – chromosome’ state. Most land plants and all animals have cells which have a double set of chromosomes (2n) called a diploid; only the sex cells use the haploid or n-chromosome so that the combination of the sperm (n) and the egg (n) from different individuals results in a diploid (2n) offspring. The life cycle of the moss starts with a spore (n) that germinates into a root-like anastomosis called a protonema if it lands in an auspicious locale. The protonema grows into what is recognizably the green, leafy, photosynthetic gametophyte (n) phase of the moss cycle. At maturity, the gametophyte develops the female sex organ called an archegonium that produces an egg (n) and a male sex organ called an antheridium that produces a number of biflagellate (two-tail) sperm (n), which must swim to the egg; the obligatory moist habitat of the mosses is thus manifest – no water, no sex, no moss. Fertilization can occur with both sex organs on the same plant, which is called monoicous (the same as monoecious for flowering plants) or with the sex organs on different plants which is called dioicous (dioecious). In either case, the fertilized egg, which is now a 2n sporophyte, will grow to maturity, a process that can take up to six months. The sporophyte rises above the gametophyte on a stalk with the spore-bearing sporangium at its apex. At maturity, the 2n mother cells in the sporangium undergo the meiosis of reduction division to produce the n spores which are shed through a hole called an operculum to disseminate and continue the cycle.
Mosses are thought to be one of the first plants to emerge from the “primordial swamp” to establish a terrestrial presence. However, due to their herbaceous nature, the fossil record is relatively devoid of any evidence of their evolution and their provenance must remain speculative. The earliest fossil remnants are of liverworts from the Upper Devonian Period when the majority of the vascular land plants first appear (400 million years ago). Moss first appears in the known fossil record in the Permian Period some 100 million years later. However, there are vestigial structural aspects of moss physiology that support the primordial hypothesis. A logical progression would have been for the green algae of the oceans to establish a terrestrial beachhead. The gametophyte and the sporophyte then evolved independently with erect stems and green leaves and eventually merged to a single entity. This is suggested by the fact that the sporophyte has chlorophyll and plastids (food storage bodies) and that the gametophyte has stomata (breathing pores) on its leaves; these would indicate that they were both at one time independent entities. The protonema which is the key to the life cycle grows in a branching and threadlike manner that is very similar to the growth of green algae, its likely progenitor.