Polytrichium: hairy cap moss
Polytrichium is a common moss that occurs across all of North America. It is large for a moss and regularly exhibits both the haploid and the diploid phases of its life cycle. Thus it is a useful example of what mosses are about. It is generally found in forests but can also be found on soil or gravel in open areas.
Phylogeny and taxonomy
In the past, the three groups of non-vascular plants (mosses, liverworts and hornworts) were put together in a taxonomic unit (Division Bryophyta = ‘Bryophytes’ = non-vascular plants), but there is little evidence that this is a unified group other than they are all plants. Mosses are distinct from the other two non-vascular groups and vascular plants are probably more closely related to liverworts than to mosses. Mosses are now generally put into their own division, the Bryophyta, distinct from the division of liverworts (Hepatophyta) and hornworts (Anthocerophyta). Within the Bryophyta there are around 12,000 species.
Structure
The gametophyte (haploid form) of mosses is the form that is usually seen. It is green, (photosynthetic) and lives for several years. Polytrichium has an erect unbranched stem with small pointed ‘leaves’ emerging off the sides. Technically it lacks ‘true’ vascular tissue because it lacks lignin. But it does produce cells, comparable to vascular cells, that are specialized for transport. Even without lignin for support Polytrichium can produce a stalk standing 4-10 cm tall. The plants have thread-like rhizoids emerging from the base of plant and attaching it to the substrate.
The gametophyte gets its name because it produces gametes. Polytrichium, but not all mosses, is dioecious, meaning that it has separate male and female plants. The gamete-producing organs appear at the tips of the stems, in structures (antheridia) that produce many mobile (flagellated) sperm on the male plants and structures (archegonia) on the female plants in which are produced a single, immobile egg.
The diploid form of the plant is called a sporophyte and it grows out of the structure that produces the egg (the archegonium). It has a typical spore-producing structure (cf. that of the cellular slime mold, Dictyostelium, or mushrooms) with a stalk (often over 5 cm) elevating a spore producing capsule at its top. The elevation provided by the stalk (seta) allows the spores to be more readily dispersed by the wind. Most moss capsules have one or two rings of teeth surrounding the opening of the capsule that can open and close, releasing spores under favorable (dry) conditions when they will be transported further by the wind.
Unlike familiar animals who reproduce by directly making replicas of themselves, plants, including mosses like Polytrichium, alternate between two forms: the sporophyte makes gametophytes and the gametophytes make sporophytes. Note that, if one rigidly holds to a definition of organisms being entities distinct in space, then the sporophytes produced by gametophytes