Text: pages 57 - 73
- approximately 19 genera, and about 370 species
- one order Polytrichales, two families Polytrichaceae, and
Dawsoniaceae (one genus, ~12 species)
- they grow well on acidic and nutrient-poor substrata, thus
they can be abundant in open terrain
- Arctic to Antarctic from sea level to high alpine
Characteristics:
(a) spore
- unicellular when shed
- extremely small (50um in Dawsonia >65 mill
spores in one sporangium)
(b) protonema
- spore germinates to produce a protonema
- protonema (chloronema and caulonema)
- protonema may coat soil and then disappear after
gametophores are produced, some persist for many years (Pogonatum)
(c) Gametophyte
- the gametophore varies considerably in size.
- some gametophores are a few millimeters tall, while others
may exceed a decimeter (Dawsonia >60cm tall)
- still others, if one considers the accumulated growth of
preceding years, can form dense turfs more than a meter deep (P. strictum).
- Most are dioicous (antheridial and archegonial plants are
separate).
- both sexes have an erect stem that is unbranched or little
branched (some exceptions of course)
- the lower part of the stem is attached to the substratum
by a network of rhizoids.
(d) Stem
- in cross-section, has stereid cells in the cortical area,
many parenchyma cells, and often a central cylinder of hydroids that make up
the hydrome, an internal conducting system,structurally and functionally
resembling the xylem of vascular plants.
- the hydrome consists of elongate dead cells.
- surrounding the hydrome is an often ill-defined layer of
living cells that transport metabolites, this consists of leptoids, and
constitutes the leptome.
- this internal conducting system is often connected to the
hydroids and leptoids of the leaves, and thus forms an effective, if rather
inefficient, internal vascular system
- water uptake in the hair cap mosses is effected by the
water held in capillary spaces among the rhizoids, enclosed by sheathing leaf
bases, and spaces among the lamellae.
- water can, therefore, enter the chlorophyllose leaf cells
directly, can enter the stem through the cortical cells, or can enter the
interior portion of the stem from the part of the stem that is embedded in the
soil.
- water conduction up and down the stem is both external
(ectohydric) and internal (endohydric) and is very slow.
- in subterranean parts of the stem you see the same
conductive and supportive cells, but there is an interesting single layer of
cells within the stereids which are enlarge.
It is speculated that they serve as an endodermis
(e) Leaves
- 6-8 leaves spirally arranged on the stem and divergent
when wet and contorted or imbricate when dry
- usually long and lanceolate, sometimes toothed, often has
an expanded colorless sheath, often abrupt "hinge" inward when dry
- usually have a costa, and on this midrib are often the
lamellae on adaxial surface of leaf.
- lamellae and most of the leaf blade (except the midrib)
are a single cell in thickness (unistratose).
- What is the
function of the lamellae?
- in some species of Polytrichum
and Dawsonia the only photosynthetic
cells are found in the lamellae
- the upper edge of the lamellae and often the
undersurface of the leaf, especially on
the midrib are heavily cutinized.
- increased phtosyntheitc surface area
- Atrichum has
reduced lamellae, no sheathing base, entire blade is photosynthetic
- Alophosia -
entire blade is bistratose
- the midrib of the leaf is usually reinforced by
thick-walled cells, the stereids. The
supportive cells give the leaf rigidity and are also significant in water
transport.
- the midrib often has hydroid cells, and sometimes
leptoids.
- there are some species with very reduced gametophore
perigonium and perichaetium often protonema is perennial (Pseudoracelopus, Racelopus, Racelpodopsis)
(f) Reproduction
- asexual reproduction by gemmae only found in one genus (Alophosia) disc-like gemmae,
subterranean fragmentation, can see Atrichum
leaves in the soil with protonema sticking out
- the antheridial plant, when sexually mature, is
terminated by a cluster of expanded leaves, usually with little chlorophyll,
and often pigmented with brown or reddish colour.
- this cup-like mass of leaves forms the perigonium and
encloses, at the axils of each of the leaves, clusters of filaments, the
paraphyses, and antheridia. The
antheridia are elongate. In the centre of the perigonium an apical cell remains
meristematic.
- the paraphyses - multiseriate and club-shaped in some genera
while in others uniseriate
- mucilage is produced (females too) which may attract
insects to attract invertebrates for sperm dispersal
- the perigonium is thus shaped like a splash-cup, and water
droplets can eject mature sperm some distance from the perigonium, therefore
closer to the female gametophore, and shorten the swimming distance for the
sperm to reach the egg.
- apical cell is not used up and new growth occurs at the
apex..may see a series of perigonial gamtophores like beads on a string.
- The female gametophores, when mature are terminated
by perichaetia formed of perichaetial leaves enclosing paraphyses and
archegonia and retain moisture within the capillary spaces along them.
- the perichaetial leaves are narrower and more elongate
than the vegetative leaves ( in some species the perichaetial leaves do not
have lamellae)
- the perichaetium, however, exhausts the apical cell in its
production, therefore any further apical growth of this gametophore is through
the production of a new apical cell below the perichaetium; this innovation
allows elongation of the stem via a lateral bud (innovations)
The erect
male and female gametophores often form dense turfs with the rhizoids firmly
intertwined among gametophores and with the soil.
Calyptra is hairy (hairs point downward)
How do the Polytrichums prevent water stress?
- cuticular resistance
- internal replenishment of water
- a drop in the ratio between carbon dioxide uptake and boundary layer resistances afforded by
mesophyll-like tissues
- turgor-driven leaf folding
Sporophyte:
The
sporophyte in the hair-cap mosses, like most mosses, consists of a foot
embedded in the gametophore, and a seta terminated by the sporangium. The foot possesses transfer tissue that
allows for translocation of water (and its dissolved minerals), plus
metabolites produced by the gametophore, to the sporophyte.
- the whole sporophyte, when immature, is an effective
photosynthetic organ, gaining its water, minerals and much of its metabolite
from the gametophore. It is unable to
absorb water directly, therefore depends on the gametophore.
(a) seta
- The length of
seta and sporangium size show similar diversity, but rarely does the seta
exceed 10 cm and the sporangium is usually less than 5 mm long.
Fig 6-9
- in cross-section, consists of highly cutinized epidermal
cells enclosing a layer of stereid cells and internal parenchyma.
- a central strand of hydroids encircled by a layer of
leptoids complete the anatomy of the seta.
- the internal conducting system allows for more efficient
transfer to the top of the sporophyte, and to the elongation of the sporophyte
preceding its differentiation of the terminal sporangium.
(b) sporangium
- The calyptra is often covered in hairs (branched,
uniseriate hairs), hence the name, hair-cap mosses. (note an exception Atrichum)
Hand-out
- stomata often on sporangial wall, often in grooves between
the apophysis and the rest of the sporangium,
sometimes the guard cells may not be completely separated (therefore the
stoma is a single cell with a slit in the middle). These enhance gas exchange
in the young sporophyte and undoubtedly are important in the upward movement of
water and metabolites from the gametophore to the sporophyte.
- the sporangium, when mature, has an operculum
Polytrichaceae
- within the sporangium the cylindric columella, usually
expanded at its apex beneath the operculum, forms the epiphragm, and is
surrounded by the cylinder of sporogenous cells.
- air chamber with filaments for support
- the peristome teeth that ring the internal margin of the
sporangium mouth, overarch the epiphragm, and are attached to its surface.
- this results in tiny holes between the teeth and their
attachment to the epiphragm.
- the calyptra falls and exposes the operculum. When the
sporangium dries, the operculum falls off and exposes the peristome teeth and
epiphragm. Any pressure on the
epiphragm or sporangium wall, or any jarring of the sporangium, puffs the
spores through these tiny holes, and disperses them into the air.
- amphithecial in origin, the two
inner layers undergo some periclinal divisions to produce a layer about 4 cells
thick
- the teeth are made up of uniformly thickened cells
- the cells at the base of the tooth are more or less
isodiametric, while they gradual become more elongate as you go up.
- in the
Polytrichaceae there are generally 32-64 solid teeth (rarely 16)
- some species have no peristome (Alophosia, Bartramiopsis, Lyelia)
Dawsoniaceae
- NewZealand and Polynesian islands
- sporangium is lunate in cross-section
- in Dawsonia
(Dawsoniaceae) the peristome is made up of elongated thick-walled cells which
remain separate, or united only in small groups, thus resulting in the shaving
brush" appearance
- many long, multicellular filaments formed by asymmetric
cell divisions in about 8 concentric layers of cells in radial rows (inner
amphithecium)
- no epiphragm
- there is some
controversy about the homology between these two types of teeth, but it is
generally accepted that at least the cells at the base of the teeth are
homologous. The endothecium of the Dawsonia may be incorported into the
teeth (Edwards doesn’t think so), whereas the teeth of the Polytrichaceae is of
amphithecial origin. (abscission layer of the inner side of the teeth in P. commune is the innermost
amphithecium.
- interesting that taxonomy of the bryophytes is based on
sporophytic material. These are so
different, but gametophytically very similar!
Should this be reflected taxonomically?????