Like all
bryophytes, the dominant life phase of a hornwort is the
haploid gametophyte. This stage usually grows as a thin
rosette or ribbon-like
thallus between one and five centimeters in diameter. Hornworts have lost two
plastid division-associated genes, ARC3 and FtsZ2, and have just a single
chloroplast per
cell (monoplastidy), with the exception of the genus
Megaceros and some species in the genera
Nothoceros and
Anthoceros, which have more than one chloroplast per cell (polyplastidy). In the polyplastidic species, and also some of the monoplastidic species, a cellular structure called a
pyrenoid is absent. The pyrenoid is a liquid-like
organelle which enables a more efficient photosynthesis, has evolved independently five to six times in hornworts and is present in half of the roughly 200 species. It is formed by the fusion of the chloroplast with other organelles and is composed predominantly of
RuBisCO, the key enzyme in carbon fixation. By using inorganic carbon transporters and carbonic anhydrases, up to a 50-fold increase in levels can be achieved. This particular feature is very unusual in land
plants, unique to hornworts, but is common among
algae. They are also the only group of land plants where
flavonoids are completely absent. Many hornworts develop internal
mucilage-filled cavities or canals when groups of cells break down. These cavities secrete hormogonium-inducing factors (HIF) that stimulate nearby, free-living
photosynthetic cyanobacteria, especially species of
Nostoc, to invade and colonize these cavities. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. Symbiotic
cyanobacteria have not been reported in
Megaceros or
Folioceros. There may also be small
slime pores on the underside of the
thallus. These pores superficially resemble the
stomata of other plants. The horn-shaped
sporophyte grows from an
archegonium embedded deep in the gametophyte. The growth of the hornwort sporophyte happens from a persistent basal
meristem, in contrast to the sporophyte of moss (apical growth) and liverworts (intercalary growth). Unlike
liverworts, hornworts have true
stomata on their sporophyte as most mosses do. The exceptions are the species
Folioceros incurvus, the genus
Notothylas and the three closely related genera
Megaceros,
Nothoceros and
Dendroceros, which do not have stomata.
Notothylas also differ from other hornworts in having a reduced sporophyte only a few millimeters tall. The sporophyte in hornworts is unique among bryophytes in being long-lived with a persistent photosynthetic capacity. The sporophyte lacks an
apical meristem, an
auxin-sensitive point of divergence with other land plants some time in the
Late Silurian/
Early Devonian. When the sporophyte is mature, it has a multicellular outer layer, a central rod-like
columella running up the center, and a layer of
tissue in between that produces spores and
pseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters of
liverworts. They have
helical thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80
μm in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the
proximal surface, and with a
distal surface ornamented with bumps or spines. == Life cycle == The life of a hornwort starts from a
haploid spore. The spores can be yellow, brown or green. Yellow and brown spores have a thicker wall and contain oils that both protect against desiccation and function as a nutrient storage, allowing them to survive for years. The species
Folioceros fuciformis and the genera
Megaceros,
Nothoceros and
Dendroceros have short-lived spores with thin and colorless walls that appear green due to the presence of a chloroplast. In most species, there is a single cell inside the spore, and a slender extension of this cell called the
germ tube germinates from the proximal side of the spore. The tip of the germ tube divides to form an
octant (solid geometry) of cells, and the first
rhizoid grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid
protonema. By contrast, species of the family
Dendrocerotaceae may begin dividing within the spore, becoming
multicellular and even
photosynthetic before the spore germinates. In either case, the protonema is a transitory stage in the life of a hornwort. From the protonema grows the adult
gametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin
rosette or ribbon-like
thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the
chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant. When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are
monoecious, with both sex organs on the same plant, but some plants (even within the same species) are
dioecious, with separate male and female gametophytes. The female organs are known as
archegonia (singular archegonium) and the male organs are known as
antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells. The biflagellate
sperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form a
zygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is
longitudinal. Further divisions produce three basic regions of the sporophyte. At the bottom of the
sporophyte (closest to the interior of the gametophyte), is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the foot), is a
meristem that will continue to divide and produce new cells for the third region. This third region is the
capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce
pseudo-elaters and
spores. These are released from the capsule when it splits lengthwise from the tip. ==Evolutionary history==