CLASS ANTHEROCEROTAE
(hornworts)

INTRODUCTION

- single order Antherocerotales, single family Anthocerotaceae (some put Notothylas in its own family, Notothyaceae)
- 5 to 9 genera include Anthoceros (250 species), Megaceros (46 species), Dendroceros (51 species), Phaeoceros  = Aspiromitus  or Anthoceros (30 species), Folioceros (19 species), and Notothylas (10 species)
- widely distributed in temperate and tropical latitudes
- all genera found in tropics and subtropics
- Anthoceros, Phaeoceros, and Notothylas widespread in temperate climates
- most found on moist mineral soil of banks and cliffs, among grasses, and along streams

CHARACTERISTICS

Gametophyte:

(a) spores
- adhere in tetrads until shed
- spores may be multi- or unicellular when shed
- some are green (Megaceros, Dendrocero), but all others are yellow to blac
  - ornamentation variable
  - some species the spores are viable for many years

(b) germination - three main patterns:

                  (i) short uniseriate germ tube that becomes two to four cells in length, apical cell devlops (Anthoceros),

                  (ii) spore wall ruptures irregularly forming a multicellular mass from which the thallus develops (Notothylas)

                  (iii) chlorophyllose and multicellular when they are shed and initiate the apical cell directly develops from mass (Dendroceros)

(c) thallus
- dark-green, succulent brittle, dorsiventrally flattened, composed of thin-walled cells (parenchymatous)
- apical cell of most with four cutting faces (Dendroceros have three)
- multistratose at least near the middle (Dendroceros has a multicellular ribbon-like midrib and thin ruffle on either side)
- attached to substratum by smooth rhizoids (branched at tips)
- each cell has a single disc-shaped chloroplast (except in Megaceros where you may find 12),
- the chloroplasts often have pyrenoids (contains RUBISCO
- ventral side of gametophyte with mucilage cleft (stoma-like)
- plants of temperate plants often with mucilage-filled cavities formed by the breakdown of groups of cells, these cavities are often invaded by blue-green algae, Nostoc which access through the mucilage cleft  (fungal associations also occur)
- no oil bodies

(f) reproductive structures
- development is the same in all hornworts
- in monoicous plants the antheridia develop and mature prior to the development of the archegonia

         (i) antheridia:
- embedded in the upper layers of the thallus when mature, they are formed from superficial cells within the mucilage cavity
- numerous antheridia often originate within a single antheridial chamber (single antheridia per sac in Megaceros and Dendroceros)
- secondary antheridia are developed from the basal cells of the primary antheridia
- mature antheridia are spherical and have a short stalk
- as they elongate they push through the roof of the cavity and expel the sperm
- the antherida occur in scattered patches ont he thicker portion of the thallus, often behind the growing point

         (ii) archegonia:
- archegonia are not discrete organs (no stalk), but are represented by neck canal cells and an egg surrounded by essentially undifferentiated cells of the thallus
- surface cells above the neck cell become mucilage papillae (canal cells) which when disintegrate expose the neck canal
- develop usually behind growing point

(f) asexual reproductive structures
- some thalli (Anthoceros) produce internal "tubers" that survive unfavourable seasons
- Anthoceros also produces gemmae at the thallus margin
- older parts can decay away and leave the younger portions to become separated and act as diasporas
- asexual reproduction is especially important for dioicous species

(g) calyptra????
- as sporophyte develops there are cell divisions of adjacent cells in the thallus, often called a thallus-calyptra (involucre-calyptra), because cells of the thallus are involved in the area of where the venter would be

Sporophyte:

(a) sporophyte
- may reach lengths of 12 cm

         (i) early development
- first division of zygote is by a longitudinal line (different from ALL other bryophytes)
- endothecium  becomes the columella and the amphithecium produces the sporogenous layer and jacket - unlike all other bryophytes in which the endothecium produces sporogenous layer (Sphagnidae is the exception; the amphithecium produces the sporogenous layer)
- the sporogenous layer overarches the columella early in development

         (ii) foot and involucre
- the foot produces short haustorial projections that push among the cells of the thallus, increase absorptive capacity of foot, transfer cells are only on gametophyte side
- the involucre persists at the base of the sporangium

         (iii) seta
- the seta is an undefined zone (some say absent)

         (iv) basal meristem
- possesses a basal (intercalary) meristem that continously develops new sporangial tissue (indeterminate growth), the sporangium does not grow indefinitely though, the gametophyte will be expended or the seasons will cause cessation

         (v) mature sporangium
- sporophyte usually grows throughout the favorable season, shedding spores at the apex and differentiating new spores from the basal meristem
- multistratose jacket with stomata (except in Dendroceros and Notothylas where they are absent and rudinmentary in Megaceros)
- hypoepidermal cells with two chloroplasts per cell
- outer layer sometimes with thickened transverse walls
- sporogenous layer unistratose until meiosis
- archesporial tissue gives rise to (a) "elater" mother cells (small nuclei), (b) spore mother cells (large nuclei) - alternate in tiers
- there is usually a cylindrical columella in the sporangium
- the columella may have  spiral and annular thickenings (Dendroceros), probably important in conduction

         (vi) pseudo-elaters
- does not have true elaters.because of development, not from same mother cell as spores, pseudo-elaters are unicelluar and multicellular, they are variable in shape.  Some have no ornamentation, some have thickenings, some long, some short, thought to perhaps be important in water conduction (Proskauer) as well as dispersal, may also serve as internal tapetum

         (vii) sporophyte nutrition
- photosynthetic
- columella and pseudo-elaters nutrition function

(b) dispersal
- the sporangium usually opens by one or two longitudinal lines, initially the split does not extend to the apex of the sporangium, the jacket walls are often hygroscopic
- Notothylas dehisces by decay of the jacket
- as the apex of the sporangium matures and dries out, the slits gape open and expose the spores and pseudo-elaters
- pseudo-elaters twist as they dry, in some, the jacket is hygroscopic
- dehiscence continues downward as the sporangium matures and new spores are ready for dispersal