Anthocerotopsida
A small class of the plant division Bryophyta, made up of plants commonly called hornworts. There is a single order with six genera.
Gametophyte structure
The gametophytes are flat thalli, often forming rosettes of uniform thickness or, in Dendroceros, with a thickened midrib and crisped unistratose wings. Ventral scales are absent. Rhizoids are unicellular, with thin, smooth walls. The thallus of undifferentiated tissue is sometimes provided with cavities containing mucilage (and often occupied by Nostoc colonies) and ventral pores leading to them. The cells are thin-walled and lack oil bodies; at least those at the surface have one, two, or more large chloroplasts with a central pyrenoid. Slime papillae (or mucilage hairs) are lacking. The stalked antheridia develop from internal cells and occur singly or in groups in cavities beneath the upper surface of the thallus and become exposed by the rupture of overlying tissue. The archegonia, also endogenous, are embedded in the dorsal tissue of the thallus. Paraphyses are lacking. The sporophyte is surrounded at the base by a tubular involucre (or completely surrounded in Notothylas). A calyptra is lacking.
Sporophyte structure
The sporophyte consists of a massive foot and an erect, long-cylindric, green capsule (see illustration), though in Notothylas the capsule is horizontal and spindle-shaped. The capsule, indeterminate in growth owing to a basal meristem, dehisces from the apex into two valves which are usually twisted when dry. The wall consists of several cell layers in a more or less solid tissue having one, two, or more chloroplasts per cell. Stomata with two guard cells are usually present. The spore sac, derived from the amphithecium, surrounds and overarches the slender columella (lacking in Notothylas). Spore maturation proceeds from the apex downward. The tetrahedral spores are mingled with pseudelaters of one to several cells, with or without spiral bands. A protonema is lacking. The haploid chromosome number is 5 or 6. See also Reproduction (plant).
The elongate capsule dehiscing by two valves and its indeterminate growth from a basal meristematic tissue are unique, as are the large chloroplasts with a central pyrenoid and the endogenous origin of sex organs. (The chloroplasts are sometimes single in the gametophyte and paired in the sporophyte.) The spore mother cells undergo meiosis directly, but the elater mother cells usually undergo several mitotic divisions before differentiation as pseudelaters of an unreduced chromosome number. As a result, the pseudelaters may be more numerous than the spores. (In the Hepaticopsida, by contrast, the spore mother cells undergo meiosis directly or undergo several mitotic divisions before meiosis, whereas the elater mother cells mature directly into diploid elaters.) See also Andreaeopsida; Bryophyta; Bryopsida; Hepaticopsida; Sphagnopsida.
