<P> The plant body of a hornwort is a haploid gametophyte stage . This stage usually grows as a thin rosette or ribbon - like thallus between one and five centimeters in diameter . Each cell of the thallus usually contains just one chloroplast . In most species, this chloroplast is fused with other organelles to form a large pyrenoid that both manufactures and stores food . This particular feature is very unusual in land plants, but is common among algae . </P> <P> Many hornworts develop internal mucilage - filled cavities when groups of cells break down . These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc . Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue - green color . There may also be small slime pores on the underside of the thallus . These pores superficially resemble the stomata of other plants . </P> <P> The horn - shaped sporophyte grows from an archegonium embedded deep in the gametophyte . The sporophyte of a hornwort is unusual in that it grows from a meristem near its base, instead of from its tip the way other plants do . Unlike liverworts, most hornworts have true stomata on their sporophyte as mosses do . The exceptions are the genera Notothylas and Megaceros, which do not have stomata . The sporophyte of most hornworts are also photosynthetic, which is not the case with liverworts . </P> <P> 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 </P>

Where does the sporophyte of a hornwort develop