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 half of the roughly 200 hornwort species, this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food. The pyrenoid is comprised predominantly of RuBisCO, the key enzyme in carbon fixation. By using inorganic carbon transporters and carbonic anhydrases, up to a 50-fold increase in CO2 levels can be achieved. This particular feature is very unusual in land plants, unique to hornworts, but is common among algae.