Bryophyte

Hepatophyta means "liver plant" and refers to the body of some common species of liverworts, whose lobing is reminiscent of a liver. During Medieval and earlier times, many people followed the doctrine of signatures—a belief that the superficial resemblance of a plant to some part of the human anatomy indicated that the plant possessed medicinal properties related to the organ it resembled. Liverworts are the simplest of the living plants, and range in size from minuscule, leafy filaments less than 0.02 in (0.5 mm) in diameter, to plants exceeding 8 in (20 cm) in size. Liverworts lack specialized conducting tissues, cuticles, and stomates, and their rhizoids are always unicellular. The gametophytes arise directly from spores in most species. Most liverworts (75%) have nine chromosomes in their haploid cells. There are two kinds of liverworts based on body form: thallose and leafy.

Thallose liverworts have gametophytes with an undifferentiated body called a thallus which has a ribbon-like appearance. Marchantia is one of the most widely distributed thallose liverworts, especially in habitats that provide ideal conditions of light and high humidity. The body is typically 30 cells thick at the midrib, and only 10 cells thick elsewhere. A thin, upper, green layer contains chlorophyll-rich cells, arranged in polygonal or diamond-shaped patterns each centered on a permanently open pore. Below each polygon is an airspace connected to the outside by the pore, and within the chamber are erect threads of photosynthetic cells. Below the relatively thin, upper photosynthetic layer is a lower layer that is colorless and stores the products of photosynthesis. Most reproduction is asexual by fragmentation, usually caused by wind or by animals breaking the plants apart while eating them or when stepping on them or when trampling them. Thallose liverworts also commonly reproduce asexually by producing small balls of cells called gemmae, within bowl-like structures called gemma cups. The balls become detached and are splashed out by raindrops, dispersing away to colonize favorable habitats.

Leafy liverworts grow in wet or humid habitats, and are especially common in the tropics and subtropics, although they also occur in temperate areas. They are the simplest of the plants with leaflike structures. Their leaves lack vascular tissue, each is deeply cleft so as to appear two-lobed, and they are arranged in two rows along a much branched stem. Unlike the true mosses, which typically appear somewhat erect, leafy liverworts form small, flat mats. Asexual reproduction by fragmentation is common.