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Liverwort

Life Cycle

The general features of the liverwort life cycle are the same as in mosses. Both have a characteristic alternation of generations in which the multicellular diploid sporophyte is dependent on the green and "leafy" haploid gametophyte. As with mosses, the gametophyte of liverworts is the form most commonly seen in nature.

In most species, a haploid liverwort spore germinates and gives rise to a single-celled protonema, a small filamentous cell. In general, the haploid gametophyte develops from the protonema. In most liverworts, the gametophyte is procumbent, although in some species it is erect. Typically, the gametophyte has a subterranean rhizoid, a specialized single-celled structure which anchors the liverwort to its substrate and takes up nutrients from the soil.

Male and female reproductive organs, the antheridia and archegonia, grow from the gametophyte. These arise directly from the thallus or are borne on stalks. About 80% of the liverwort species are dioecious (male and female on separate plants) and the other 20% are monoecious (male and female on the same plant). Each archegonium produces a single egg; each antheridium produces many motile sperm cells, each with two flagella. The sperm cells must swim through water to reach the archegonium. Then, the sperm fertilizes the egg to form a diploid cell. This eventually develops into a multicellular diploid sporophyte.

The sporophyte of liverworts, like that of mosses, has a terminal capsule borne on a stalk, known as a seta. As the sporophyte develops, haploid spores form inside the capsule. In general, the sporophytes of liverworts are smaller and simpler in morphology than those of mosses. Another difference is that the liverwort seta elongates after capsule maturation, whereas the moss seta elongates before capsule maturation.


Additional topics

Science EncyclopediaScience & Philosophy: Linear expansivity to Macrocosm and microcosmLiverwort - General Characteristics, Life Cycle, Spore Dispersal, Evolution - Asexual reproduction