There are about 8,000 known species of liverworts. The most common and widespread liverworts have flattened, lobed, somewhat leaflike bodies called thalli (singular: thallus). The thalloid liverworts, however, constitute only about 20% of the species. The other 80% are "leafy" and superficially resemble mosses.
Liverworts differ from mosses in several details and are considered less complex. Their thalli or "leafy" stages (game-tophytes) develop from spores. When the spores germinate, they may produce a protonema, which is an immature gameto-phyte consisting of a short filament of cells. Typically, the pro-tonema develops further into a more extensive gametophyte, or in some instances, the protonemal stage may be absent, the mature gametophyte then developing directly from the spore. Thalloid liverworts have smooth upper surfaces with various
Figure 20.4 Marchantía (a thalloid liverwort). A. Thalli with male gametophores. B. Thalli with female gametophores.
1. The Doctrine of Signatures held that any plant part that resembled a human body part would be helpful in healing that body part if it was diseased or hurting.
markings and pores, and the corners of cell walls of most liverworts are specially thickened. The lower surfaces have many one-celled rhizoids. Growth is prostrate instead of upright, and the rhizoids, which look like tiny roots, anchor the plants.
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