Bryophytes

Liverworts

Liverworts (Hepaticophyta) are viewed as the plants most closely related to the ancestor that moved to land. Liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species (Figure). Some gametophytes form lobate green structures, as seen in Figure. The shape is similar to the lobes of the liver, and hence provides the origin of the name given to the phylum.

 The illustration shows a variety of liverworts, which all share a branched, leafy structure.
This 1904 drawing shows the variety of forms of Hepaticophyta.
 Photo shows a liverwort with lettuce-like leaves.
A liverwort, Lunularia cruciata, displays its lobate, flat thallus. The organism in the photograph is in the gametophyte stage.

Openings that allow the movement of gases may be observed in liverworts. However, these are not stomata, because they do not actively open and close. The plant takes up water over its entire surface and has no cuticle to prevent desiccation. Figure represents the lifecycle of a liverwort. The cycle starts with the release of haploid spores from the sporangium that developed on the sporophyte. Spores disseminated by wind or water germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Male and female gametangia develop on separate, individual plants. Once released, male gametes swim with the aid of their flagella to the female gametangium (the archegonium), and fertilization ensues. The zygote grows into a small sporophyte still attached to the parent gametophyte. It will give rise, by meiosis, to the next generation of spores. Liverwort plants can also reproduce asexually, by the breaking of branches or the spreading of leaf fragments called gemmae. In this latter type of reproduction, the gemmae—small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus (shown in Figure)—are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes.

 The liverwort has a flat, leaf-like structure haploid (1n) called a thallus. Root-like rhizoids grow from the bottom of the thallus. A slender stalk extends from the thallus, and an archegonial head sits at its top. The archegonial head has fronds, like a palm tree. The underside of the archegonial head contains protrusions called archegonia, which house the eggs. Sperm enter through a hole in the bottom of the archegonium and fertilize the egg to produce a diploid (2n) embryo. The embryo grows into a stalk. Meiosis produces haploid (1n) spores in a sac at the tip of the stalk . The sac bursts open, releasing the spores. The spores sprout, producing a new thallus and rhizoids.
The life cycle of a typical liverwort is shown. (credit: modification of work by Mariana Ruiz Villareal)
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