Liverworts

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 The illustration shows a variety of liverworts, which all share a branched, leafy structure.
Liverworts. This 1904 drawing shows the variety of forms of Marchantiophyta. Source: OpenStax Biology 2e

OpenStax Biology 2e

Liverworts (Marchantiophyta) are currently classified as the plants most closely related to the ancestor of vascular plants that adapted to terrestrial environments. In fact, liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species. Lobate liverworts form a flat thallus, with lobes that have a vague resemblance to the lobes of the liver, which accounts for the name given to the phylum. Leafy liverworts have tiny leaflike structures attached to a stalk.

Photo shows a liverwort with lettuce-like leaves. The gemma cup, described in the paragraph and subsequent life-cycle diagram, is highlighted.
Liverwort gametophyte. A liverwort, Lunularia cruciata, displays its lobate, flat thallus. The organism in the photograph is in the gametophyte stage, but has not yet produced gametangia. Lunularia gametophytes produce crescent-shaped gemmae (circled), which contain asexual spores. The tiny white dots on the surface of the thallus are air pores. Source: OpenStax Biology 2e

Openings in the thallus that allow the movement of gases may be observed in liverworts. However, these are not stomata, because they do not actively open and close by the action of guard cells. Instead, the thallus takes up water over its entire surface and has no cuticle to prevent desiccation, which explains their preferred wet habitats. The image below represents the lifecycle of a lobate liverwort. Haploid spores germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Stalk-like structures (gametophores) grow from the thallus and carry male and female gametangia, which may develop on separate, individual plants, or on the same plant, depending on the species. Flagellated male gametes develop within antheridia (male gametangia). The female gametes develop within archegonia (female gametangia). Once released, the male gametes swim with the aid of their flagella to an archegonium, and fertilization ensues. The zygote grows into a small sporophyte still contained in the archegonium. The diploid zygote will give rise, by meiosis, to the next generation of haploid spores, which can be disseminated by wind or water. In many liverworts, spore dispersal is facilitated by elaters—long single cells that suddenly change shape as they dry out and throw adjacent spores out of the spore capsule. 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 — 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.
Reproductive cycle of liverworts. The life cycle of a typical lobate liverwort is shown. This image shows a liverwort in which antheridia and archegonia are produced on separate gametophytes. (credit: modification of work by Mariana Ruiz Villareal)

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

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