CLADES 1-3: HAPLOMITRIOPSIDA, THE MOSS-LIKE LIVERWORTS

The plants are moss-like with gametophore axes bearing leaves that are not similar to those of the leafy liverworts. Also, the leaves tend to be relatively massive. That is, they are made of multiple cell layers, at least at the base. The gametophore stems also tend to be mucilaginous. The antheridia and archaegonia are not clearly differentiated from the surrounding vegetative tissue. The sporophyte is robust, and the capsule opens along four lines of dehiscence.

Plants in this class are sisters to all of the other liverworts and similar to the mosses (Forrest et al. 2006). The class is not very diverse, having only 3 genera. The following genera represent their respective subclasses: Treubiidae and Haplomitriidae.

Treubia (Figure 2) is a plant whose gametophore is leafy and prostrate. The leaves are lobed, two-ranked, and generally unistratose (1-layered); however, they are multi stratose at the leaf base which is very wide and becomes confluent with the stem. The stem has scattered rhizoids. Capsules are egg-shaped and multistratose.

Haplomitria (Figure 3) has a complex leafy gametophore whose leaves are unlobed, and three-ranked. The leaves are multistratose at the base. The underground portion of the stem is leafless. The capsules are cylindrical and unistratose.

CLADES 4-6 MARCHANTIOPSIDA, THE THALLOID LIVERWORTS

The plants of this class are thalloid, though some are leafy, and generally quite diverse. The thallus typically has a region that is photosynthetic, a region of tissue that stores photosynthate. and a region that attaches to the substrate. The ventral part of the thallus has scales and rhizoids that can be complex. Usually, the thallus serves as the protonema from which unisexual gametophores arise (Figure 4-A-J). Sporophytes (Figure 4-K) are somewhat simple with a short seta and a unistratose capsule that releases spores after dehiscence (by four slits, a lid, or by tearing). Some produce asexual spores called gemmae, which usually occur in specialized cup-like structures that resemble small small birds nests. When dislodged by wind or rain, gemmae are dispersed and begin to grow as a small thalli.

Plants in this class appear to be the least complex of the embryophytes. However, according to Forrest et al. (2006), it is a derived condition. The taxa are unevenly divided between two subclasses: Blasiidae and Marchantiidae.

Blasiidae: This sister group to the Marchantiidae has only two genera: Blasia, and Cavicularia.

Cavicularia (Figure 5) shows little dorsiventral differentiation and no oil bodies. The ventral part of the thallus typically has simple scales associated with Nostoc-containing structures. Sexes are separate, but they do not have gametophores. Both archaegonia and antheridia occur on the dorsal surface of the respective plant. Antheridia develop singly within an antheridial chamber. The sporophytes, after they develop, are enclosed by a portion of the thallus.

Marchantiidae: Plants in this subclass are thalloid, most of which have a clear differentiation between the dorsal and ventral portions of the thallus. The upper surface usually has air pores that lead to air chambers, making the photosynthetic tissue somewhat spongy.

Marchantia (Figures 4 and 6) is a common hepatic that grows on moist soil and dripping walls that are protected from the sun. I have seen whole walls of Marchantia growing on such protected wet limestone outcrops in Arkansas. Marchantia also has a pleasant citrus-like odor when handled. It is typical of the thallose taxa. The prostrate, dichotomously branching lobes have several layers of cells. The upper surface of the thallus shows a regular pattern of polygons, each with a pore in the center. This represents the structure of the air chambers, each connected to the atmosphere by a chimney of cells. It is surrounded by epidermal cells (few chloroplasts per cell) with photosynthetic filaments (many chloroplasts per cell) in the air chambers. Beneath that is a nonphotosynthetic parenchymatous layer that lies atop a layer of scales and rhizoids (text with tooltip) Thread-like growths, simple or branched, which serve for absorption and anchorage. that serve to anchor the plant and wick water into the tissue. Growth is apical by division of a single large meristematic cell and the differentiation of its progeny into the tissues of the gametophyte. It will also make upright gametophores. The antheridiophores are umbrella-like structures borne aloft by a stalk (Figure 4-B). The antheridia develop near the upper surface, which, when hit by a rain drop, tears and releases swimming sperm cells (Figure 4-D,E,&G). The archegoniophores look like the antheridiophores but the top looks like many fingers arranged radially (Figure 4-A). Under the fingers are the archegonia with the ends aimed down (Figure 4-C&F). So, when a drop of sperm-laden rainwater adheres to the archegoniophore, the sperm find the end of the archegonium, swim past the neck canal cells and fertilize the egg in the venter (text with tooltip) The base of the archegonium; holds the egg. . Quickly, the sporophyte develops, but its foot remains within the archegonium. Spores are shed after the sporangial wall weakens and tears; the dispersal is facilitated by elaters (text with tooltip) In the sporangia of liverworts and horsetails, small twisted cells that push the spores out of the sporangium. (Figure 4-F-K). With some modifications, the vegetative description and life history of most thalloid hepatics [e.g. Riccia (Figure 7) and Monoclea (Figure 8)] are like that of Marchantia.

Sphaerocarpos (Figure 9), though also in the Marchantiopsida, is somewhat different. For example, all cells except the rhizoids are green. Also, they exhibit a decided differentiation between antheridial plants (smaller and purplish) and archegonial plants (larger and green). In general, they grow as small bottle-like thalli that surround the gametangia. Although species occur from freshwater to most terrestrial environments, the semi desert forms are the most impressive.

CLADE 7. JUNGERMANNIOPSIDA

The leafy liverworts (also known as the scale mosses), sometimes look like small leafy plants (Clade 11). However, unlike mosses, liverwort “leaves” usually are unistratose (text with tooltip) Comprised of a single-cell layer, e.g. the leaves of most bryophytes. , have no costae, are asymmetrical, and 3-ranked. It is these leafy gametophores are the dominant vegetative forms. However, many taxa are thalloid in the vegetative state. They are defined as members of the class Jungermanniopsida not by their vegetative features, but by the details of their sporophytes [see Figure 10 for details of the life history, particularly the of the mature sporophyte]. The main morphological synapomorphies are:

• The embryo develops first as a filament and then emerges from the archaegonium by a long seta.
• The capsule wall is always made of two or more cell layers and dehisces along four lines.

This class is made up of three subclasses: Pelliidae, Metzgeriidae, and Jungermanniidae. There are no clear synapomorphies that define or separate the Pelliidae and Metzgeriidae. Indeed, the analyses of He-Nygren et al. (2006) and Forrest et al. (2006), both show that the Pelliidae, as defined by Crandall-Stotler et al. (2009), is paraphyletic. Thus, we use Crandall-Stotler et al. (2009) as a temporary system until that problem is resolved. We will consider the thalloid jungermanniopsids as a whole. Jungermanniidae, on the other hand is well-supported as a monophyletic group.

Pelliidae and Metzgeriidae (Clades 10-12)- the thalloid jungermanniopsids: These superficially resemble members of the Marchantiopsida. However, the thalloid jungermanniopsids have simple thalli that lack air chambers.

Pellia (Figure 11) is thalloid genus that superficially resembles Marchantia, but its sporophyte resembles those of the leafy liverworts.

Metzgeria (Figure 12) is quite variable in form. This species resembles Riccia in its growth habit. However, its sporophyte with an unusually long seta and a capsule that opens by multiple elongate slits show that it is a jungermanniopsid. This one is an epiphyte in Olympic National Park.

Pleurozia (Figure 13) is a sac-bearing hepatic that superficially resembles Sphaerocarpos. However, Barthlott et al. (2000) and Hess et al. (2005) provide evidence that Pleurozia together with Colura is zoophagous. They both produce unusual elongate cup-like structures that most have assumed to be for catching and storing water. However, these are not xeric taxa like Spherocarpos. That these grow in relatively cool and moist environments (e.g. northern Scotland) where many other mosses thrive, might suggest that the sacs provide an additional advantage. The authors documented the occurrence of many ciliates and other protists that grow in the sacs. Indeed, the types of animals in the sacs suggested that they were lured there. Whether the liverworts actively digest the protists or allow the ubiquitous bacteria to do it for them, Hess et al. (2005) argue that the liverworts would benefit from uptake of the organic matter released into the water.

Jungermanniopsida- the leafy liverworts: The leafy liverwort subclass Jungermanniidae is by far the most diverse group of all the hepatics. Some of them produce short, filamentous protomenata. Most are leafy with asymmetrical bilobed leaf-like structures, which are spirally arranged on the stem (usually in three rows) with 2 rows of lateral leaves and one row of under leaves or amphigastrea (text with tooltip) An amphigastrea is a small leaf on the under side of the stem in the leafy liverworts (Also called an underleaf) . The archaegonia are on stalks and clustered at the tip of the stem, while the antheridia occur in the axils of the leaves. Their sporophytes are typical of the leafy liverworts. They usually grow in dense mats, especially in areas of high rainfall where they can be found on moist soil, bark, and fallen logs.

Porella (Figure 14) can be found growing along the edges of streams where it can be periodically inundated. Also, some Porella species have symbiotic relationships with Nostoc, which allows them to fix nitrogen. In the tropics, some leafy jungermannids are epiphytic (text with tooltip) A plant that grows on another plant but does not derive nourishment from it. .

Scapania (Figure 15) is a typical leafy liverwort with bilobed leaves.

Radula (Figure 16) also is an epiphyte. Its leaves are bilobed, but the lobes are similar in shape unlike the “leaves of Scapania and Porella.