Additional morphology links:
Simple thalloids (I)
Simple thalloids and Pleurozia (II)
Haplomitrium and Treubia
Complex thalloids and Blasia
Leafy liverworts (I)
Leafy liverworts (II)
Liverworts possess the unique features of 1) non-photosynthetic sporophytes that mature completely before elongation (see also Life Cycle); 2) elaters that are produced by meiosis, and 3) oil bodies, which are special membrane-bound organelles with interesting biochemistry.
Liverworts can be grouped into three major groups according to their morphology: the simple thalloid (Metzgeriidae), the complex thalloid (Marchantiopsida), and the leafy liverworts (Jungermanniidae). According to recent phylogenetic work, the simple thalloids do not form a monophyletic group, but are the ancestral group from which the other two lineages arose. The leafy and complex thalloid groups appear to be monophyletic.
Pallavicinia subciliata, a simple thalloid liverwort (photo by L. Zhang)
The simple thalloids display perhaps the greatest diversity in morphology of the three groups. As the name implies, most have flattened, even ribbon-like bodies, but never are divided into multiple, differentiated cell layers as seen in the complex thalloids. However, some have leafy morphologies, or deeply dissected thalli that look leafy. Perhaps it should not come as a surprise that this ancient, morphologically diverse group of liverworts is not monophyletic.
Most of the simple thalloids form a grade that is the sister group to the leafy liverworts, and the larger group that includes these simple thalloid taxa and the leafy liverworts (Jungermanniopsida) has several defining characteristics (Crandall-Stotler and Stotler, 2000):
- Antheridial development from two primary androgonial cells (never four)
- Capsule walls composed of 2 or more layers of cells
- Lobed spore mother cells
- And in the simple thalloids only: Anacrogyny (archegonia originating sub-apically)
Recent phylogenetic studies have shown that Pleurozia, formerly thought to be a highly derived member of the leafy liverwort group, belongs to a clade of simple thalloids.
Haplomitrium and Treubia form a monophyletic group that is sister to the rest of the liverworts, and are thus the oldest living lineage of liverworts. The two genera share (Forrest et al., 2006):
- Tetrahedral apical cells
- Stalked slime papillae
- Calyptra or perigynium without perichaetial scales
Each genus also has numerous, unique oddities, both morphologically and genetically.
Barbara Crandall-Stotler and Ray Stotler (Southern Illinois University, Carbondale; see Personnel) are LiToL’s taxonomic specialists for this group.
Conocephalum conicum, a complex thalloid liverwort. Note the air pores on the surface (photo by L. Zhang).
The complex thalloids all have flattened bodies without any leaf-like structures. The most common body type in the group involves variations of a “complex thallus,” with multiple cell layers (an upper layer with air pores, a middle layer with air pockets and cells containing many chloroplasts, and a lower carbohydrate storage layer). The Blasiales, the Sphaerocarpales, and Monoclea, however, do not possess this body type.
The Marchantiopsida are characterized by (Crandall-Stotler and Stotler, 2000):
- Dorsal tilt to apical cell
- Four primary androgonial cells
- Six rows of archegonial neck cells
- Unistratose capsule jacket
- Unlobed spore mother cells
Other common Marchantiopsida characters include:
- “Pegged” and smooth rhizoids (not Sphaerocarpales or Monoclea)
- Multistratose gametophytes with air chambers, idioblastic oil-cells (not Sphaerocarpales) and ventral appendages (not Sphaerocarpales or Monoclea)
- Ventral scales (many exceptions)
- Complex, upright branches housing the archegonia and antheridia (many exceptions)
Recent phylogenetic studies have shown that Blasia also belongs to the complex thalloids. This genus, previously thought to belong to the simple thalloid group, is the earliest diverging lineage in the Marchantiopsida.
David Long (Royal Botanic Gardens, Edinburgh; see Personnel) is LiToL’s taxonomic expert for this group, and he and his lab have taken primary responsibility in LiToL for the sequencing related to this group of liverworts.
Plagiochila, a leafy liverwort (photo by C. Davis)
As the name implies, this group contains plants that have leafy gametophytes. These leafy plants may have two or three rows of leaves (2 lateral, one ventral), and may be upright or very prostrate in habit. The group displays a wide diversity in leaf shape and lobing, as well as in branching types and protective structures around the archegonia and antheridia. Taken together, the leafy liverworts are distinguished from the simple thalloids by (Crandall-Stotler and Stotler, 2000):
- Tetrahedral apical cells
- Leaves formed from 2 initials
- Capsules always dehiscing into 4 valves
- Acrogyny (archegonia terminating the main stem)
Leaf insertion is an important characteristic in the leafy liverworts. Leaves may be transverse (with leaves coming off at right angles to the stem), incubous (the apex-facing edge of the leaf overlapping the base of the leaf above it), or succubous (the opex facing edge of the leaf overlapped by base of the leaf above it); see Figure 1, below. In the figure, the horizontal parallel lines represent the stem (dorsal and ventral sides labelled), while the lines along the stem represent the insertion angle of the leaves.
The leafy liverworts are estimated to contain 85% of the species diversity of liverworts.
|Figure 1. Diagrammatic representation of leaf insertion types in the leafy liverworts.|
John Engel and Matt von Konrat (Field Museum, Chicago; see Personnel) are LiToL’s taxonomic experts for this group.