NON-VASCULAR EMBRYOPHYTES

Hepatophyta (he-pa-TA-fa-ta) is made of two Greek roots that mean liver (hepato -ηπατό); and plant (phyto -φυτό). The reference is to the lobed appearance of the prostrate, thalloid liverworts. This hearkens back to a time when physicians believed that God put a signature on particular plants (the doctrine of signatures) to indicate what they were meant to heal. In this case, a liverwort like Marchantia (Figure A) looked like a diminutive sick liver; so, it was meant to cure sick livers.

The liverworts, though generally considered to be the most structurally simple of the embryophytes, are quite diverse and can be found in almost all terrestrial and freshwater environments. All have a photosynthetic gametophyte, usually with indeterminate growth that develop from a protonema. Almost all produce gametophores that rise from the protonemata and bear the gametangia, antheridia and archegonia. The sporophyte is short-lived and determinant in its growth. It develops within the archegonium forming a foot. It rises out of the archegonial neck by a seta that is crowned by a relatively simple capsule in which sporogenous tissue undergoes meiosis to form spores, usually in tetrads. Some capsules tear open, but most have capsules that open along determinate lines of dehiscence.

In a broad sense there are two types of liverworts: the thallose and leafy hepatics. These categories are generally reflected in the systematic structure of the phylum. The thallose hepatics have the greatest structural simplicity of both the gametophyte and sporophyte (Figures A-D) of any of the other bryophytes. Marchantia (Figure A) 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 tha air chambers. Beneath that is a nonphotosynthetic parenchymatous layer that lies atop a layer of scales and rhizoids 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. The antheridia develop near the upper surface, which, when hit by a rain drop, tears and releases swimming sperm cells. The archegoniophores look like the antheridiophores but the top looks like many fingers arranged radially. Under the fingers are the archegonia with the ends aimed down. 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. 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 [See the life history of Marchantia]. With some modifications, the vegetative description and life history of most thalloid hepatics (ex. Figures B and C) are like that of Marchantia. These taxa also produce small birds nest structures called gemmae on the upper surfaces of their thalli. These are asexual structures that, when dislodged by wind or rain, are dispersed and begin to grow as a small thalli.

Sphaerocarpos (Figure D), 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.

The leafy liverworts (also known as the scale mosses), sometimes look like small leafy plants. However, unlike mosses, liverwort "leaves" usually are unistratose, have no costae, are asymmetrical, and 3-ranked (Figures E-M). It is these leafy gametophores are the dominant vegetative forms. However, many taxa are thalloid (e.g. Figures F and H). They are defined as members of the class Jungermanniopsida not by their vegetative features, but by the details of their sporophytes. In general, the leafy liverworts have very long setae and capsules that dehisce along four longitudinal lines [See the life history of a generalized leafy liverwort].

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 assymetrical 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. 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 typically grow in dense mats, especially in areas of high rainfall where they can be found on moist soil, bark, and fallen logs. Porella (Figure K) 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.

Barthlott et al. (2000) and Hess et al. (2005) provide evidence that the unusual water sac bearing hepatics, Colura and Pleurozia (Figure M) are 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 occurence 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.

A. Marchantia is a common thalloid liverwort with typical dichotomous branching of the thallus.

B. Monoclea resembles Marchantia, but it is much larger and restricted to Central and South America.

C. Riccia shows the dichotomously branching growth habit with narrow thalli.

D. Sphaerocarpos is a thalloid liverwort that is modified as bottle-like thalli that surround the gametangia.

E. Haplomitrium is a leafy liverwort, but the "leaves" are somewhat thalloid in that they have multiple cell layers and are unlobed.

F. Cavicularia is somewhat thalloid with sac-like gemmae structures at the ends of certain of their thalli.

G. Treubia has "leaves" that have a large point of attachment with the stem.

H. Pellia is thalloid genus that superficially resembles Marchantia, but its sporophyte resembles those of the leafy liverworts.

I. Metzgeria also is a leafy liverwort. This one is an epiphyte in Olympic National Park.

J. Scapania is a typical leafy liverwort with bilobed "leaves".

K. Porella is a leafy liverwort. This particular species is an epiphyte that has a symbiotic relationship with Nostoc, which allows it to fix nitrogen.

L. Radula also is an epiphyte. Its "leaves" are bilobed, but the lobes are similar in shape unlike the "leaves of Scapania and Porella.

M. Pleurozia has "leaves" whose dorsal lobes are modified into water sacs, which may function as animal traps.

Images taken from:
A: http://www.botany.ubc.ca/bryophyte/march2.htm
B: http://bryophytes.plant.siu.edu/images/monoclea2.jpg
C:http://www.csun.edu/~hcbio028/Riccia.jpg
D: http://www2.una.edu/pdavis/images/liverworts/sphaec3.jpg
E: http://bryophytes.plant.siu.edu/images/Haplomitrium_mnioides.jpg
F: http://www.digital-museum.hiroshima-u.ac.jp/~museum/habit/hepa_habit/Cavicularia%20densa/Cavicularia_densa04L.jpg
G: http://www.kaimaibush.co.nz/Bryophyta/Liverworts/Treubia.html
H: http://www.science.siu.edu/landplants/Hepatophyta/images/Pellia.epiph.JPEG
I: http://www.nps.gov/olym/crypto/V_MECO.htm
J: http://www.uni-koeln.de/math-nat-fak/botanik/lehre/exkursionen/kleineexkursionen/moose/scapania/scapania.jpg
K:http://academic.reed.edu/biology/Nitrogen/Nfix1.html
L: http://www.anbg.gov.au/cryptogams/underworld/panel-7/images-large/5-7.jpg
M: http://bryophytes.plant.siu.edu/pleupic.html

The liverworts represent the minimalist embryophyte group and likely are little-changed from the algal-land plant transition (Graham 1985 and Graham et al. 1991). Graham (in a series of publications that include Graham 1985; Graham et al. 1991; and Graham and Wilcox 2000) insists that the Coleochaetales are the closest of the charophyte green algae to the hepatics. Molecular work, like that of Marin and Melkonian (1999), however, suggests that another group gave rise to the embryophytes. Nevertheless, such molecular work does confirm the basal position of the liverworts such that the structural simplicity is, indeed, primitive. The liverworts produce a thallial/protonematal stage and upright gametophores (in most taxa). The liverworts exist in two gametophyte forms: thallose liverworts and leafy liverworts, groups that I recognize as the classes Marchantiopsida and Jungermanniopsida, respectively. The thallose-leafy dichotomy really is not very clear, though. The Jungermanniopsida produces capsules that dehisce by more than one longitudinal line. The capsules of the Marchantiopsida simply tear open.

The traditional classification system of the hepatophytes was quite simple in that each of the two classes was divided into three orders (e.g. Scagel et al. 1982; and Bold et al. 1987). The system of Crandall-Stottler and Stottler (2000) maintains the two class system (see below). However, the structure of the lower taxa is much more complex and reflects the development of molecular taxonomy of the liverworts (and bryophytes in general) over the past ten years (see review of the literature in Crandall-Stottler and Stottler 2000). He-Nygren et al. (2006) in a comprehensive analysis of four genome regions and 90 morphological characters confirms the monophyly of the hepatophytes, but separates them into 3 classes with a more complex hierarchical structure. Tentatively, I use the system of Crandall-Stottler and Stottler (2000) until I can see the evaluation of the He-Nygren et al. (2006) system in the literature.

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