ORDER METZGERIALES

(multiform thallose liverworts)

INTRODUCTION

- may be the most ancient of the hepatics

- the earlies hepatic within the fossil record (Pallaviciniites devonicus related to Pallavicinia, a living genus)

Consider what we know about early plant evolution based on fossils:

Oldest algae known – 450 mill years ago

Oldest vasclular plant – Cooksonia – 400 million years ago (late Silurian)

Fossil bryophytes – Palliviciniites – about 350 million years ago during the late Devonian

- no more bryophytes until the Carboniferous (Metzgeriales and first acrocarpous mosses)

- 30 genera and ~ 300 species (many genera only represented by one species)

- widely distributed from the arctic to the tropics, and from sea leavel to alpine elevations. diversity greatest in the humid subtropics and tropics.

- most are terrestrial, some are epiphytic, and one is a subterranean saprophyte (Cryptothallus) - endogenous fungi

- nearly half of the genera possess a single species and tend to show a relatively restricted geographic distribution

n = 8, 9, 10 (or multiples)

CHARACTERISTICS

Gametophyte:

- variation - most thallose, leafy ruffles

- bilaterally symmetrical (even when leafy)

- little to no internal differentiation (no pores)

- never has trigone

- 1 cm to more than 10 cm in length and up to cm wide

- Hattorianthus, Podomitrium, and Hymnophyton, thallus is structurally similar to sporophytes of the fern family Hymenophyllaceae

- the Metzgeriales apparently commonly associated with endophytic fungi .

- endogenous colonies of the blue-green alga (Nostoc) present in Blasia and Cavicularia

- Cryptothallus lives underground, not photosynthetic, heterothallus. This is thought to be the most derived genus in this group

(a) spores

- usually unicellular (exception: Pellia spores are multicellular)

- surfaceornamentation is variable

- generally larger than the spores of the Jungermanniales

(b) protonema/sporeling -

Germination patterns:

(a) filamentous protonema, apical cell of filament (or of a branch) gives rise to a new gametophore. (14-3A,B,E)

(b) stap-shaped unistratose protonematal phase, short uniseriate germ tube produces a biseriate or multiseriate protonema, its apicl cell produces a b

(c) cylindrical or globose exosporic protonematal phase, in which the germinating spore ruptures the spore coat and produces a multistratose cell mass form which the gametophore is initiated. (14-3F-J)

(d) globose multicellular endosporic protonematal phase, in which the spore undergoes several cell divisions before the spore coat is ruptured....sometimes the spores are multicellular within the spore capsule. The multicelluar mass produces an apical cell which gives rise tot he gametophore (14-3CD)

(c) rhizoids

- unicellular, smooth, usually clear, some may be pigmented purply red (Fossombronia)

- scattered rather than clumped like the Jungermanniales

- some genera lack rhizoids (Metzgeria)

- In Treubia the rhizoids may have constrictions similar to those of Marchantiales (Moerkia, Blasia)

- scales are also found in two or more dorsal rows (Apotreubia), ventral ones are only found in Blasia and Cavicularia which have a row on both sides of midrib.

- when leafy the scales are associated with each lobe

(d) stem

- greatest diversity in morphology

- most are thalloid - the simplest are like Pellia, or Riccardia, in which a thickened central band of the thallus thins gradually toward the margins, all cells parenchymatous

- some have a terete axis with unistratose wings on wither side (Rhizoids or hairs are confined to the margins and to the ventral surface of the midrib.

- Apometzgeria, unicellular hairs on both surfaces if the wings (Petalophyllum has lamellae on the upper surface of the thallus

- some are leafy

- many oil bodies per cell (although Treubia has one single)

- mainly thin-walled cells

- occasionally an internal conduction strand is present (Pallavicinia)

apical most are lenticular, with two cutting faces growing point often protected by mucilage papillae or filaments

- branching is irregular, but in some (Metzferia, Hymenophyton) branching is pseudodichotomously

- Riccardia - pinnate, sometimes bipinnate.

- branches are usually terminal but may be intercalary, and they originate ventrally some distance behind apex.

- oil bodies, variable, not all have them, some have numerous oil bodies, some have a single large ones (Treubia).

(e) leaves

- when present never in three rows

- never show bilobed ontogentic origin like in the Jungermanniales)

(f) asexual reproductive structures

- not as common as in the Jungermanniales

- posterior decay

- gemmae - originate within cells of the dorsal surface of the thallus in some species of Riccardia, cell contents round up and divide to form an ovoid two-celled gemma, the endogenous gemma is released when the wall fo the cell containing gemma is ruptured.

- gemma in others is exogenous (Blasia) but occurs withing a long-necked flask on the sorsal surface of the thallus, the gemmae are budded off from cells within the flask, when mature mucilage within the flask imbibes moisture, and the gemmae are extruded through the neck. (Blasia - stellate)

- some generalk (Metzgeria) bud off gemmae on the thallus margins

- gemmae seem to occur in juvenile plants (prior to sporophyte production)

(f) reproductive structures

(a) archegonia

- archegonia are lateral, the apex of the stem continues indeterminate growth and is not utilized directly in teh production of the archegonia (anacrogynous)

- in some genera the sex organs are restricted to reduced branches (Metzgeria, Podomitrium)

- the archegonia are usually protected by a sheath of tissue (involucre or a stem calyptra) or they are embedded in the thallus

- in Fossombronia and Treubia, the sex organs are exposed on the dorsal sideof the thallus

(b) antheridia

- may be naked onver surface (primitive)

- in some protected by scales

- in pockets, flasks, usually near the apex of thallus lobe

(g) calyptra

Sporophyte:

(a) sporangium -

- spherical to cylindrical

- jacket 2 - 5 cells thick, epidermal cells often show semiannular darkly pigmented thickenings

(b) seta

- very similar to the other leverworts

- may be photosynthetic when young, but colorless when mature

(c) spore dispersal

- elaters usually long, spirals vary from one to three.

- opens by four longitudinal lines, irregular decomposition, 2 dehiscence lines

- Moerckia, single slit

- in some there is a small cylinder of sterile tissue within the sporangium to which some elaters may be attached, this is called an elatophore

- in several genera, including Riccardia, the elatophores hangs down inside the apex of the sporangium, and thus, when the sporangium ruptures by the four slits, each is terminated by a tuft of laters attached to the quarteredelatophore.

- Pellia - dimorphic elaters, short ones attached to elaters

EVOLUTIONARY RELATIONSHIPS

Very difficult to interpret this order.

1. irregular branching is more generalized than pseudodichotomous or pinnate branching

2. exposed sex organs rather than those protected in a specialized strucutre or embedded within the thallus are the more generalized

3. numerous oil bodies in a chlorophyll-containing cell appears to be more generalized than the restriction of a single oil body to cells where chloroplasts are lacking. The absence of complex oil bodies may be a specialized feature.

4. Exogenous gemmae appear to be more generalized than endogenous gemmae.

5. A distinctive stem with leaves, lobes, or "wings" aoppears to be less generalized than a stem lacking a stem.

6. An elongate seta appears to be more generlaized than a short one.

7. The saprophytic condition is specialized.

They appear to be most closely related to the Jungermanniales

They share most sporophytic characteristics, even to the lobed spore mother cells, and cytological details

Anacrogynous condition and generally thallose gametophore spearate the order ans an independent evolutionary line.

Even the thallose members of the Jungermanniales are decidedly different from he metzgeriales and appear to represent specialization within the Jungermannialian line rather than the progenitor to the Metzgeriales

Is Metzgeriales a natural unit? Probably, but is made up of the relics.

Metzgeriales and Jungermanniales differ in both ecology and distribution. There are epiphytic species (Metzgeria and Riccardia which are fairly drought resistant) not many of this order are

Evolutionary relationship is puzzlig.

If we assume a monophyletic origin: If we assume that the Calobryales are the most primitive group. Ancestral type would be isophyllous, erect. However the earliest fossils found are all thallose: Devonian - Pallavivniites devonicus, Penssylvanian - Treubites lobatus, Metzgerites

some fossils found represent extant species.

Why would you conclude the relationship between the Calobryales and the Metzgeriales is loer than with the Jungermanniales? A number of features are shared with the Calobryakes: anacrogynous, large chromosomes, large sex chromosomes, two nucleolate in haploid. Calobryales do not represent the extinct form because the unistratose sporangial jacket is considered derived. Other data on sperm confirms this.

Perhaps a polyphyletic origin

morphological and anatomically diverse group

- classified under 7 suborders