| Haplomitriopsida | |
|---|---|
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| Haplomitrium hookeri | |
Scientific classification | |
| Kingdom: | Plantae |
| Division: | Marchantiophyta |
| Class: | Haplomitriopsida Stotler & Stotl.-Crand., 1977 |
| Type genus | |
| Haplomitrium Nees, 1833 | |
| Subgroups | |
| Synonyms | |
| |
Haplomitriopsida is aclass ofliverworts comprising fifteen species in threegenera. Recentcladistic analyses ofnuclear,mitochondrial, andplastid gene sequences place thismonophyletic group as the basalsister group to all other liverworts.[1][2][3][4] The group thus provides a unique insight into the earlyevolution of liverworts in particular and ofland plants in general.
Plants ofTreubia grow as a prostrate leafythallus. Thebifid leaves extend like wings on either side of the midrib, or may be folded upwards and pressed close together, giving the plants a ruffled appearance. By contrast,Haplomitrium grows as asubterraneanrhizome with erect leafy stems. The thin, rounded leaves are arranged around the upright stems, giving the appearance of a soft moss. The speciesHaplomitrium ovalifolium of Australia often has bifid leaves that areasymmetrical, somewhat like those inTreubia.[5]
Haplomitrium has a number of unique characters that distinguish it from other liverworts, such as lacking rhizoids. The vegetative stems possess a central water-conducting strand with large perforations derived fromplasmodesmata.[6] This central strand is surrounded by a cylinder of cells that conduct food throughout the plant. Such an arrangement is evocative of thexylem and phloem found invascular plants. Although some thalloid liverwort species in thePallaviciniaceae also possess a central conducting strand,[7]Haplomitrium differs in having a food-conducting layer and in producing nocallose.
Treubia also has features that differ from those found in other bryophytes,[8] such as the differentiation of five identifiable zones in the stem midrib. Unlike other leafy species, theoil bodies in itscells are restricted to certain clusters of cells, as they are in theMarchantiopsida. These oil body clusters appear as dark spots in the leaves when the plant is held up to the light.[9]
Living representatives of the group exhibit an essentiallyGondwanan distribution with its center ofdiversity inAustralasia. Such a distribution implies that the modern genera radiated prior to the beginning of theCretaceous when Gondwana broke apart. Schuster proposes that species distributed in the northern hemisphere "rafted" on the Indiansubcontinent to Asia, then spread across theBering Strait into North America.[10]
Most species in the Haplomitriopsida are found in south of the equator, though there are northern ones. The genusTreubia is restricted to the southern hemisphere, whileApotreubia has one species inNew Guinea and another disjunct between easternAsia andBritish Columbia. The genusHaplomitrium exhibits a wider distribution, with species in both North and South America, northern and central Europe, the Himalayas, Japan, and Australasia.
The orders, families, and genera within class Haplomitriopsida are as follows:[11]
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Data related toHaplomitriopsida at Wikispecies