Haplomitriopsida
| Haplomitriopsida Temporal range: Early Permian–Recent |
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|---|---|
| Scientific classification | |
| Kingdom: | Plantae |
| Division: | Marchantiophyta |
| Class: |
Haplomitriopsida Stotler & Stotl.-Crand. |
| Subgroups | |
| Synonyms | |
|
Treubiopsida |
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Treubiopsida
Haplomitriopsida is a newly recognized class of liverworts comprising fifteen species in three genera. Recent cladistic analyses of nuclear, , and plastid gene sequences place this monophyletic group as the basal sister group to all other liverworts. The group thus provides a unique insight into the early evolution of liverworts in particular and of land plants in general.
Plants of Treubia grow as a prostrate leafy thallus. The 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 a rhizome with erect leafy stems. The thin, rounded leaves are arranged around the upright stems, giving the appearance of a soft moss. The species Haplomitrium ovalifolium of Australia often has bifid leaves that are asymmetrical, somewhat like those in Treubia.
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 from plasmodesmata. This central strand is surrounded by a cylinder of cells that conduct food throughout the plant. Such an arrangement is evocative of the xylem and phloem found in vascular plants. Although some thalloid liverwort species in the Pallaviciniaceae also possess a central conducting strand,Haplomitrium differs in having a food-conducting layer and in producing no callose.
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