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Concept
Velloziaceae
Résumé
Velloziaceae is a family of monocotyledonous flowering plants. The APG II system, of 2003 (unchanged from the APG system, 1998), also recognizes this family, and assigns it to the order Pandanales.
By contrast to other members of Pandanales the family demonstrates stricter flower morphology. Despite that, the flower structure is still quite variable and with a lot of specifics. Thus morphological analyses are not able to uncover realistic phylogenetical relationships neither appropriate taxonomy. Embryological development places the family among the amaryllids while the composition of the ovary puts it near the distinct Hypoxidaceae. However, a study regarding the pollen structure in two genera from Velloziaceae was found to be an important character and suggests some correlation but this is still a variable trait.
Molecular analyses recognize five distinct genera (by including Talbotia in Xerophyta and Nanuza in Vellozia thus forming two sister groups) and show that the monotypic genus Acanthochlamys is sister to the rest of the members. The family originated in Gondwana. The crown group is dated to be very young - 14 Mya (Mid Neogene) but the stem group was found to be much older - 108 Mya (Mid Cretaceous). Velloziaceae has a total of 306 known species.
Members of Velloziaceae are distributed both in the Old and the New world. Genera as Vellozia, Nanuza and Barbacenia are found in South America (Brazil, Cerrado). Xerophyta and Talbotia are growing across Africa. Plants from the family are also found in Madagascar and the southern part of the Arabian peninsula. One distinct species (Acanthochlamys bracteata) is restricted to China as the only member found in Asia.
Velloziaceae includes different xerophytes inhabiting open and dry habitats. These plants exhibit various adaptations against desiccation as for example less-densely distributed stomata, decreased surface area by the use of developed furrows (as in many cacti) and expansive leaf sheets that cover the stem or the aerial roots until they grow long enough to reach the ground level.
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