Mesangiospermae
Flower of Liriodendron tulipifera, a Mesangiosperm
Scientific classification
Kingdom:	Plantae
(unranked):	Angiospermae
(unranked):	Mesangiospermae
Clades
Ceratophyllales Chloranthales Eudicots Magnoliidae Monocots

Mesangiospermae is a clade of flowering plants, informally called "mesangiosperms". They are one of four groups of Angiosperms.^[1] There are about 350,000 species of mesangiosperms.^[2] The mesangiosperms contain about 99.95% of the flowering plants, assuming that there are about 175 species not in this group^[3] and about 350,000 that are.^[2]

The mesangiosperms are usually recognized in classification systems that do not assign groups to taxonomic rank. Whenever the mesangiosperms are given taxonomic rank, that rank is between class and division.

Besides the mesangiosperms, the other groups of flowering plants are Amborellales, Nymphaeales, and Austrobaileyales. These are collectively called basal angiosperms. The order names, ending in -ales are used here without reference to taxonomic rank because these groups contain only one order.

Contents 1 Description 2 History 3 References 4 External links

[edit] Description

In molecular phylogenetic studies, the mesangiosperms are always strongly supported as a monophyletic group.^[4] There is no characteristic which is found in all mature mesangiosperms but which is not found in any of the basal angiosperms. Nevertheless, the mesangiosperms are recognizable in the earliest stage of embryonic development.^[3][5] The ovule contains a megagametophyte,also known as an embryo sac, that is bipolar in structure and contains 8 cell nuclei. The antipodal cells are persistent, and the endosperm is triploid.

[edit] History

The oldest known fossils of flowering plants are fossil mesangiosperms from the Hauterivian stage of the Cretaceous period.^[6]

Comparisons of DNA sequences indicate that the mesangiosperms originated between 140 and 150 Mya(million years ago) near the beginning of the Cretaceous period.^[7] This was about 25Mya after the origin of the angiosperms in the mid-Jurassic.^[8]

By 135Mya, the mesangioisperms had radiated into 5 groups: Chloranthales, Magnoliids, Monocots, Ceratophyllales, and Eudicots.^[8] When taxonomic rank is assigned, these groups are designated as classes. The radiation into 5 classes probably occurred in about 4 million years.

Because the interval of this radiation (about 4 million years) is short in proportion to its age (about 145 million years), it had long appeared that the 5 classes of mesangiosperms had arisen simultaneously. In 2007, two studies attempted to resolve the phylogenetic relationships among these 5 classes by comparing large portions of their chloroplast genomes.^[8][9] These studies agreed on the most likely phylogeny for the mesangiosperms. This is the same phylogeny shown at the Angiosperm Phylogeny Website. However, this result is not strongly supported. The approximately unbiased topology test showed that some of the other possible phylogenies had more than 5% probability of being correct. The major weakness of these 2 studies was the small number of species whose DNA was being used in the phylogenetic analysis, 45 in one study and 64 in the other.^[8] This was unavoidable, because complete chloroplast genome sequences are known for only a few plants.

[edit] References

[edit] External links

• Angiosperm Phylogeny at Missouri Botanical Garden
• Divergence Times T. Jonathan Davies et alii. (2004). PNAS 101(7):1904-1909.
• chloroplast genome list In: Montreal genomics