| Tetracarpaea | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Order: | Saxifragales |
| Family: | Tetracarpaeaceae Nakai |
| Genus: | Tetracarpaea Hook. |
| Species: | T. tasmannica |
| Binomial name | |
| Tetracarpaea tasmannica Hook. | |
Tetracarpaea is the onlygenus in theflowering plantfamilyTetracarpaeaceae.[1][2] Sometaxonomists place it in the familyHaloragaceaesensu lato, expanding that family from its traditionalcircumscription to includePenthorum andTetracarpaea,[3] and sometimesAphanopetalum as well.[4]
Tetracarpaea has onespecies,Tetracarpaea tasmannica, anevergreen, bushyshrub fromsubalpine areas ofTasmania.[1] It is variable in height, from 1.5 to 10dm. The leaves are shiny and small, with prominent veins, and the ends of the branches are crowded with small, white flowers. It is not known incultivation, but has beengrown fromcuttings.[5]
Tetracarpaea has an odd mix of characters, and during the nineteenth and twentieth centuries, itsaffinities remained obscure. It was variouslyclassified by different authors, usually with considerable uncertainty.[6]Molecular phylogenetic analysis ofDNA sequences has shown thatTetracarpaea is a member of the Haloragaceae alliance, aninformalgroup composed of the families Aphanopetalaceae, Tetracarpaeaceae, Penthoraceae, and Haloragaceae.[2] These are four of the fourteen[7] or fifteen[4] families in theorderSaxifragales.
The following description is based on information from several sources.[1][2][6][8][9]
Tetracarpaea tasmannica is aglabrous,evergreen,erect and bushyshrub. It is variable in height, usually from 1.5 to 6dm, but sometimes attaining a height of 1 m and a width of 7 dm.
Theleaves are elliptic tooblanceolate, about 25 mm long and 8 mm wide, on apetiole about 2 mm long. Theveins are prominent and end near the margin. Themargins areserrate orcrenate. On both surfaces, theepidermis is covered by a thickcuticle.
Theinflorescences are dense, erect,terminalracemes, up to 5 cm long. Theflowers appear inautumn. They are bisexual,actinomorphic, and 5 to 10 mm wide. The 4sepals persist to thematurity of the fruit. The 4petals arewhite andspatulate in shape.
Thestamens are either 4 or 8 in number. If 4, they are opposite (along the same radii as) the sepals. Theanthers arebasifixed.
Theovary issuperior and consists of 4carpels that are large compared to the rest of the flower. The carpels are usually separate, but occasionally 2 or 3 of them are fused at the base, or rarely, as far as halfway up. They are erect andstipitate with asuture along the ventral side. Aplacenta runs along each side of the suture and bears 1 to 3 rows of numerous, tinyovules. The ovules have been described as having oneintegument[8] or two.[6]
The ovary hardly enlarges afteranthesis. Thefruit consists of 4follicles joined at the base. The seeds are numerous and about ½ mm long.
Named byWilliam Jackson Hooker in 1840, the nameTetracarpaea refers to the four conspicuous and separate carpels.[10] At that time, he wrote:
This beautiful little shrub is altogether new to me: but much as it differs in certain characters, both of the foliage and fructification, from the OrderCunoniaceae, I think it may safely be referred to it. The 4 carpels, which have suggested the Generic name, are perfectly free even in the earliest state of the ovary.
Hooker did not use the modern system of suffixes fortaxonomic ranks. He placedTetracarpaea in what would later be known as the familyCunoniaceae. From that time, until the end of the twentieth century, most authors put it in either Cunoniaceae,Escalloniaceae, orSaxifragaceae.[6] It was believed that these three families were related, but today, they are placed in separate orders.[11]
It is now known that Cunoniaceae belongs inOxalidales.[12] Escalloniaceae is even more distant fromTetracarpaea, being a member of anasterid group called thecampanulids.[13] In theAPG III system, it is assigned to themonofamilial orderEscalloniales.[7]Phylogenetic analysis ofDNA has placedTetracarpaea in the orderSaxifragales, and in the "core Saxifragales", but no closer to Saxifragaceae.[4] The family Saxifragaceae is now defined much more narrowly than it had been until 2001.[14]
After William Jackson Hooker namedTetracarpaea and placed it in Cunoniaceae,[10] he was followed byGeorge Bentham, who placed it in the same family.[15] Bentham mistakenly attributed the name to "Hook.f." (Joseph Dalton Hooker), an error that is still repeated today.[6] In 1865, George Bentham and Joseph Hooker movedTetracarpaea from Cunoniaceae to Escalloniaceae.[16]
Adolf Engler putTetracarpaea in Saxifragaceae, butdefined the latter so widely that it included what is now Escalloniaceae as a subfamily. Engler first placedTetracarpaea in subfamily Escallonioideae, then later moved it to its own subfamily.[17]
In 1943,Takenoshin Nakai putTetracarpaea in a family to itself and was the first to use the term "Tetracarpaeaceae".[18][19]John Hutchinson did not follow suit, but placed it in Escalloniaceae.[20]
Arthur Cronquist putTetracarpaea inGrossulariaceae.[21] This family is now understood to contain only the genusRibes and issister to Saxifragaceae.Armen Takhtajan has at different times putTetracarpaea in Escalloniaceae and in Tetracarpaeaceae. Most recently, in 2009, he put it in Tetracarpaeaceae.[8]
In 1988, Matthew H. Hils,et alii, did a detailed study of theanatomy of thewood and leaves ofTetracarpaea.[6] They concluded thatTetracarpaea was much closer to Saxifragaceae than to Cunoniaceae or Escalloniaceae.
The firstmolecular phylogenetic studies of the order Saxifragales were inconclusive because their results had only weakstatistical support. In 2008, by comparingDNA sequences of the entireinvert repeat region of thechloroplastgenome, Shuguang Jian et alii were able to determine the position ofTetracarpaea within Saxifragales. These results had strongbootstrap support.[4]
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