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The subgenus Siphisia is characterized by the presence of a 3-lobed gynostemium, whereas subgen. Aristolochia usually presents 5-6 lobes and subgen. Pararistolochia 6-24.
From an evolutionary viewpoint this subgenus is even older and more primitive than the 'classic' subgen. Aristolochia representing 'living fossils' in that context.
Thus subgen. Siphisia is not a species-rich lineage comparably and can be further divided into the following sections:
restricted to S-China and few adjacent countries.
named by reason of similarities to species of Nepenthes pitcher plants and mainly consisting of highland species from the Himalaya region. According to the latest discoveries its range of occurrence is to be expanded to Vietnam and Hainan.
interestingly consists of species from Asia (Russia to Indonesia) as well as about two dozen ones from North America.
Mexico is the centre of diversity once again but the southernmost species even reaches Panama.
Recent investigations led to the conclusion these separate distribution hotspots resulted from the colonization of North America via the Bering Strait and extinctions within the last Ice Age leading to the withdrawal to more temperate/tropical areas again.
Sect. Siphisia species are known for their disjunct trans-Pacific distribution range. While the northernmost species in Asia reaches the Mandshurian region, the very same in North America is restricted to S-Canada and southwards.
Nevertheless species from both continents share outstanding similarities in morphology which has resulted in several investigations during the past decades.
Interestingly these revealed the two groups are very variable yet closely related and lead to the conclusion they must have conquered America via the Bering Strait to diversify southwards again just like in Asia.
Although many species show a strong resistance regarding colder temperatures, constant humidity and wrong cultivation conditions their diversity hotspots are to be found in the lowland tropics of both SE-Asia and SE-Mexico.
Exceptions prove the rule of course - there's quite a number of frost-resistant highland species scattered across the Himalaya & adjecent regions plus a number of hardy species from the USA.
Especially regarding the tropical regions more research would be appropriate to understand their outstanding radiation since flower characteristics and plant habits diversify extremely and may justify the separation of new subtaxa in the future.
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