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The Pandanales order is distinctive with its highly variable and hardly definable floral morphology,[2] especially the number of stamens and their structure as well as many other characteristics. In some of the members, different interpretations exist regarding the composition and organization of the reproductive structures.
The order includes plants with traits that seem atypical when compared to other groups of monocots. A good example is the female reproductive organ and its position relative to other parts of the flower. Some of the species included in the families Pandanaceae and Stemonaceae show flowers formed from only one carpel, while in the Triuridaceae, a family that lacks chlorophyll, the carpels are free from each other. In fact, the Triuridaceae possess the least "typical" flower morphology in the order.[citation needed]
Taxonomy
History
The components of the order sensu APG have been difficult to place consistently, and historically have been associated with a number of other groupings.[2] The Bentham & Hooker system (1883) had a similar order under the name Nudifloreae, incorporating:
In the classification system of Dahlgren the Pandanales were in the superorder Pandaniflorae (also called Pandananae) with the single family Pandanaceae.
Angiosperm Phylogeny Group
The APG III system (2009) places the Pandanales in the monocots. Both the APG III and APG II systems include five families in this order.[1]
Since the morphology of the order varies on such scale, its classification and phylogeny are based on genetic analyses.
Inside the order, some doubt remains about the position of the entirely mycoheterotrophic family Triuridaceae, since it is the only one on which genetic analyses have not yet been applied. With high probability, the family may be sister to the Velloziaceae, but similarities with the family Zingiberaceae (which is a part of a whole different order – Zingiberales) do not exclude the chance for a different phylogeny. The family Velloziaceae on its own is placed at the base of the tree. The Pandanaceae and Cyclanthaceae are sister groups, and they form a clade which on its own is sister to the Stemonaceae (a family composed of two more clades).
Evolution
The order Dioscoreales holds sister relationship with Pandanales by diverging from them around 121 million years ago in the mid-Cretaceous. The formation of the crown groups took place with a difference of 2 million years between the orders - 116 Mya for the Dioscoreales and 114 Mya for the Pandanales.
However, the stem group of the Pandanales is much older and goes back to 130 Mya in the early Cretaceous.[3]
Phylogeny
Dioscoreales
Pandanales
Velloziaceae
Triuridaceae
Stemonaceae
Pandanaceae
Cyclanthaceae
Subdivision
The composition of the order in APG III and APG II were slightly from that in the 1998 APG system, which used the circumscription[1]
The species are members of various ecological groups, including tropical shrubs, lianas and trees, xerophytic plants, mycoheterotrophs, as well as different herbaceous representatives.
Rudall, Paula J.; Bateman, Richard M. (1 April 2006). "Morphological Phylogenetic Analysis of Pandanales: Testing Contrasting Hypotheses of Floral Evolution". Systematic Botany. 31 (2): 223–238. doi:10.1600/036364406777585766. S2CID86213914.
Rudall, Paula J.; Cunniff, Jennifer; Wilkin, Paul; Caddick, Lizabeth R. (1 August 2005). "Evolution of Dimery, Pentamery and the Monocarpellary Condition in the Monocot Family Stemonaceae (Pandanales)". Taxon. 54 (3): 701–711. doi:10.2307/25065427. JSTOR25065427.
