Pandanales, the pandans or screw-pines, is an order of flowering plants placed in the monocot clade in the Angiosperm Phylogeny Group and Angiosperm Phylogeny Web systems. Within the monocots Pandanales are grouped in the lilioid monocots where they are in a sister group relationship with the Dioscoreales. Historically the order has consisted of a number of different families in different systems but modern classification of the order is based primarily on molecular phylogenetics despite diverse morphology which previously placed many of the families in other groupings based on apparent similarity. Members of the order have a subtropical distribution and includes trees, shrubs, and vines as well as herbaceous plants. The order consists of 5 families, 36 genera and about 1,610 species.
Description
thumb|right|[[Xerophyta elegans]]
Pandanales are highly diverse including large arboraceous plants of tropical rainforests and coastal areas, climbing vines and lianas, as well as very small achlorophyllous (mycoheterotrophic) and saprophytic herbaceous forest floor species. This has made it difficult to reliably define synapomorphies, but the loss of trimery distinguishes many of them from other lilioid monocots.
The Pandanales order is distinctive with its highly variable and hardly definable floral morphology, 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.
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. The Bentham & Hooker system (1883) had a similar order under the name Nudifloreae, incorporating:
- Pandaneae
- Cyclanthaceae
- Typhaceae
- Aroideae
- Lemnaceae
The Wettstein system (1935) placed the order in class Monocotyledones and used a different circumscription, incorporating:
- Pandanaceae
- Sparganiaceae
- Typhaceae
The Cronquist system (1981) placed the order in subclass Arecidae in class Liliopsida [=monocotyledons] with only one family:
- Pandanaceae
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.
Phylogeny
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