Dilleniales

▪ plant order

      order of dicotyledonous (dicotyledon) flowering plants, comprising one family (Dilleniaceae), with 10 genera and about 300 species of trees, shrubs, and woody vines (or rarely herbs) of the tropics and subtropics.

      Dilleniales has had a checkered history. It used to be considered an evolutionary link between the more “primitive” Magnoliales, or magnolia order, and several more “advanced” orders such as Theales, the tea or Camellia order, and Violales, or the violet (Viola) order. However, in the Angiosperm Phylogeny Group II botanical classification system (APG II), Theales has been moved to Ericales, Violales has been moved to Malpighiales, and none of the three orders (Magnoliales, Ericales, and Malpighiales) is thought to be particularly close to Dilleniales. Dilleniales has sometimes included Paeoniaceae, or the peony family, but this family is now treated as a member of Saxifragales, or the saxifrage order, so that Dilleniales now includes only Dilleniaceae. Its position within the core eudicots is uncertain, but it may be fairly close to Caryophyllales, or the carnation order. A few features, such as stomata type, sepals that are persistent in fruit, and perhaps seeds with long embryos, may link the two. For more information on the APG II system, see angiosperm.

      Larger genera in the order include Hibbertia (115 species), which grows from Madagascar to Fiji (more than 100 species grow in Australia), Dillenia (60 species), growing from Madagascar to Australia, Tetracera (40 species), growing through much of Indo-Malesia (see Malesian subkingdom (biogeographic region)), and Doliocarpus (40 species) and Davilla (20 species), both restricted to the Neotropics. Dilleniaceae are recognizable by the often strong and parallel secondary veins that proceed straight into the leaf teeth. Ladderlike fine venation is quite common in the family; the leaf blade is often rough; and the bark is often rich brown. The pedicels persist after the flowers (flower) fall off. The flowers are radially symmetric and usually bisexual, with three to many (but usually five) overlapping sepals, usually five overlapping petals that are crumpled in bud, numerous stamens, and several separate ovule-bearing structures (carpels), each developing into a shiny follicle with one to many seeds covered by a fleshy aril.

      Members of Dilleniaceae are quite variable in how they grow, and this is especially true of Hibbertia. Some species of Hibbertia are plants of dry and open habitats; they have very reduced leaves and flattened stems that are the main photosynthetic (photosynthesis) organ of the plant. Hibbertia also has exceptionally variable flowers. In general, flowers of Dilleniaceae lack nectaries and are probably usually visited by rather generalist pollinators. The arillate seeds are probably dispersed by birds, but in some species of Dillenia the fruits are completely surrounded by the sepals, which grow enormously after flowering. D. indica has fruits the size of cannonballs, which may be dispersed by elephants.

      A few species of Dillenia are useful for their timber and as a source of tannin. D. indica, a tree native to Southeast Asia and western Malesia but widely planted elsewhere, is valued for its scented flowers and lemon-flavoured fruits used in jellies and curries. Fruits of other species of the genus have similar uses. Several species of Hibbertia are grown as ornamentals, especially H. scandens, a woody vine with yellow ill-smelling flowers, which is grown mainly in warm areas or in greenhouses.

Paul E. Berry

 

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