DILLENIIDAE

subclass of woody or herbaceous dicotyledonous flowering plants. The flowers of plants belonging to the Dilleniidae usually contain numerous petals, stamens, and staminodes. The carpels of the gynoecium are united in a compound ovary, which often rises above the floral envelope. The subclass Dilleniidae consists of 13 orders, of which the majority of species are contained in the orders Violales, Capparales, Ericales, Theales, Malvales, Primulales, and Ebenales. The remaining six orders within the Dilleniidae, together constituting less than one-fourth of the species in it, are Nepenthales, Lecythidales, Salicales, Batales, Diapensiales, and Dilleniales. subclass of flowering plants belonging to the class Magnoliopsida. The subclass Dilleniidae in the system used in this article contains 13 orders, 78 families, and about 25,000 species. Among its members are such plants as the peony, cacao, kapok (also known as the silk-cotton tree), mallow, brazil nut, nearly all the genera of pitcher plants except for Cephalotus (Rosidae), the sundew, violet, papaya, cucumber, begonia, willow, caper, mustard, heath, ebony, and primrose. The largest orders are the Violales, Capparales, Ericales, Theales, and Malvales, each of which has more than 3,000 species. Apart from certain relatively small families that may ultimately be found to belong to another subclass of Magnoliopsida, the members of the Dilleniidae form a coherent taxon. It is more advanced in some respects than the subclass Magnoliidae, but less advanced than the subclass Asteridae. Unlike most members of the subclass Magnoliidae, the carpels (ovary components) in most Dilleniidae are fused (syncarpous), the exception being the Dilleniales. The pollen of the Dilleniidae has three pores and/or slits (triaperturate) or a derivation of this type, which is an advancement over the single pore that is seen frequently in the pollen of the Magnoliidae. Unlike Magnoliidae, the Dilleniidae have few alkaloids and are completely devoid of the benzylisoquinoline alkaloids characteristic of the Magnoliidae. They contain ellagic acid and raphides, all of which are concerned with defense against predators and are lacking in most of the Magnoliidae. The Dilleniidae, unlike the Caryophyllidae, do not contain betalains (red to blue flower colour being produced by anthocyanins), nor do they exhibit free-central placentation, except in the relatively advanced order Primulales (e.g., the primroses). The boundary between the members of Dilleniidae and those of Rosidae is more difficult to fix, however. Most Dilleniidae have simple leaves; those members with compound leaves, especially those with distinct leaflets, are derived from various ancestors that had simple leaves. Compound leaves, on the other hand, are a common and probably basic feature in the Rosidae. In the Dilleniidae, the flower petals are partly or completely fused (sympetalous) in about a third of the group, although petalless flowers (apetalous) are also found, as in Glaux (Primulaceae) and the family Salicaceae (poplars and willows). In nearly all Rosidae, on the other hand, the petals are separate and unfused (polypetalous), although they may be absent or inconspicous. Parietal placentation (in which the ovules are directly attached to the inner wall of the ovary) and latex secretion are common in the Dilleniidae and relatively rare in the Rosidae. The nectary disc (the nectar-secreting structure) that is so frequent in the Rosidae is absent in the Dilleniidae, but other nectar-secreting structures do occur. The marginal teeth (serration) of leaves belonging to Dilleniidae are basically narrow, whereas in the Rosidae the leaf teeth are initially narrow and then become broad. The major distinguishing characteristic between the Dilleniidae and Rosidae, however, is the direction of stamen development in flowers with a large number of stamens: in Dilleniidae it is usually from the inside outward, whereas in Rosidae it is usually from the outside inward. Additional reading The subclass is addressed in Friedrich Ehrendorfer, New Ideas About the Early Differentiation of the Angiosperms, Plant Systematics and Evolution, Supplementum, 1:227234 (1977), which suggests that the Dilleniales have originated from the Hamamelididae by secondary polyandry (multiplication) of 10 single stamens in response to visits from pollen-collecting insects. Alternative views based on various vegetative characters include, on leaf venation and marginal toothing, R. Melville, Leaf Venation Patterns and the Origin of the Angiosperms, Nature, 224(5215):121125 (1969); L.J. Hickey and J.A. Wolfe, The Bases of Angiosperm Phylogeny: Vegetative Morphology, Annals of the Missouri Botanical Garden, 62(3):538589 (1975); and J.A. Wolfe, Leaf-architecture Analysis of the Hamamelididae, in Peter R. Crane and Stephen Blackmore (eds.), Evolution, Systematics, and Fossil History of the Hamamelidae, vol. 1 (1989), pp. 75104; and, on fasciculate (bundled) stamens, C.L. Wilson, The Floral Anatomy of the Dilleniaceae: I. Hibbertia Andr., Phytomorphology, 15:248274, and The Floral Anatomy . . . : II. Genera Other Than Hibbertia, Phytomorphology, 23:2542 (1973); and N.K.B. Robson, Evolutionary Recall in Hypericum (Guttiferae), Transactions of the Botanical Society of Edinburgh, 41(3):365383 (1972), and Studies in the Genus Hypericum L. (Guttiferae): 2. Characters of the Genus, Bulletin of the British Museum (Natural History), Botany Series, 8(2):55226 (March 1981).