Proteaceae

The Proteaceae /ˌproʊtiˈeɪsiː/ are a family of flowering plants predominantly distributed in the Southern Hemisphere. The family comprises 83 genera with about 1,660 known species. Together with the Platanaceae and Nelumbonaceae, they make up the order Proteales. Well-known genera include Protea, Banksia, Embothrium, Grevillea, Hakea and Macadamia. Species such as the New South Wales waratah (Telopea speciosissima), king protea (Protea cynaroides), and various species of Banksia, Grevillea, and Leucadendron are popular cut flowers, while the nuts of Macadamia integrifolia are widely grown commercially and consumed. Australia and South Africa have the greatest concentrations of diversity. The name Proteaceae by Antoine Laurent de Jussieu in 1789 was based on the genus Protea, which in 1767 Carl Linnaeus derived from the name of the Greek god Proteus, a deity that was able to change between many forms. This is an appropriate image, seeing as the family is known for its astonishing variety and diversity of flowers and leaves. The genera of Proteaceae are highly varied, with Banksia in particular providing a striking example of adaptive radiation in plants. This variability makes it impossible to provide a simple, diagnostic identification key for the family, although individual genera may be easily identified. Plant stems with two types of radii, wide and multi-serrated or narrow and uni-serrated, phloem stratified or not, trilacunar nodes with three leaf traces (rarely unilacunar with one trace), sclereids frequent; bark with lenticels frequently horizontally enlarged, cork cambium present, usually superficial. Roots lateral and short, often grouped in bundles (proteoid roots) with very dense root hairs, rarely with mycorrhiza. Generally speaking, the diagnostic feature of Proteaceae is the compound flower head or, more accurately, inflorescence. In many genera, the most obvious feature is the large and often very showy inflorescences, consisting of many small flowers densely packed into a compact head or spike. Even this character, however, does not occur in all Proteaceae; Adenanthos species, for example, have solitary flowers. In most Proteaceae species, the pollination mechanism is highly specialised. It usually involves the use of a 'pollen-presenter', an area on the style-end that presents the pollen to the pollinator. Proteaceae flower parts occur in fours, but the four tepals are fused into a long, narrow tube with a closed cup at the top, and the filaments of the four stamens are fused to the tepals, in such a way that the anthers are enclosed within the cup. The pistil initially passes along the inside of the perianth tube, so the stigma, too, is enclosed within the cup. As the flower develops, the pistil grows rapidly. Since the stigma is trapped, the style must bend to elongate, and eventually it bends so far, it splits the perianth along one seam. The style continues to grow until anthesis, when the nectaries begin to produce nectar. At this time, the perianth splits into its component tepals, the cup splits apart, and the pistil is released to spring more or less upright. Many of the Proteaceae have specialised proteoid roots, masses of lateral roots and hairs forming a radial absorptive surface, produced in the leaf litter layer during seasonal growth, and usually shrivelling at the end of the growth season. They are an adaptation to growth in poor, phosphorus-deficient soils, greatly increasing the plants' access to scarce water and nutrients by exuding carboxylates that mobilise previously unavailable phosphorus. They also increase the root's absorption surface, but this is a minor feature, as it also increases competition for nutrients against its own root clusters. However, this adaptation leaves them highly vulnerable to dieback caused by the Phytophthora cinnamomi water mould, and generally intolerant of fertilization. Due to these specialized proteoid roots, the Proteaceae are one of few flowering plant families that do not form symbioses with arbuscular mycorrhizal fungi. They exude large amounts of organic acids (citric acid and malic acid) every 2–3 days in order to aid the mobilization and absorption of phosphate. Many species are fire-adapted (pyrophytes), meaning they have strategies for surviving fires that sweep through their habitat. Some are resprouters, and have a thick rootstock buried in the ground that shoots up new stems after a fire, and others are reseeders, meaning the adult plants are killed by the fire, but disperse their seeds, which are stimulated by the smoke to take root and grow. The heat was previously thought to have stimulated growth, but the chemicals in the smoke have now been shown to cause it. There are four dioecious genera (Aulax, Dilobeia, Heliciopsis and Leucadendron), 11 andromonoecious genera and some other genera have species that are cryptically andromonoecious: two species are sterile and only reproduce vegetatively (Lomatia tasmanica, Hakea pulvinifera). The species vary between being autocompatible and autoincompatible, with intermediate situations; these situations sometimes occur in the same species. The flowers are usually protandrous. Just before anthesis, the anthers release their pollen, depositing it onto the stigma, which in many cases has an enlarged fleshy area specifically for the deposition of its own pollen. Nectar-feeders are unlikely to come into contact with the anthers themselves, but can hardly avoid contacting the stigma; thus, the stigma functions as a pollen-presenter, ensuring the nectar-feeders act as pollinators. The downside of this pollination strategy is that the probability of self-fertilisation is greatly increased; many Proteaceae counter this with strategies such as protandry, self-incompatibility, or preferential abortion of selfed seed. The systems for presenting pollen are usually highly diverse, corresponding to the diversification of the pollinators. Pollination is carried out by bees, beetles, flies, moths, birds (honeyeaters, sunbirds, sugarbirds and hummingbirds) and mammals (rodents, small marsupials, elephant shrews and bats. The latter two means were evolutionarily derived from entomophily in different, independent events. The dispersion of some species exhibit the curious phenomenon of serotiny, which is associated with their pyrophytic behaviour: these trees accumulate fruits on their branches whose outer layers or protective structures (bracts) are highly lignified and resistant to fire. The fruit only release their seeds when they have been burnt and when the ground has been fertilized with ashes from the fire and is free from competitors. Many species have seeds with elaiosomes that are dispersed by ants; the seeds with wings or thistledown exhibit anemochory, while the drupes and other fleshy fruit exhibit endozoochory as mammals and birds ingest them. Some African and Australian rodents are known to accumulate fruit and seeds of these plants in their nests in order to feed on them, although some manage to germinate.