Proteaceae

The Proteaceae form a family of flowering plants predominantly distributed in the Southern Hemisphere. The family comprises 83 genera with about 1,660 known species. Australia and South Africa have the greatest concentrations of diversity. Together with the Platanaceae, Nelumbonaceae and in the recent APG IV system the Sabiaceae, they make up the order Proteales. Well-known Proteaceae genera include Protea, Banksia, Embothrium, Grevillea, Hakea, and Macadamia. Species such as the New South Wales waratah, king protea, and various species of Banksia, Grevillea, and Leucadendron are popular cut flowers. The nuts of Macadamia integrifolia are widely grown commercially and consumed, as are those of Gevuina avellana on a smaller scale.

Etymology

The name Proteaceae was adapted by Robert Brown from the name Proteae coined in 1789 for the family by Antoine Laurent de Jussieu, based on the genus Protea, which in 1767, Carl Linnaeus derived from the name of the Greek god Proteus, a deity who was able to change between many forms.

Taxonomy

First described by French botanist Antoine Laurent de Jussieu, the family Proteaceae is a fairly large one, with around 80 genera, but less than 2,000 species. It is recognised by virtually all taxonomists. Firmly established under classical Linnaean taxonomy, it is also recognised by the cladistics-based APG and APG II systems. It is placed in the order Proteales, whose placement has itself varied.
A classification of the genera within Proteaceae was made by Lawrie Johnson and Barbara Briggs in their influential 1975 monograph "On the Proteaceae: the evolution and classification of a southern family", until it was largely superseded by the molecular studies of Peter H. Weston and Nigel Barker in 2006. Proteaceae are now divided into five subfamilies: Bellendenoideae, Persoonioideae, Symphionematoideae, Proteoideae and Grevilleoideae. In 2008 Mast and colleagues updated Macadamia and related genera in tribe Macadamieae. Furthermore, Orites megacarpus was found not to be within the genus Orites, nor in the tribe Roupaleae, instead in the tribe Macadamieae, hence given the new species name Nothorites megacarpus. The full arrangement, according to Weston and Barker with the updates to genera from Mast et al., is as follows:
File:Banksia coccinea.jpg|thumb|right|Flowers, leaves and fruit of Banksia coccinea, from Ferdinand Bauer's 1813 flora ''Illustrationes Florae Novae Hollandiae''

Description

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.

Proteaceae range from prostrate shrubs to tall forest trees of in height, and are usually of medium height or low or perennial shrubs, except for some Stirlingia species that are herbs. Some species are facultatively deciduous, rarely acaulescent, the cauline portion of the collar is often thickened. Indumentum of three-celled hairs, sometimes glandular, rarely absent, the apical cell is usually elongated, acute, sometimes equally or unequally bifid.
Leaves rarely aromatic, usually alternate, and in a spiral, rarely opposed, or verticilate; coriaceous, rarely fleshy or spinescent, simple or compound, entire edge to pinnatisect ; rarely divided dichotomously, often remotely toothed, crenate or serrated, seated or stalked; the petiole frequently with a swollen base but rarely sheathed, without stipules; pinnate sometimes palmate or parallel venation, brochidodromous or reduced to a single prominent vane, vernation normally conduplicate; anisophylly often occurs during the different growth periods; leaf blade dorsiventral, isobilateral or centred; mesophyll tissue usually with sclerenchymatous idioblasts, rare secretory cavities. Brachy-paracytic stomata.

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, sclereids frequent; bark with lenticels frequently horizontally enlarged, cork cambium present, usually superficial. Roots lateral and short, often grouped in bundles with very dense root hairs, rarely with mycorrhiza.

Plants usually hermaphroditic, more rarely monoecious, dioecious or andromonoecious.
Inflorescences very variable, simple or compound, axillary or terminal, lateral flowers solitary or in pairs, rarely with a terminal flower, racemiform, paniculate or condensed, usually with bracts, sometimes converted into leaves or squamiform, forming a type of cone, or with bright colours, forming an involucre or pseudanthium, the peduncles and pedicels sometimes contracted, compacted with the rachis, in some cases the congested inflorescences form super inflorescences ; very rarely the flowers are solitary and axillary near the end of branches; in species with lignotubers the flowers sometimes grow from these and pass through the soil.
Flowers are usually perfect, actinomorphic, or zygomorphic, hypogynous, frequently large and showy. Flat or oblique, sometimes forming a gynophore. Hypogynous disk present and extrastaminal or absent. Perianth of 4 tepals, in 1 valvate whorls, sometimes elongated in a basal sack, free or fused in different ways, or even connivent by marginally interdigitate papillae forming a tube or a bilabiate structure, zygomorphic, sometimes opening laterally in a variety of ways. Haplostemonous androecium, usually isostemonous, opposititepalous of 4 stamens, all fertile or some converted into staminodes, usually filamentous, filaments partially or totally fused to the tepals, rarely free, basifixed anthers adnate, ditheous, tetrasporangiate, sometimes unilocular and bisporagiate, introrse to latrorse, expanded connective, usually with apiculus, dehiscence along longitudinal tears. Hypogynous glands 1–4, squamiform or elongated, fleshy, free or fused forming a lunate or annular nectary over the receptacle. Superior gynoecium of 1 apocarpous carpels, sessile or stipitate, sometimes not completely closed, style usually developed, stigma small or in the shape of a terminal or sub terminal disk or even lateral and oblique, often indented, papilous, moist or dry, ovules 1–100 or more per carpel, anatropous, hemianatropous, amphitropous or orthotropous, mostly hemitropous, bitegmic, crassinucellate, chalaza with a ring of vascular bundles, the funiculus is occasionally absent and the ovule is fused to the placenta, marginal placentation with various dispositions or apical.
Fruit dehiscent or indehiscent, in achene or nucule, follicle, drupe or falsely drupal, sometimes similar to a caryopsis as it is fused to the wall of the ovary and the testa, often lignified and serotinous; the fruit from the same inflorescence are sometimes fused forming a syncarp.
Seeds 1-many, sometimes winged, flat to rounded, with endosperm absent, present in Bellendina, endotesta with an unusual layer containing crystals of calcium oxalate that is rarely absent, well differentiated embryo, straight, dicotyledonous, but often with 3 or more large cotyledons, often auriculate.
Pollen in monads, triangular in polar view, 3-aperturate, usually isopolar and triporate, biporate in Embothrium and the tribe Banksieae, colpoidate in Beauprea, spherical in Aulax and Franklandia or strongly anisopolar in some species of Persoonia; the openings of the former's tetrads follow Garside's Law.
Chromosomal number: n=5, 7, 10–14, 26, 28; sizes range from very small to very big according to species; x=7, 12.
Flowers

Generally speaking, the diagnostic feature of Proteaceae is the compound pseudanthium. 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. This character does not occur in all Proteaceae, however; 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. 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, the perianth is split 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.

Ecology

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 every 2–3 days in order to aid the mobilization and absorption of phosphate. Many species are fire-adapted, 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, 11 andromonoecious genera and some other genera have species that are cryptically andromonoecious: two species are sterile and only reproduce vegetatively. 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 and mammals. The latter two means were evolutionarily derived from entomophily in different, independent events. The dispersion of some species exhibit serotiny, which is associated with their pyrophytic behaviour. These trees accumulate fruits on their branches whose outer layers or protective structures 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.