Eudicots

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Eudicots are composed of three major groups: caryophyllids (a single order, Caryophyllales), rosids (13 orders) and aster-ids (nine orders). In addition to these (the core eudicots), there are a number of smaller families and orders that form a grade with respect to the core eudicots. The largest of these are Ranunculales, which include the buttercups (Ranunculaceae) and poppies (Papaveraceae), and Proteales, which include the plane tree (Platanaceae), lotus (Nelumbonaceae) and protea (Proteaceae) families. The last is an important family in South Africa and Australia where they are one of the dominant groups of plants. The placement of the lotus (Nelumbo) in this order was one of the most controversial aspects of the early phylogenetic studies based on DNA sequences, but subsequent studies have demonstrated that this is a robust result. The lotus is a 'waterlily' (an herbaceous plant with rhizome and round leaves attached to the stem in their middle), but its similarities to the true waterlilies are due to convergence.

The so-called 'basal' eudicots (i.e. Ranunculales and Proteales) have flowers that lack the organization typical for the larger group. The strict breakdown into sepals, petals, stamens and carpels is not obvious in many of these taxa. Some have what appears to be a regular organization, but upon closer inspection this breaks down. For example, some Ranunculaceae have whorls of typical appearance, but the sepals are instead bracts and the petals are most likely derived from either sepals or stamens. Numbers of parts are also not regular, and fusion within whorls or between whorls is rare, whereas in the core eudicots flowers take on a characteristic 'synorganization' in which numbers are regular and whorls of adjacent parts are often fused or otherwise interdependent. This is not to say that there are not complicated flowers in these basal lineages because there are some rather extraordinary ones: for example, in Ranunculaceae, there are Delphinium species with highly zygomorphic flowers in which the parts are highly organized. None the less, synorganization is typically the hallmark of the core eudicots.

Caryophyllids

The flowers of Caryophyllales (29 families; APG, 2003) often look like those of other core eudicot families, and thus some of the members of this order were previously thought to be rosids (e.g. the sundews, Droseraceae, which were thought to be related to Saxifragaceae) or asterids (e.g. the leadworts, Plumbaginaceae, which many authors thought were related to Primulaceae because of their similar pollen and breeding systems with stamens of different lengths). The core Caryophyllales have a long history of recognition, and in the past they have been called the Centrospermae because of their capsules with seeds arranged on centrally located placenta. This group was clearly identified in the first DNA studies (Chase et al, 1993), so previous workers were correct in recognizing this group, but the DNA analyses placed a number of additional families with the core Caryophyllales. In addition to their fruit characters, Centrospermae also have betalain floral pigments that have replaced the antho-cyanins typically found in angiosperms. Another common characteristic is anomalous secondary growth; such plants are woody and often small trees or shrubs, but the way in which they make wood does not follow the typical pattern for angiosperms, which is probably an indication that these plants are derived from herbs that lost the ability to make woody growth. None the less, some of these groups do make wood that appears to be typical, so it is not yet clear whether or not Caryophyllales are ancestrally herbaceous. Good examples of this anomalous woodiness are the cacti (Cactaceae). Well-known examples of core Caryophyllales families include Amaranthaceae (which include spinach and beets), Caryophyllaceae (carnations), Cactaceae and Portulacaceae (pusley and spring beauty). Cactaceae and several other families adapted to arid zones are known to be closely related to various members of Portulacaceae, but a formal transfer of these families to the last has not yet been proposed (although it will almost certainly be treated this way in a future update of the APG system).

In the DNA studies, Centrospermae (core caryophyllids) were found to have a number of previously undetected relatives. Many of these have chemical and pollen similarities to the core group, and some have anomalous secondary growth as well. The core set of families are well known for their abilities to adapt to harsh environments, particularly deserts and salty sites, and their newly discovered relatives are similarly adapted. For example, the tamarisks (Tamaricaceae) and frankenias (Frankeniaceae) have salt-secreting glands, and jojoba (Simondsiaceae) grows in the arid zones of western North America along with cacti. The leadworts (Plumbaginaceae) and jewelweeds

(Polygonaceae) also include a number of plants adapted to dry and salty conditions. The ecological diversity displayed by these plants was increased by the recognition that several families of carnivorous plants are members of Caryophyllales. These are the sundews and Venus fly trap (Droseraceae) and the Asian pitcher plants (Nepenthaceae).

Carnivory evolved several times in the angiosperms, and there are members in each of the major groups: Brochinnia (Bromeliaceae) in the monocots, the Australian pitcher plants (Cephalotus,

Cephalotaceae) in the rosids and the blad-derworts (Lentibulariaceae) and New World pitcher plants (Sarraceniaceae), each related to different groups of the asterids. Botanists had debated the affinities of each of these groups of carnivorous plants for many years, and most had proposed multiple origins. However, there was little agreement about which of the carnivorous plants might be closely related and with which other families they shared a common history. DNA data were crucial to establish patterns of relationships (Albert et al., 1992) because the highly modified morphology of these plants as well as the diversity of floral types made assessments of their relationships largely a matter of intuitive weighting of the reliability of these characters.

Santalales

Before turning to the rosids, I would like to mention briefly two APG orders of core eudicots that have not been placed in the three major groups because they have yet to obtain a clear position in the results of the DNA studies. The first of these are Santalales (six families), which include a large number of parasitic plants, all of which are photosynthetic but none the less obligate parasites. Some, like the sandalwood family (Santalaceae), attach to their hosts via underground haustoria, whereas others, like the mistletoes (Loranthaceae), grow directly on the branches of their woody host plants. Although most are parasites on woody species, some, such as the Western Australian Christmas tree (Nuytsia), attack herbaceous plants (they are one of the few trees in the areas where they grow). Santalales have a long history of recognition as a group, and nearly all proposed classifications have included them, more or less with the same circumscription as in APG (1998, 2003). Like other core eudicots, species in Santalales have organized flowers, but they have unusual numbers of whorls. Rosids and caryophyllids generally have one whorl each of calyx (sepals), corolla (petals) and carpels, whereas there are two whorls of stamens (sometimes with an amplification of these). Asterids are similar except that there is a single whorl of stamens. Santalales have typically many whorls of some parts, particularly stamens (up to as many as 16 in some cases), so they clearly deviate from the main themes of the core eudicots. It is likely that Santalales evolved before the number of whorls became fixed or that they have simply retained a degree of developmental flexibility that was lost in the other major groups.

Saxifragales

Unlike Santalales, Saxifragales (12 families) is a novel order in the APG system (1998, 2003). The name has been used previously by some authors, but the circumscription of the order is different. Some of the families are woody and wind-pollinated, for example the witch hazel family (Hamamelidaceae, although some genera are pollinated by insects) and the sweet gum family (Altingiaceae), and these were previously considered to be related to the other windpollinated families (see Hamamelidae below). Others are woody and insect-pollinated, for example the gooseberry and currant family (Grossulariaceae), and yet others are herbaceous and insect-pollinated, for example the stonecrops (Crassulaceae), peonies (Paeoniaceae) and saxifrages (Saxifragaceae). The order has many species with a particular type of vein endings in their leaves, but in general they are diverse in most traits. If not thought to be related to Hamamelidae, then they were thought to be related to the rosids in Rosales and clustered near Saxifragaceae. New results have shown that a small tropical family, Peridiscaceae, are also related (Davis and Chase, 2004).

Dilleniaceae

This small family is only mentioned here because, although it is an unplaced-to-order core eudicot, it is the namesake of subclass Dilleniidae, which figured importantly in many previous systems of angiosperm classification (e.g. Cronquist, 1981). They occupy a potentially critical position within the core eudicots as sister to one of the other major groups (i.e. asterids, caryophyllids or rosids) or perhaps to a pair or all three, so, when they are placed, an understanding of their floral organization might be key to understanding floral evolution of the eudicots in general. In the three-gene analysis of Soltis et al. (2000), they were sister to Caryophyllales but this was not a clear result. If additional gene data also place them in this position, they will be included in Caryophyllales.

Rosids

Like Carophyllales, rosids and asterids have a long history of recognition, and similarly the DNA sequence studies have considerably enlarged the number of groups associated with them (see below). In contrast to the Caryophyllales and the asterids, many groups of plants long thought to be rosids have been demonstrated to have relationships to the first two groups, and thus the rosids have somewhat fewer families than in many systems of classification. The additional families have come mostly from the group called by many previous authors the dilleniids (e.g. in Cronquist, 1981, subclass Dilleniidae) and hamamelids (subclass Hamamelidae, sensu Cronquist). Before discussing the rosids, it is appropriate to first discuss these two groups that are not present in the APG system.

Hamamelidae (Cronquist, 1981) contained nearly all of the families of wind-pollinated trees, including such well-known families as the beeches and oaks (Fagaceae), birches (Betulaceae) and plane tree (Platanaceae). They were often split into 'lower' and 'higher' Hamamelidae, in recognition of their degree of advancement. The syndrome of wing pollination is highly constraining of floral morphology on a mechanical basis, and convergence in distantly related families was always suspected. Nevertheless, since the syndrome is one associated with either great modification or loss of many floral organs (e.g. petals are nearly always absent and stamens are held on long filaments so that they can dangle in the wind), determination of other relationships was made difficult, leading most workers to place them together. DNA studies have been of major significance in sorting out the diverse patterns of relationships; some families are now placed among the non-core eudicots (e.g. Platanaceae in Proteales; Trochodendraceae, unplaced to order), Saxifragales (e.g. Daphniphyllaceae and Hamamelidaceae), rosids (most of the 'higher' Hamamelidae such as Betulaceae and Fagaceae in Fagales, see below) or even asterids (e.g. Eucommiaceae in Garryales).

At least in the case of Hamamelidae, botanists had the characters associated with wind pollination as the basis for placing the families in one taxonomic category, but the basis for Dilleniidae was always much weaker and less consistent among the authors who recognized the group. Basically (and explaining their characters in APG terminology), they were core eudicots that tended to have many petals and stamens, with the latter maturing centrifugally. In all other respects, they were diverse and difficult to place. With respect to the APG system (1998, 2003), families of this subclass are now placed in either the rosids (e.g. Brassicaceae, Clusiaceae, Cucurbitaceae, Malvaceae and Passifloraceae) or asterids (Ericaceae, Primulaceae and Theaceae). The only exceptions to this are Paeoniaceae and Dilleniaceae, which are Saxifragales and unplaced in the core eudicots thus far, respectively. Thus with respect to all previous systems of angiosperm classification, that of APG (1998, 2003) does not contain in any form two of the previously recognized major taxa, which have been shown by DNA studies to be polyphyletic (Chase et al., 1993; Savolainen et al., 2000; Soltis et al., 2000).

Within the rosids, there are still several orders not yet placed to either of the two larger groups, eurosid I and II: Crossosomatales, Geraniales and Myrtales. Crossosomatales are a small order, with three families, none of which is well known. It is another of the APG orders that no one had predicted. Geraniales have four families, of which only Geraniaceae are well known (the temperate genera Geranium and largely South

African Pelargonium, the 'geranium' of commerce, which are both important horticultur-ally). On the other hand, Myrtales (13 families) have several important families, including the combretum family (Combretaceae), the melas-tome family (Melastomataceae), the myrtle and guava family (Myrtaceae) and the fuchsia and evening primrose family (Onagraceae). Melastomataceae and Myrtaceae are both large and ecologically important in the tropics, whereas Onagraceae are horticulturally important. Onagraceae have been studied for many years by several American botanists and have become a minor model family.

The remainder of the rosids are split into two major clades, which have been referred to as eurosid I and eurosid II. Alternative names, fabids and malvids, have also been suggested for these two clades, respectively. Celastrales (three families) are another order unique to the APG classification (in the sense of their circumscription). The reasonably large spindle family, Celastraceae, is the only one of any particular note in this order, which is sister to one of the larger orders, Malpighiales (28 families). Malpighiales and Celastrales share a particular seed type with a fibrous middle layer. Seed characters appear to be significant tax-onomic characters in the angiosperms as a whole, but unfortunately they are relatively poorly studied. Within Malpighiales, the most important families are the mangosteen family (Clusiaceae or Guttiferae), a large tropical family with several species important for their fruit or timber, the spurge family (Euphorbiaceae), the passionfruit family (Passifloraceae) and the violet family (Violaceae). Also related to these two orders are Oxalidales (six families), in which the oxalis (Oxalidaceae) and elaeocarp (Elaeocarpaceae) families are placed. Both of these are sources of ornamentals, and some species of oxalis are important weeds. The southern hemisphere cunon family (Cunoniaceae) includes some important tree species.

The rest of the families make up a clade that has been termed the 'nitrogen-fixing clade' (Soltis et al, 1995) because at least some members of each order are known to harbour nitrogen-fixing bacteria in root nodules. This trait is important because these plants can thereby grow on poorer soil and enrich it (e.g. farmers alternate crops so that in some years they plant legumes, one of the major nitrogen-fixing families). It has been hypothesized (Soltis et al., 1995) that this trait evolved in the common ancestor of this clade and then was lost in many of the genera, although the reasons why such a valuable trait would be lost is not clear. The alternative hypothesis, and perhaps the more likely one, is that there are some preconditions that are required for the trait to evolve and these were present in the common ancestor; possession of the preconditions then made it more likely that the trait would evolve. If nitrogen fixation can be engineered in plants that currently are not capable of this, then it is more likely that this will be possible in non-fixing species in this clade than those in other clades.

Cucurbitales (seven families) contain the familiar cucumber and melon family (Cucurbitaceae) as well as the begonia family (Begoniceae), which is common in our gardens. They are sister to Fagales (seven families), which are important (mostly) north temperate forest trees. These include the birch family (Betulaceae), the she-oak family (Casuarinaceae, one of the tropical members of this order), the beech and oak family (Fagaceae, with some tropical genera), the walnut, pecan and hickory nut family (Juglandaceae) and the southern beeches (Nothofagaceae). These families are well known for their timbers as well as their fruits (nuts), and they are dominant members of many temperate and tropical ecosystems.

Fabales (four families) are important because they include the legume family, which, as mentioned above, are capable of fixing nitrogen and thus enriching many of the soils in which they grow. They are also important as food-producing plants and are grown as crops throughout the world. Beans and pulses are a good source of protein; soybean is widely grown and soya is a widely used meat substitute. Many legumes, both herbs and woody species, are also common ornamentals, and some of the tropical genera are timber species. Most are ecologically important throughout the world. The other large family of Fabales is the milkwort family, Polygalaceae. Both of these families have highly characteristic zygomorphic flowers of similar general construction, although no previous author had suggested that they were closely related. DNA studies were the first to place these families in one clade (Chase et al., 1993). Milkworts curiously are unable to fix nitrogen.

Rosales (nine families) in the APG circumscription (2003) are radically different from those of most previous systems (e.g. Cronquist, 1981). Among the important families are the rose family (Rosaceae), which include many ornamentals as well as fruit-bearing species, such as apples, cherries, peaches, plums, raspberries and strawberries. A few are also important timbers (e.g. cherry and white beam). Both Rosaceae and Rhamnaceae include a number of nitrogen-fixing genera, and the latter include a number of timber species as well as some minor fruit-bearing genera (e.g. jujube). Circumscription of the last set of families in Rosales is in flux, but these have long been recognized as a natural group. Relative to their limits as used in APG (2003), the marijuana and hops family (Cannabaceae) should now include the hop-hornbeam family (Celtidaceae), which has been split from Ulmaceae. The nettle family (Urticaceae) have a number of temperate herbs of minor importance and a larger number of tropical trees that are timber species; many are sources of fibres. The fig and mulberry family (Moraceae) are a mostly tropical group, which are important ecologically and as a source of fruits. Figs are well known for their symbiotic relationships with their pollinators, fig-wasps, each species of which generally has a one-to-one relationship with a species of fig. This relationship is one of the longest enduring known; it probably dates back to 90 million years ago, when the first fig-wasp fossils are known.

In the second major clade of the rosids, there are only three orders, Brassicales, Malvales and Sapindales. Brassicales (15 families, most of them small) include all of the families that produce mustard oils, but their morphological traits were so diverse that only one author ever previously included them in a single order (Dahlgren, 1980). This circumscription was so highly criticized by other taxonomists that in this next classification, he split them again into several unrelated orders. The basis for including them in a single order was simply due to the presence of mustards oils, which involve a complicated biosynthetic pathway for their synthesis; chemists interested in plant natural chemistry had long believed that it was highly unlikely that such a process could have evolved so many times in distantly related groups (up to six times if you consider the placement of these families in the system of Cronquist, 1981). Thus DNA data figured importantly in the recognition of this circumscription of the order. The largest family in the order is the mustard family, Brassicaceae (Cruciferae), which include the well-known broccoli, Brussels sprouts, cabbage and cauliflower, all of which are selected forms of the same species. In APG (2003), the circumscription of Brassicaceae included the caper family (Capparaceae), but recent studies have shown that by segregating a third family, Cleomaceae, it would then be appropriate to reinstate Capparaceae as a recognized family. Other commonly encountered families of Brassicales are the papaya (pawpaw) family (Caricaceae) and the nasturtium family (Tropaeolaceae).

Malvales (nine families) are well known for their production (in various parts of the plants) of mucilaginous compounds (e.g. the original source of marshmallow is the marsh mallow, a species in Malvaceae; sugar mixed with these polysaccharides is what was originally used to make the candy, but it is now artificially synthesized). Nearly all of the nine families produce at least some of these compounds. The best-known family of the order is the mallow and hibiscus family (Malvaceae), which before the application of DNA data was typically split into four families, Bombacaceae, Malvaceae, Sterculiaceae and Tiliaceae. Chocolate is also a commercial product from a species in the family, and okra is an edible fruit of a species of hibiscus. A number of ornamentals are found in the next largest family in Malvales, the thymelea family (Thymelaeaceae), which include the daphne, whereas the next largest family is the dipterocarps (Dipterocarpaceae), which is the most important family of the Old World tropical forests and produces timbers.

The last order of the eurosid II clade is Sapindales (nine families), which are nearly all woody species, whereas Brassicales and to a lesser degree Malvales have many herbaceous species. The largest family of the order is that of the maple and litchi (Sapindaceae), which is a largely tropical group; the well-known north temperate maples and horse chestnuts (buckeyes) are two exceptions to this distribution. Another important family of tropical forest trees is the mahogany family (Meliaceae), from which also comes an important insecticide, neem. The citrus or rue family (Rutaceae) is also an important woody group, but there are some herbaceous species, such as rue itself, which is a temperate genus. Grapefruit, lemons, limes and oranges, as well as a number of minor fruits, are important commercially. The poison ivy and cashew family (Anacardiaceae) is another largely tropical group; the family is well known for its highly allergenic oils, which cause severe and sometimes fatal reactions in many people. Cashews, mangoes and pistachios are important commercial members of the family.

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