Why is an Oak like a Cassowary?

Beach casuarina Casuarina equisetifolia bearing flowers and cones, copyright Atamari.

When one thinks of the Australian vegetation, one might think of towering eucalypts or hardy acacias. One might contemplate unwelcoming spinifex or vibrant grevilleas. But perhaps few groups of plants are so distinctively Australian as the Casuarinaceae, the casuarinas or she-oaks. Members of this family are also found in south-east Asia and the Pacific Islands, but it is in Australia that they reach their highest diversity.

Casuarinas are also unmistakable. They are flowering plants, but they are wind-pollinated and the flowers are highly reduced, being borne in small clusters or spikes. The clusters of fruits, when mature, look more like a miniature pine cone than anything else. The trees that bear these cones also look a bit like pines themselves, with their narrow photosynthetic branches (cladodes) bearing a superficial resemblance to pine needles. The leaves proper are reduced to tiny teeth arranged around nodes or joints on the branches. The outer layer of the cladodes is composed of a thick cortex which together with the needle-like morphology helps resist desiccation. The name of the family refers to the resemblance of their branches to the hair-like feathers of a cassowary Casuarius. Casuarinas are so distinct from other flowering plants that their affinities were long uncertain, though more recent studies have suggested a relationship to other wind-pollinated trees in families such as the Betulaceae (Steane et al. 2003).

In line with their drought-resistant mien, casuarinas are most often found growing in arid and/or coastal regions. The most widespread species, the beach she-oak Casuarina equisetifolia, is found along coastlines from the Bay of Bengal to Polynesia. Their persistance in harsh conditions is also assisted by the presence of nodules on their roots containing bacteria of the genus Frankia, that function like the Rhizobium in root nodules on legumes to fix nitrogen from the atmosphere. Casuarinas also resemble pine trees in forming a mat around their base of fallen cladodes that restricts the growth of competing vegetation.

Desert oaks Allocasuarina decaisneana, copyright Cgoodwin.

Until relatively recently, casuarinas were all classed in a single genus but most authors now recognise four genera in the family. The most distinctive, whose position as sister to the remaining genera is confirmed by molecular analyses (Steane et al. 2003), is Gymnostoma, which contains eighteen species found from south-east Asia to Queensland and Fiji. Whereas other genera of Casuarinaceae have the stomata on the cladodes hidden within deep longitudinal grooves, Gymnostoma has much shallower grooves on the cladodes and the stomata more or less exposed. As such, it is less resistant to desiccation than the other genera. Gymnostoma has four of these grooves on each cladode, corresponding to four leaf-teeth around each node, so the cladodes also tend to have a squarish cross-section.

The second-most divergent genus, Ceuthostoma, contains just two species found from Palawan and Borneo to New Guinea. Ceuthostoma resembles Gymnostoma in having four teeth around each node, but resembles the remaining two genera, Casuarina and Allocasuarina, in having the stomata hidden within deep grooves. In Casuarina and Allocasuarina, the number of teeth around each node is generally increased (up to twenty in Casuarina), meaning that the cladodes are more rounded than square. As noted by Steane et al. (2003), rounder cladodes with more grooves mean that the opening of each groove is narrower, further improving desiccation resistance. Casuarina and Allocasuarina are most readily distinguished by the appearance of their seeds, which are paler and dull in Casuarina but dark brown or black and shiny in Allocasuarina. Allocasuarina is the most diverse genus of the family, with over fifty species endemic to Australia. Casuarina contains fewer species but is more widespread. Steane et al.'s molecular analysis suggested a division within Casuarina between two main clades, one of which was restricted to Australia while the other was primarily composed of Indomalesian species (as well as C. equisetifolia which, as noted above, is found damn near everywhere).

Borneo ru Gymnostoma nobile, from natureloveyou.sg.

Fossils of Casuarinaceae date back to the Palaeocene epoch, and indicate that the family was more widespread in the past with species known from the Eocene of South America and the Miocene of New Zealand. Casuarinaceae-like pollen is also known from the Palaeogene of southern Africa and Antarctica. The South American species have been assigned to the living genus Gymnostoma; the New Zealand species, though originally assigned to Casuarina, is probably also more closely related to Gymnostoma (Zamaloa et al. 2006). Though dominant in the modern flora, the drought-resistant clade of the other three genera is probably of more recent origin, and has probably only ever been unique to the Australasian region.

And I've just realised that I haven't answered the question in the title to this post. As I noted above, an alternate vernacular name for these trees to 'casuarina' is 'she-oak'. I used to wonder why this should be, seeing as casuarinas look about as unlike oaks as you might care to imagine. A good summary of the solution can be found in this newspaper column from the Western Mail of 1914. Though some have suggested that 'she-oak' may be a corruption of an Aboriginal word (despite no such word having been put on record), the more simple explanation is that even if the tree itself doesn't look like an oak, the wood that comes out of it does.

REFERENCES

Steane, D. A., K. L. Wilson & R. S. Hill. 2003. Using matK sequence data to unravel the phylogeny of Casuarinaceae. Molecular Phylogenetics and Evolution 28: 47–59.

Zamaloa, M. del C., M. A. Gandolfo, C. C. González, E. J. Romero, N. R. Cúneo & Peter Wilf. 2006. Casuarinaceae from the Eocene of Patagonia, Argentina. International Journal of Plant Sciences 167 (6): 1279–1289.

I Said Primrose-Willows, Darling

The plant shown in the photo above (copyright Forest and Kim Starr) is Ludwigia octovalvis, commonly known (along with other species in the same genus) as primrose-willow. This is a very common plant in tropical and subtropical regions around the world; indeed, it is so widespread that we have little idea where in the world it originated*. The name 'primrose-willow' derives, of course, from its combination of primrose-like flowers with willow-like leaves, but it is no close relation to either. Primrose-willows belong to the Onagraceae, the same plant family as evening primrose or fuchsias. Ludwigia octovalvis is a shrubby plant, sometimes growing up to four metres in height. Lower parts of the stem may become woody with age, but the greater part of the plant is herbaceous. It prefers to grow in damp habitats, in swampy soil or alongside streams, even rooted in ponds. The seeds are minute and easily spread by water or mixed in with other materials. They are also durable: Raven (1977) refers to the possibility of propagating Ludwigia from seeds preserved in herbarium specimens.

*Which, if one were of a panbiogeographical bent, might be taken to indicate that it has survived unchanged since the Triassic at least.

Ludwigia octovalvis may even grow as floating mats upon the surface of water. The floating roots then produce spongy, upright branches called aerophores. These have been interpreted as floatation devices, but the plant is apparently perfectly buoyant without them. It is more likely that they allow oxygen to reach the waterlogged roots. The lower part of the stem may also become covered in aerenchyma, porous tissue that also aids in the diffusion of gases. If conditions dry up and the plant becomes rooted in the ground, the aerophores disappear and the roots resume their normal rootly business.

Close-up of flower of Ludwigia octovalvis, copyright Bob Peterson.

Primrose-willows are generally toxic to humans. In the usual way, I have come across references to Ludwigia octovalvis being used folk-medicinally, mostly to help with ailments of the digestive tract such as diarrhoea and worms. A quick look through Google Scholar indicates that this has lead to a certain degree of pharmaceutical research, but so far this doesn't seem to have lead to much major commercial application. At the present point in time, the main economic impact of Ludwigia octovalvis is as a weed. It can grow mixed in with fields of crops such as rice and taro, or particularly lush patches of primrose-willow may clog up waterways. On the flipside, I did find this summary of the species that notes that, "Its yellow flowers add a splash of color to areas often devoid of colorfully flowering plants".

REFERENCES

Raven, P. H. 1977. Onagraceae. Flora Malesiana, ser. I, 8 (2): 98–113.

Melastomes and Pals

Princess flower Tibouchina heteromalla, copyright João Medeiros.

For most botanists currently working on flowering plants, the default taxonomic framework for their studies is the classification published by the Angiosperm Phylogeny Group. This is not the only classification for angiosperms proposed in recent years, but it is the most widely recognised, and it is the one that all its competitors are compared to. If there is one major deficiency of the Angiosperm Phylogeny Group classification, it is that it eschews the use of formal categories between 'orders' (which it tends to define broadly) and 'families'. As such, there are a number of well-supported clades that require one to turn to alternative classifications for suitable names.

Crypteronia paniculata, copyright Tony Rodd.

The Melastomatineae, as recognised by Reveal (2012), for instance, is a clade of mostly tropical and subtropical plants within the Myrtales commonly recovered by molecular phylogenetic analyses. Most of its members are included in the pantropical family Melastomataceae, but it also includes three much smaller and more localised families: the southeast Asian Crypteroniaceae, the western Neotropical Alzatea and the African Penaeaceae. Some authors also divide the Melastomataceae into two families Melastomataceae and Memecylaceae, but as the two groups are universally accepted as sister clades this is purely a matter of taste. Morphological characters uniting the families are few (see the Angiosperm Phylogeny Website). Many accumulate aluminium in the leaves, to the extent that the leaves of the small tree Memecylon edule were used in India as a mordant for fixing dyes to cloth. The flowers lack nectaries, and when nectar is produced in the Melastomataceae it exudes from locations such as the anthers or the corolla. Some Olisbeoideae (the Memecylaceae of other classifications) produce oil from glands on the anthers that is collected by pollinators in lieu of nectar. In the South American genus Axinaea, the anthers have a sugary appendage that is eaten by tanagers; as they attack it, a puff of pollen dusts their head. The anthers of Melastomataceae are often distinctly coloured from the rest of the flower, and arranged in a distinctive seried row on one side of the flower. The Melastomatoideae (i.e. Melastomataceae sensu stricto) also have distinctive leaves, with three or more strong longitudinal veins arising from the leaf base and connected by cross-veins.

Mountain hard pear Olinia emarginata, copyright H. Robertson. Olinia leaves smell of almonds when crushed, due to the presence of a cyanogenic compound.

Though Melastomataceae are often significant members of the tropical forest understorey, they tend not to have much direct economic significance for humans. Some, such as the princess flowers or glory trees of the genus Tibouchina, are grown as ornamentals. The hard pear Olinia ventosa of the Penaeaceae (no, I don't know why it's called that) is grown as a shade tree in South Africa. A few species of Melastomataceae are significant invasive weeds in warmer regions, including such luminaries as the Straits rhododendron Melastoma malabathricum and the evocatively-named Koster's curse Clidemia hirta. The velvet tree Miconia calvescens has earned itself the label of the 'purple plague' in Hawaii, where it over-runs native forest.

REFERENCE

Reveal, J. L. 2012. An outline of a classification scheme for extant flowering plants. Phytoneuron 37: 1–221.

Malpighiales: A Glorious Mess of Flowering Plants

Ixonanthes reticulata, from here.

There is no denying that the advent of molecular analysis revolutionised the world of plant phylogeny. Previously an uncertain landscape of shifting sands, beset by the eroding forces of convergent evolution and morphological plasticity, the higher relationships of flowering plants have begun to resolve into a much clearer view than before. But some of the revealed vistas have been unexpected, and have led to quagmires of their own.

The Malpighiales are one clade that has become generally recognised as a result of molecular analyses, but remain almost impossible to characterise morphologically. Part of that difficulty is a consequence of sheer diversity: the clade includes about 16,000 species worldwide. The bulk of these species are tropical; it has been estimated that 40% of the world's tropical rain-forest understory is composed of Malpighiales (Xi et al. 2012). Only a relative minority of Malpighiales are found in more temperate parts of the world, though that minority still includes such familiar plants as violets, willows and spurges. The ranks of Malpighiales include some of the most bizarrely modified of all flowering plants: the endoparasitic Rafflesiaceae and the aquatic Podostemaceae.

Fruit of the jellyfish tree Medusagyne oppositifolia, photographed by Christopher Kaiser-Bunbury. The jellyfish tree is restricted to the Seychelles and critically endangered; the few surviving trees occupy marginal habitat where seedling germination seemingly cannot occur.

Though molecular analyses have been fairly consistent in supporting the Malpighiales as a whole, relationships within the Malpighiales long proved more recalcitrant. As a result, its species have been placed in up to 42 different families, these families varying wildly in diversity. At one end of the scale, the Euphorbiaceae has been home to over 5700 species, even in its modern restricted sense (earlier botanists recognised a Euphorbiaceae that was considerably larger). At the other end, the Malesian vine Lophopyxis maingayi and the jellyfish tree Medusagyne oppositifolia of the Seychelles have each been considered distinctive enough and of uncertain enough affinities to be placed in their own monotypic families. Many of these families could only be placed within the Malpighiales as part of a great polytomy, an unresolved mess of relationships at the base of the clade.

Herbarium specimen of Centroplacus glaucinus, from here. This species has a restricted range in West Africa; its closest relatives belong to the genus Bhesa in south-east Asia.

A major advance in our understanding of malpighialean phylogeny was made just recently by Xi et al. (2012), who were able to obtain a more resolved phylogenetic tree than previous studies through the use of data from a large number of genes (they also ignored the Rafflesiaceae; those guys just cause trouble). Their results suggested a division of the Malpighiales between three basal clades. The smallest of these includes relatives of the families Malpighiaceae and Chrysobalanaceae. Few members of this clade are familiar outside the tropics. Some are known for their edible fruit, such as the coco plum Chrysobalanus icaco, the nance Byrsonima crassifolia, the Barbados cherry Malpighia emarginata and the butter-nut Caryocar nuciferum. In contrast, the southern African gifblaar Dichapetalum cymosum contains toxic sodium monofluoroacetate and is regarded as a serious threat to livestock.

Small individual of the mangrove Kandelia candel, photographed by Dans.

The next clade includes families relatied to the Clusiaceae, Ochnaceae and Erythroxylaceae. The latter family is closely related to (and sometimes synonymised with) the Rhizophoraceae, a small but significant family that dominates among the tropical mangroves. The Erythroxylaceae is itself most notorious for including the coca plant Erythroxylum coca, the source of the drug cocaine*. The clusioid families include the Clusiaceae, Hypericaceae and Calophyllaceae, treated in older sources as a single family Guttiferae but currently treated as separate families owing to the paraphyly of such a grouping to the families Bonnetiaceae and Podostemaceae. The name 'Guttiferae' refers to the production of resin by many clusioids. In some species, these resins are produced in the flowers in lieu of nectar and are collected for nest-building by visiting bees. Economically significant clusioids include the mangosteen Garcinia mangostana and the St John's wort Hypericum perforatum, which has been grown commercially in some parts of the world for its supposed medicinal properties but is regarded in other parts of the world as a highly undesirable weed.

*Not raisins.

Flower of Rhizanthes infanticida, a smaller relative of Rafflesia, growing from host roots on the forest floor in Thailand, from here. Further buds are visible as reddish balls closer to the tree.

The third clade, and the largest by a considerable margin, includes such families as the Euphorbiaceae, Violaceae and Salicaceae. Noteworthy examples of this clade also include the passion fruit Passiflora edulis, and the flax plant Linum usitatissimum. The Euphorbiaceae, as alluded to above, were previously considered to include taxa more recently treated as the separate families Putranjivaceae, Phyllanthaceae, Picodendraceae and Peraceae. The Putranjivaceae were placed by Xi et al. (2012) in the Malpighiaceae-Chrysobalanaceae clade, and so are not close relatives of the Euphorbiaceae sensu stricto. The remaining families are closer, but modern authors would prefer to keep them separate as the demands of monophyly would then require the Euphorbiaceae be further enlarged to include the Linaceae and Rafflesiaceae. Nobody wants a Rafflesia in their family.

REFERENCE

Xi, Z., B. R. Ruhfel, H. Schaefer, A. M. Amorim, M. Sugumarane, K. J. Wurdack, P. K. Endress, M. L. Matthews, P. F. Stevens, S. Mathews & C. C. Davis. 2012. Phylogenomics and a posteriori data partitioning resolve the Cretaceous angiosperm radiation Malpighiales. Proceedings of the National Academy of Science of the USA 109 (43): 17519-17524.

The Wool Plants

Vegetable lamb, as illustrated in The Travels of Sir John Mandeville (ca 1360).

Medieval legend in Europe spoke of a strange animal that could supposedly be found far off in central Asia: the vegetable lamb. According to legend, this was an animal much like an ordinary sheep except that it grew directly from a plant, to which it remained attached by the umbilical cord. The vegetable lamb would sustain itself by grazing on nearby vegetation but when this was depleted, as the lamb could not move away from the plant to which it was attached, the lamb would die. How such a pointlessly self-defeating organism was supposed to persist does not appear to have concerned the medieval lexicographers; presumably it was supposed to be allegorical of something.

Opening fruit of Gossypium hirsutum, photographed by B. P. Schuiling.
Part of the reason for the legend's persistence, however, was that there was indeed a form of 'wool' that came from a plant: cotton. The cotton genus Gossypium comprises about fifty species found in tropical and subtropical regions around the world (Wendel et al. 2010). Members of the genus vary from herbaceous perennials to small trees. The genus is divided into four subgenera, most of which are geographically distinct. The subgenus Gossypium is found in Africa and Arabia, subgenus Sturtia in Australia, and subgenus Houzingenia in the Americas. These three subgenera between them include the diploid cotton species; the fourth subgenus Karpas is also found in the Americas but differs in containing tetraploid species. Genetic evidence indicates that the subgenus Karpas arose at some point in the very recent past (within the last one or two million years) from a single hybridisation event between a species of subgenus Gossypium and one of Houzingenia, probably as a result of some chance dispersal event from Africa. Gossypium seeds seem well suited to dispersal: seeds of the Hawaiian Island species G. tomentosum have apparently germinated after being kept immersed in artificial seawater for three years (Wendel et al. 2010)! This same predicection for dispersal has resulted in the tetraploid species rapidly becoming widespread despite their recent origin, and in producing two species in remote locales: the Hawaiian G. tomentosum is directly related to the mainland G. hirsutum, while the Galapagos G. darwinii is sister to the mainland G. barbadense.

Levant cotton Gossypium herbaceum, photographed by H. Zell.

Commercial cotton is grown from four species of Gossypium, which may have each been domesticated independently in prehistoric times. All Gossypium species produce seeds with a covering of fuzzy hairs, but seeds of the two Old World diploid species G. herbaceum and G. arboreum also possess an outer layer of longer, flatter hairs that can be woven into thread. It was one of these two species, or possibly some now-extinct close relative, that made the crossing over the Atlantic to become one ancestor of the tetraploid species; as a result, the tetraploid species also possess these long outer hairs. Two of the tetraploid species, G. barbadense and G. hirsutum, were also domesticated, and the latter of these is now by far the most abundant cotton species in cultivation*.

*In case you were wondering, no-one seems to have suggested that the island species related to the two American domesticates might have been human-dispersed.

Sturt's desert rose Gossypium sturtianum, from here.

Other diploid Gossypium species do not possess this longer outer hair layer, only the inner short layer, and are not sources of commercial cotton (though hybrids with some of these species have been used to breed desirable genetic traits into the commercial species). In one group of Australian species (the section Grandicalyx) found in the Kimberley region of northern Western Australia, the hair layer has become very sparse and the seeds are almost hairless. These seeds also possess fatty bodies called eliosomes that are attractive to ants, and the plants are dispersed by having hungry ants carry their seeds away. Grandicalyx species are seasonal herbs, dying off above ground during droughts only to resprout from their thick root-stock. Other Australian species include the Sturt's desert rose Gossypium sturtianum, the floral emblem of Australia's Northern Territory.

Gossypium gossypioides, from here.

As with other plant groups, hybridisation appears to have been a recurring factor in the evolution of Gossypium. The diploid Gossypium species have been divided between eight genome groups, hybrids between which are generally not viable (though not unknown: the parents of the tetraploid lineage, for instance, belonged to separate groups). However, genetic studies of some Gossypium species have identified discrepancies where a species may possess the nuclear genome of one group, but the chloroplast genome of another. For instance, the North American species G. gossypioides resembles other New World species in its nuclear genome, but has chloroplasts related to those of G. herbaceum or G. arboreum (which it may have acquired as a result of the same hybridisation event that produced the tetraploid species*). This phenomenon, which has been called cytoplasmic introgression, may have arisen in cotton through a process called semigamy. Semigamy is a particular form of apomixis (reproduction without fertilisation) in which sperm and egg cells fuse cytoplasmically, but their nuclei remain distinct (Curtiss et al. 2011). These nuclei will eventually be segregated by cell division, resulting in offspring that are mosaics of male- and female-line genomes. Over time, selection or drift may produce a homogenous population that retains the nuclear genome of one ancestor, but the cytoplasmic heritage of the other.

*The American parent of the tetraploids has more usually been identified as G. raimondii, a South American species, but G. raimondii is the direct sister species of G. gossypioides. It may be that G. gossypioides is the true parent of the tetraploids, or it may be that it too is derived from G. raimondii or its parent stock).

REFERENCES

Curtiss, J., L. Rodriguez-Uribe, J. McD. Stewart & J. Zhang. 2011. Identification of differentially expressed genes associated with semigamy in Pima cotton (Gossypium barbadense L.) through comparative microarray analysis. BMC Plant Biology 11: 49.

Wendel, J. F., C. L. Brubaker & T. Seelanan. 2010. The origin and evolution of Gossypium. In: Stewart, J. McD., D. Oosterhuis, J. J. Heitholt & J. R. Mauney (eds) Physiology of Cotton pp. 1-18. Springer.

The Range of Lotus

For today's post, I'll be focusing on the lotus. And having said that, how many of you instantly thought of something like this:

To which I can only say: you should be ashamed of yourself. That is not a lotus, that is some wierd aquatic poppy-type thing called Nelumbo nucifera. This is a lotus:

Bird's-foot trefoil Lotus corniculatus, from Lyndon's Garden.

To clarify, Lotus is a genus of over a hundred species of herbaceous legumes native mostly to Eurasia and northern Africa, with smaller numbers of species in sub-Saharan Africa and Australia (Kirkbride 1999). About forty or so species have also been assigned to this genus from the Americas (particularly western North America), but all recent analyses have agreed that the New World species are not immediately related to the Old World species (Allan & Porter 2000; Arambarri et al. 2005) and they have been reclassified as genera Hosackia, Acmispon, Ottleya and Syrmatium—we shall not speak of them again. How the name 'lotus' came to be simultaneously applied to two such different plants as pictured above, I couldn't say, but the practice goes back a long time: the elder Pliny was referring to both sweet clover and a water lily as lotus in the first century AD (Kirkbride 1999). He also used the name 'lotus' for jujubes and (possibly) pomegranates, so he evidently had a certain affection for the word.

Greater lotus Lotus uliginosus, photographed by Forest & Kim Starr.

Lotus species are commonly known as trefoils, in reference to the leaves being divided into three leaflets. As it happens, most Lotus species actually have leaves with five leaflets, but two of those are separated from the others by an extended midrib. Pea-shaped flowers are borne in small terminal clusters; these are most commonly yellow, though some species produce red flowers. A few species have gone by the vernacular name of 'bacon and eggs' in reference to their producing flowers which are a combination of the two colours. Seeds are produced in long straight pods, and the appearance of the clustered pods is responsible for another vernacular name, bird's-foot trefoil. In one group of species, commonly separated as a genus Tetragonolobus but phylogenetically nested among other Lotus (Allan & Porter 2000), the pods bear four longitudinal wings. Even excluding the New World species, the exact number of species recognised in Lotus varies between authors, primarily due to disagreement over the appropriate treatment of segregates of the more widespread and variable taxa.

Young pod and flower of asparagus pea Lotus tetragonolobus, from here.
A number of Lotus species, particularly L. corniculatus and the greater lotus L. uliginosus*, are used as pasture legumes and have become established around the world as a result. Though arguably less productive than alternative legumes such as clover, they are often able to tolerate more marginal habitats (particularly waterlogged ground). Some species do contain secondary metabolites that can produce cyanide, but concentrations are not usually high enough to be a concern. The asparagus pea Lotus tetragonolobus (also known as Tetragonolobus purpureus) is grown for more direct human consumption, with the pods being eaten before they reach maturity. The name 'asparagus pea' is supposed to refer to their flavour, but this website expressed the opinion that: "If you have an excessively moist mouth, and are looking for something to suck all the moisture out and leave you all pasty, then asparagus peas are the vegetable for you."

*There seems to be some disagreement out there about whether Lotus uliginosus should be recognised as distinct from L. pedunculatus. Kirkbride (1999) uses L. uliginosus as a distinct taxon.

REFERENCES

Allan, G. J., & J. M. Porter. 2000. Tribal delimitation and phylogenetic relationships of Loteae and Coronilleae (Faboideae: Fabaceae) with special reference to Lotus: evidence from nuclear ribosomal ITS sequences. American Journal of Botany 87 (12): 1871-1881.

Arambarri, A. M., S. A. Stenglein, M. N. Colares & M. C. Novoa. 2005. Taxonomy of the New World species of Lotus (Leguminosae: Loteae). Australian Journal of Botany 53: 797-812.

Kirkbride, J. H., Jr. 1999. Lotus systematics and distribution. In: Trefoil: The Science and Technology of Lotus, pp. 1-20. Crop Science Society of America and American Society of Agronomy.

Lecantheae and/or Elatostemateae

Elatostema umbellatum var. majus, photographed by Michael Becker.

The Elatostemateae is a tribe of plants in the Urticaceae, the family that includes the nettles (Urticaceae in general have been looked at in an earlier post). Elatostemateae are mostly distinguished from other members of the Urticaceae by their generally tripartite perianth in the female flowers and their brush-like stigmas (Friis 1993). There seems to be some conflict over the appropriate name to call this group: Conn & Hadiah (2009) argued for the use of Elatostemateae, but many authors had previously called this group the Lecantheae. The argument hinges purely on a question of priority a bit too tedious to examine here.

Lecanthus peduncularis, photographed by Li-Chieh Pan.

The question may or may not be moot, anyway. Recent phylogenetic analyses of the Urticaceae have generally agreed that the Elatostemateae can be divided between two clades, one including the genus Elatostema and the other the genera Lecanthus and Pilea. The two groups are morphologically as well as molecularly distinguishable: species of Pilea and Lecanthus generally have opposite leaves, but in Elatostema one of each leaf pair has been greatly reduced (or lost) so that the leaves appear alternate. However, it is currently uncertain whether these two groups together form an exclusive clade. In the analyses by Hadiah et al. (2008), trnL-F sequence data was consistent with a monophyletic Elatostemateae, but rbcL data placed the Lecanthus-Pilea group closer to the tribe Urticeae than to Elatostema. This is not inherently unreasonable: as members of the Urticeae also have brush-like stigmas like those of Elatostemateae, the main distinction between the two tribes is the plesiomorphic absence in the latter of the stinging hairs that characterise the Urticeae.

Pilea fairchildiana, previously Sarcopilea domingensis, from Javier Francisco-Ortega.

Whether monophyletic or not, the Elatostemateae are mostly a tropical and subtropical group. They are mostly herbs and subshrubs, with a smaller number of shrub species. The centre of diversity is in the Old World; only the genus Pilea is found in the Americas. Members of the Elatostema group may be either all included in a single genus or divided between genera Elatostema, Procris and Pellionia distinguished by inflorescence characters. Pilea, the largest genus in the group, includes species in both the Old and New Worlds, of which some are commonly succulent. The Hispaniolan species Pilea fairchildiana has developed a rosulate growth form, remarkably similar to members of genera in the unrelated family Crassulaceae, such as Aeonium and Sempervivum. Until very recently, this species was considered distinctive enough to be placed in a separate genus Sarcopilea; Jestrow et al. (2012) demonstrated its more nested position. Economic usages of Lecantheae species are few: the leaves of some Pilea species, such as the artillery plants* P. microphylla and P. melastomoides, have some limited use as aromatic herbs, while some species of the Elatostema group are apparently used in Indonesia as shampoo.

*The name 'artillery plant' apparently refers to the way in while the male flowers fire forth their pollen, which is supposed to resemble gunsmoke.

REFERENCES

Conn, B. J., & J. T. Hadiah. 2008. Nomenclature of tribes within the Urticaceae. Kew Bulletin 64; 349-352.

Friis, I. 1993. Urticaceae. In: Kubitzki, K., J. G. Rohwer & V. Bittrich (eds) The Families and Genera of Vascular Plants vol. 2. Flowering Plants. Dicotyledons. Magnoliid, hamamelid and caryophylliid families pp. 612-629. Springer.

Hadiah, J. T., B. J. Conn & C. J. Quinn. 2008. Infra-familial phylogeny of Urticaceae, using chloroplast sequence data. Australian Systematic Botany 21: 375-385.

Jestrow, B., J. J. Valdés, F. Jiménez Rodríguez & J. Francisco-Ortega. 2012. Phylogenetic placement of the Dominican Republic endemic genus Sarcopilea (Urticaceae). Taxon 61 (3): 592-600.

Milk-vetches, Liquorice and Locoweeds

Flowers of the weeping broom Carmichaelia stevensonii, from here.

Far in the distant past, I commented on a review of the New Zealand brooms of the genus Carmichaelia. The New Zealand brooms were placed by Wagstaff et al. (1999) in the tribe Galegeae, which includes more than 3000 species around the world. The vast majority of these are placed in the genus Astragalus, milk-vetches, which may itself have well over 2500 species and be the largest recognised genus of flowering plant. Other significant members of the Galegeae include the locoweeds (Oxytropis) and Glycyrrhiza, a small genus of less than twenty species that nevertheless deserves praise for being the source of liquorice*.

*I am reliably informed that there are people in this world who do not appreciate the flavour of liquorice. There is simply no accounting for taste.


Flowers of Astragalus monspessulanus, from here.

However, despite its wide recognition, phylogenetic studies of recent years have been unanimous in declaring the Galegeae polyphyletic. It is true that the vast majority of galegeans remain in a clade, with only a few relatively minor genera placed elsewhere (Wojciechowski et al. 2000). The problem is that one of those 'minor genera' happens to be Galega itself, the type genus and therefore sine qua non of the tribe, which is more closely related to the chickpeas of the genus Cicer. Wojciechowski et al. (2000) circumvented this issue by recognising a clade called Hologalegina, uniting the 'Galegeae' with other tribes such as Hedysareae, Trifolieae, Fabeae, Loteae and Robinieae. Within the Hologalegina, the taxa previously assigned to the Galegeae all belong to what is called the IRLC, the 'Inverted Repeat-Lacking Clade'. The name of this clade refers to the loss of a copy of the 25 kb inverted repeat in the chloroplast genome that is otherwise found in almost all other land plants. Members of the IRLC are largely herbaceous, and many of the most commercially significant legumes (such as peas, beans, lentils, clover and alfalfa) belong to this clade.


Liquorice, Glycyrrhiza glabra, photographed by J. C. Schou. The flavour comes from the roots.

With the exception of the aforementioned Glycyrrhiza (which is another phylogenetically distant genus from other galegeans), most of the 'Galegeae' are not commercially grown. They may be commercially eradicated: species of Oxytropis get their name of 'locoweed' from their toxic effects on grazing livestock, and various species of Astragalus and Swainsona are also known for their toxicity. Some species are cultivated as garden plants, such as the weeping broom Carmichaelia stevensonii shown in the top photo (though many places still list it under the older name of Chordospartium stevensonii) and the kaka beak Clianthus puniceus. One particular galegean is widely regarded as the stuff that gardeners' dreams are made of, despite being a notoriously difficult plant to grow. My partner tells me that when he lived in Kalgoorlie as a child, tourist buses would regularly appear outside his house just so that their passengers could see this plant flowering in the front yard. I speak, of course, of Swainsona formosa, Sturt's desert pea:


Photographed by Marj Kibby.

REFERENCES

Wagstaff, S. J., P. B. Heenan & M. J. Sanderson. 1999. Classification, origins, and patterns of diversification in New Zealand Carmichaelinae (Fabaceae). American Journal of Botany 86 (9): 1346-1356.

Wojciechowski, M. F., M. J. Sanderson, K. P. Steele & A. Liston. 2000. Molecular phylogeny of the “temperate herbaceous tribes” of papilionoid legumes: a supertree approach. In: Herendeen, P. S., & A. Bruneau (eds). Advances in Legume Systematics vol. 9, pp. 277–298. Royal Botanic Gardens, Kew.

Cunoniaceae and Friends

The Albany pitcher plant Cephalotus follicularis of south-western Australia. The streaked colours on the inside of the lid attract insects into the pitcher; the incurved teeth around the rim stop them from climbing back out. Photo by Holger Hennern.

The plant order Cunoniales was first established in 1926 to include plants with similar flowers to the Saxifragales but that were primarily woody rather than herbaceous (Dickison, 1975). Woodiness vs. herbaceousness is no longer considered that significant a feature in plant classification (sometimes you can find both in the same genus) and the content of the order has varied between classifications*. Today, the type family Cunoniaceae is included in the order Oxalidales and a taxon "Cunoniales" is no longer used as such. However, one of the two basal clades within the Oxalidales includes the Cunoniaceae and two ex-Cunoniales genera placed in their own families, Cephalotus and Brunellia, together with the family Elaeocarpaceae (previously in its own order) (Matthews & Endress, 2002). With the notable exception of Cephalotus, the members of this clade are mostly shrubs or trees. Economically, the clade is not overly significant: some species are used for wood; some have good reputations as honey sources for bees; a few produce edible fruits but do not appear to have been systematically cultivated for them. Members of all families bear their flowers clustered into (most often cymose) inflorescences; the size of individual flowers in the inflorescences varies between species. Brunellia and Cephalotus both produce flowers with thick sepals and no petals; in the other two families, petals may be present or absent (Matthews & Endress, 2002).

*As have most flowering plant "orders". There's a reason why order-level taxa don't get much day-to-day use among botanists compared to families.


Coachwood, Ceratopetalum apetalum, a member of the Cunoniaceae from eastern Australia. Photo by Melburnian..

The clade formed by these four families has a distinctly southern distribution in southern Africa, South and Central America, south-east Asia, Australia and New Zealand. Elaeocarpaceae are absent from continental Africa but are present in Madagascar. Also, while currently absent from India, they have been recorded from the fossil record there (Crayn et al., 2006). The Cunoniaceae include about 300 species, half in the genus Weinmannia, whereas the Elaeocarpaceae include about 600 species. Molecular studies have shown that an Australian radiation of dry-habitat shrubs previously regarded as the family Tremandraceae is in fact a subclade of the otherwise mostly rainforest-inhabiting Elaeocarpaceae (Crayn et al., 2006). Interestingly, the molecular dating study by Crayn et al. (2006) suggests that the 'Tremandraceae' developed scleromorphy (a suite of adaptations such as hardened leaves that are usually associated with arid habitats) some time before the Australian continent developed its current arid climate. While this may seem counter-intuitive, it is worth pointing out that the fossil record supports the same thing in the evolution of the genus Banksia (Mast & Givnish, 2002). It has been suggested that scleromorphy in these groups was therefore not originally an adaptation for arid living, but for growing in the poor soils of the Palaeogene Australian rainforest.


Prima donna, Elaeocarpus reticulatus. Fringed petals are characteristic of a number of species of Elaeocarpaceae. Elaeocarpaceae flowers are also adapted for buzz-pollination, where anthers do not release their pollen until vibrated by the wingbeats of their pollinating bees. Photo from Kate's Photo Diary.

The oddest member of this clade, however, has to be the Albany pitcher plant, Cephalotus follicularis. Restricted to the south-west corner of Western Australia, this plant bears a superficial resemblance to pitcher plants from other parts of the world, the mostly Indomalayan Nepenthaceae and the North American Sarraceniaceae. All three of these families have evolved the pitcher morphology independently, and can in fact be placed in separate subclasses: Cephalotus in the Rosidae, the Nepenthaceae in the Caryophyllidae and the Sarraceniaceae in the Asteridae. In the Nepenthaceae, the pitchers are developed from trendrils at the ends of the leaves; in the other two families, the entire leaves form the pitchers growing from a ground-level rhizome. In Cephalotus, only the outer leaves of an individual plant are developed into pitchers; the inner leaves remain flat and simple.

REFERENCES

Crayn, D. M., M. Rossetto & D. J. Maynard. 2006. Molecular phylogeny and dating reveals an Oligo-Miocene radiation of dry-adapted shrubs (former Tremandraceae) from rainforest tree progenitors (Elaeocarpaceae) in Australia. American Journal of Botany 93 (9): 1328-1342.

Dickison, W. C. 1975. Studies on the floral anatomy of the Cunoniaceae. American Journal of Botany 62 (5): 433-447.

Mast, A. R., & T. J. Givnish. 2002. Historical biogeography and the origin of stomatal distributions in Banksia and Dryandra (Proteaceae) based on their cpDNA phylogeny. American Journal of Botany 89 (8): 1311-1323.

Matthews, M. L., & P. K. Endress. 2002. Comparative floral structure and systematics in Oxalidales (Oxalidaceae, Connaraceae, Brunelliaceae, Cephalotaceae, Cunoniaceae, Elaeocarpaceae, Tremandraceae). Botanical Journal of the Linnean Society 140 (4): 321-381.

A Simple Stream Life

In a footnote to a recent post, I made the offhand comment that the strangest flowering plants were to be found among the Podostemoideae. Today I'd like to introduce you to them.


Unidentified Podostemaceae (perhaps Podostemon) collected from a fast-flowing stream in Venezuela. Photo by Kevin Nixon. (And yes, this is the identity of Name the Bug #17.)

Podostemaceae is a family of about 270 species of freshwater aquatic plants found mostly in tropical parts of the world. The family is divided into three subfamilies, Podostemoideae, Tristichoideae and the monogeneric Weddellina. The main difference between Tristichoideae and Podostemoideae is that the flowers of the latter developed encased in a sack-like covering called a spathella. Weddellina resembles tristichoids in lacking a spathella but some of the finer features of its flower anatomy are more like podostemoids, to which phylogenetic analysis indicates it is the sister taxon (Kita & Kato 2001).

Podostems specialise in living in fast-moving, temporary streams and waterfalls that become dry for part of the year, usually on rocky surfaces. Many podostem species are known for having ridiculously small distributions, often restricted to a single waterway (more on that later). Podostems differ from all other aquatic flowering plants in lacking aerenchyma, the gas-filled tissue I mentioned in reference to duckweeds. The primary podostem morphology involves spreading, usually flattened 'roots' that give rise to branching 'shoots' that in turn produce 'leaves' (Rutishauser 1997). However, all parts are photosynthetic and these structures probably do not correspond directly to comparable structures in other plants. The plumule and the radicle, the initial shoots in a normal germinating seed that develop into the stem and the root, respectively, are absent in most podostems (Sehgal et al. 2002). Instead, the germinating plant body develops as a lateral outgrowth of the hypocotyl, the stem that would normally support the plumule. The plant attaches itself to the rock by small rootlets growing from the 'roots' or by disk-shaped holdfasts. Either the rootlets attach themselves in pre-existing films of adherent bacteria (Jäger-Zürn & Grubert 2000) or they may secrete their own sticky mucilage (Sehgal et al. 2002). In the Indian Dalzellia zeylanica the distinction between 'roots' and 'shoots' has disappeared entirely; instead, the plant grows as a crustose thallus bearing both rootlets and 'leaves'. 'Leaves' of podostems are varied in appearance from large compound structures up to two metres long to minute scales or bundles of filaments. Species with larger leaves often have hairs, prickles or warts covering their surface.


The South American podostem Rhyncholacis penicillata in flower. Photo by Berrucomons.

The flowers of podostems, whether covered by a spathella or not, are very variable. Inflorescences of Mourera fluviatilis (the species with the two-metre leaves) can be up to 64 cm tall including the spike with up to ninety flowers. Other species (including all Tristichoideae) may bear only a single flower on a short stem. Depending on the species, podostem flowers may be wind-pollinated, insect-pollinated or self-pollinated. Podostem seeds are tiny, wind-dispersed and lack endosperm (podostems lack the double fertilisation of most flowering plants). 1g of weight may contain over a million seeds. Fruits vary greatly in size between species - Mourera fluviatilis may produce fruits containing up to 2400 seeds each while Farmeria metzgerioides produces only two seeds per fruit. In the case of the latter, the fruit doesn't break open to release the seeds; instead, the seeds germinate in place over the remains of their parent.


Podostemon in the dry season: desiccated thalli holding maturing fruits. Photo by Renato Goldenberg.

Much speculation has been conducted on why so many podostems have restricted distributions. Some have implied that many podostem 'species' may turn out to be ecological variants of more widespread species; however, multiple podostem species may be found growing in a single habitat. Others have suggested that podostems are somehow under less selective pressure morphologically than terrestrial plants, allowing a higher rate of mutational drift; however, this proposal remains untested. Interestingly, the molecular phylogenetic analysis of Kita & Kato (2001) found that the highly modified Dalzellia zeylanica was closely related to the morphologically conservative Indotristicha ramosissima. Indeed, the genetic distance between the two was little greater than that between separate populations recognised as the single species Tristicha trifaria, unusual among podostems in being found in both Africa and the Americas. This would suggest that podostems are indeed capable of rapid morphological changes—the only question is how?

REFERENCES

Jäger-Zürn, I., & M. Grubert. 2000. Podostemaceae depend on sticky biofilms with respect to attachment to rocks in waterfalls. International Journal of Plant Sciences 161 (4): 599-607.

Kita, Y., & M. Kato. 2001. Infrafamilial phylogeny of the aquatic angiosperm Podostemaceae inferred from the nucleotide sequences of the matK gene. Plant Biology 3 (2): 156-163.

Rutishauser, R. 1997. Structural and developmental diversity in Podostemaceae (river-weeds). Aquatic Botany 57: 29-70.

Sehgal, A., M. Sethi & H. Y. Mohan Ram. 2002. Origin, structure, and interpretation of the thallus in Hydrobryopsis sessilis (Podostemaceae). International Journal of Plant Sciences 163 (6): 891-905.

A South American Paradox (Taxon of the Week: Sellocharis)

Aspects of Sellocharis paradoxa (Papilionaceae) as illustrated by Polhill (1976): "1, habit; 2, node with stem cut away to show leaf-arrangement; 3, flower; 4, calyx, opened out; 5, standard; 6, wing; 7, keel; 8, stamens, spread out; 9, anthers; 10, pistil; 11, same with ovary-wall cut away to show ovules".

A brief entry today, because Taxon of the Week this week is a bit of a mystery. The southern Brazilian leguminous subshrub Sellocharis paradoxa was first described in 1889, but for a very long time was known solely from the original isotypes*. It has only been rediscovered in scrubby grasslands and rockfields of the Brazilian state of Rio Grande do Sul within the last ten years or so (Conterato et al., 2007). Without having seen the original description, I can't tell you for certain what earned Sellocharis the name of 'paradoxa', but I suspect it was probably the unique arrangement of its leaves. As you can see in the figure above**, S. paradoxa has its leaves arranged in regular whorls of five to seven. The individual "leaves" are more similar to the leaflets of other leguminous plants, and Polhill (1976) tentatively suggested that that might be what they were - that instead of having whorls of six leaves, S. paradoxa might have only a single leaf that had lost its basal stalk so completely that it had merged with the main stem.

*For the non-botanists among you, "isotypes" are two or more type specimens that have been taken from the same original individual, such as two branches from a single tree.

**Now possibly the only depiction of Sellocharis available freely online. Not that I'm bragging or anything (especially considering I just scanned it out of the original book).

As befits its unusual morphology, the relationships of Sellocharis paradoxa are similarly mysterious. The most similar genus is Anarthrophyllum, a genus of Andean 'cushion plants' in which the stipules of the often trifoliate leaves surround the stem, often forming a sheath, and most authors seem to have assumed a relationship between the two genera. Flower morphology and the presence of α-pyridone alkaloids in Anarthrophyllum suggest a position in the Genisteae, the tribe including brooms, gorse and lupins, which I described in a previous post. Within the Genisteae, the flowers of Anarthrophyllum and Sellocharis are most similar to those of the basal Argyrolobium group. A large-scale molecular analysis of Papilionaceae placed Anarthrophyllum as sister to the clade of Lupinus and Genistinae (Wojciechowski et al., 2004), which is consistent with the previously suggested position of the Argyrolobium group (Ainouche et al., 2003) though no other members of the group were included in the later analysis. However, Polhill (1976) noted that, if one interprets the 'leaves' of Sellocharis as leaflets of a single divided leaf, then they bear a certain resemblance to the leaves of lupins and may indicate a relationship to that genus instead.

A genistean position for Sellocharis and Anarthrophyllum is still not un-problematic. As described in the previous post, the Genisteae is a primarily Old World lineage. Lupinus is the only other genus of Genisteae found in the Americas, and as it is found in both the Old and New Worlds it could be a later invader of the latter. Nevertheless, other genera of the Argyrolobium group are found in southern Africa, and the ancestors of Sellocharis may have come from there as other organisms are known to have done (the ancestors of the New World monkeys being perhaps the most famous example). Also potentially problematic is that the number and morphology of chromosomes in Sellocharis is very distinct from those of any other Genisteae; however, the authors who described Sellocharis' karyotype (Conterato et al., 2007) only referred to its differences from Genisteae without comparing it to members of other tribes. Hopefully, now that Sellocharis paradoxa has been re-found, more progress can be made on establishing just what it is.

REFERENCES

Ainouche, A., R. J. Bayer, P. Cubas & M.-T. Misset. 2003. Phylogenetic relationships within tribe Genisteae (Papilionoideae) with special reference to genus Ulex. In Advances in Legume Systematics part 10, Higher Level Systematics (B. B. Klitgaard & A. Bruneau, eds.) pp. 239-252. Royal Botanic Gardens: Kew.

Conterato, I. F., S. T. Sfoggia Miotto & M. T. Schifino-Wittman. 2007. Chromosome number, karyotype, and taxonomic considerations on the enigmatic Sellocharis paradoxa Taubert (Leguminosae, Papilionoideae, Genisteae). Botanical Journal of the Linnean Society 155 (2): 223-226.

Polhill, R. M. 1976. Genisteae (Adans.) Benth. and related tribes (Leguminosae). Botanical Systematics 1: 143 - 368.

Wojciechowski, M. F., M. Lavin & M. J. Sanderson. 2004. A phylogeny of legumes (Leguminosae) based on analysis of the plastid matK gene resolves many well-supported subclades within the family. American Journal of Botany 91: 1846-1862.

Are You Sucking on a Lemon or a Lime?

The genus Citrus is one of the most significant groups of fruit trees around the world. An overwhelming diversity of fruit varieties are produced by Citrus, such as oranges, lemons, grapefruit, tangelos, citrons, bergamots, mandarins, and the wonderfully-named ugli fruit (and yes, it is). Technically, the fruit of Citrus is a specialised type of berry called a hesperidium, named after the mythical garden of the Hesperides where golden fruit were tended by airy nymphs watched over by a giant serpent, suggesting that Greek prophets had also predicted the eventual appearance of Benny Hill. The hesperidium is distinguished by its leathery, acidic rind and division into segments, and is unique to a clade containing Citrus and a few closely related genera such as Poncirus and Fortunella (kumquats*), both of which have been included by at least some researchers in Citrus (de Araújo et al., 2003). From the taxonomist's point of view, however, Citrus has always been a gigantic headache. The question of how to classify this kaleidoscopic array of varieties, most of them only known as cultivated forms with no record of their origins, presents a quandary perhaps rivalled among plants only by the similarly over-cultivated genus Brassica. In the two main classifications that have been used for Citrus, that presented by Swingle in 1943 recognised sixteen species in the genus, while that of Tanaka in 1977 recognised one-hundred and sixty-two (Jung et al., 2005).

*Some time ago, I commented on the misleadingly obscene sound of words such as "yeast", "moist" and "sphagnum". None of these words, however, comes even close in this regard to the filth innocently suggested by "kumquat".


The citrus variety known as poorman's orange (it's not an orange) or New Zealand grapefruit (it's not a grapefruit, either). Photo by Julian Sauls.

As well as their long history of cultivation, citrus classification is also handicapped by the fact that the different varieties are at one and the same time both highly interfertile and significantly reproductively isolated. This may sound like something of a paradox, but it results from Citrus' distinctive reproductive system, involving a process called nucellar embryony (Moore, 2001). The nucellus is the nutritive tissue surrounding and protecting the ovum in plant ovules. Seed development begins with the ovum being fertilised, and beginning to develop into an embryo. In Citrus, however, the nucellus itself also gives rise to a number of additional embryos that are genetic clones of the parent plant. These nucellar embryos often outcompete the sexually produced embryo, meaning that when the seed germinates the seedling that grows from it will commonly be genetically identical to its parent, and actual sexually-produced offspring are rare. In spite of this, actual direct barriers to reproduction between different species of Citrus are low, so when successful sexual reproduction does occur, the results can be unpredictable.


Kaffir lime (Citrus hystrix), an ingredient that no good curry can do without. Photo from Trade Winds Fruit.

In light of the above, it is perhaps not that surprising that studies conducted on morphology and biochemistry of Citrus species in the 1970s came to the conclusion that of the 100+ potentially recognised species of cultivated Citrus, only three represented true "species" in the sense of deriving from separate domestications of independently evolved taxa. These three primordial species were the citron (Citrus medica), pummelo (Citrus maxima) and mandarin (Citrus reticulata). All other domestic "species" are ultimately derived from crosses, re-crosses and back-crosses of these three species and their derivatives. Oranges, for instance, are probably derived from hybrids of mandarins and pummelos, while the citron is an ancestor for lemons and limes. . A fourth species, the uncultivated Citrus halimii, has also been suggested as a progenitor of cultivated varieties, but Pang et al. (2007) felt that it was not supported as such (apparently - I haven't read the paper, as our library lacks the appropriate subscription). Nicolosi et al. (2000) added another species, the papeda Citrus micrantha, to the mix, representing the subgenus Papeda that includes Citrus micrantha and its close relatives such as the kaffir lime, Citrus hystrix.


Probable relationships between citrus species, from Moore (2001).

I find it quite an impressive thought that the spectacular diversity of citrus fruits we see today should have come from so few progenitors. From pummelos to papedas to Poorman oranges, this is certainly something to keep in mind the next time you down a cocktail.

REFERENCES

Araújo, E. F. de, L. P. de Queiroz & M. A. Machado. 2003. What is Citrus? Taxonomic implications from a study of cp-DNA evolution in the tribe Citreae (Rutaceae subfamily Aurantioideae). Organisms Diversity & Evolution 3 (1): 55-62.

Jung, Y.-H., H.-M. Kwon, S.-H. Kang, J.-H. Kang & S.-C. Kim. 2005. Investigation of the phylogenetic relationships within the genus Citrus (Rutaceae) and related species in Korea using plastid trnL-trnF sequences. Scientia Horticulturae 104 (2): 179-188.

Moore, G. A. 2001. Oranges and lemons: clues to the taxonomy of Citrus from molecular markers. Trends in Genetics 17 (9): 536-540.

Nicolosi, E., Z. N. Deng, A. Gentile, S. La Malfa, G. Continella & E. Tribulato. 2000. Citrus phylogeny and genetic origin of important species as investigated by molecular markers. Theoretical and Applied Genetics 100 (8): 1155-1166.

Pang, X.-M., C.-G. Hu & X.-X. Deng. 2007. Phylogenetic relationships within Citrus and its related genera as inferred from AFLP markers. Genetic Resources and Crop Evolution 54 (2): 429-436.

In a bunch, in a bunch!

Cytisus scoparius, a widespread broom species, and the most widespread as an invasive species. Photo by Paul Slichter.

After the dummy-spit of the last post, let's get on to something a little more comforting. This is, I think, the perfect time for a botany post to calm the nerves. Which is lucky, because the Taxon of the Week post this week is on Genisteae.

Genisteae is the tribe of about 450 species of mostly Holarctic leguminous plants that includes brooms*, gorse and lupins**. The first of these are the Palaearctic brooms - in an earlier post, I wrote about the New Zealand brooms, which belong to a different legume tribe, the Galegeae, and whose broom-like appearance is convergent with that of the Genisteae brooms. However, a number of species of Genisteae have been transported by humans to many temperate regions around the world (including New Zealand), and many are familiar weed species outside their native ranges.

*The shrubs, obviously, not the things you sweep with. Though brooms the plants were often used for the making of brooms the implements, which is probably the origin of the name of one or the other.

**Monty Python fans are permitted to start humming now.


Argyrolobium zanonii, a Mediterranean species of the possibly polyphyletic genus Argyrolobium that is one of best contenders for inclusion in the Genisteae. Photo from here.

Phylogenetically, Genisteae can be divided into three well-divided groups (Ainouche et al., 2003). The probably paraphyletic Argyrolobium group includes five genera of mostly southern African and South American plants that lie outside the clade formed by the Palaearctic members of Genisteae. Previously members of this group were included in the sister tribe Crotalarieae, and their position remains unsettled. In particular, it has been suggested that Argyrolobium itself may be polyphyletic, with some species belonging to Crotalarieae and others to Genisteae. With the Argyrolobium group included, the Genisteae are characterised by a basically two-lipped calyx with a trifid lower lip, and the presence of quinolizidine alkaloids of a-pyridone type (chemical characters have proven to be very useful in plant systematics, with many groups characterised by the production of particular secondary metabolites).


Lupinus polyphyllus, a lupin species native to western North America. Photo from Wikimedia.

The genus Lupinus (the lupins) form a distinct group from the remainder of the Palaearctic Genisteae. Lupinus is by far the largest genus of Genisteae, including about half the species and the only genus to have made it to the New World, where it is found in both North and South America. Most lupins are readily distinguished from other Genisteae by their palmate, divided leaves. While the flowers are not particularly edible (despite the rumoured possibilities of lupin soup, roast lupin, steamed lupin, braised lupin in lupin sauce, lupin in the basket with sauted lupins, lupin meringue pie, lupin sorbet...), the beans of some species are eaten pickled (raw beans generally contain toxic alkaloids) in Mediterranean areas or Latin America. Lupins have been widely grown as stock feed and as ornamentals, and include many of the aforementioned weed species.


Valley in Victoria (Australia) overrun by the vile Ulex europaeus. Photo by Kate Blood.

The remaining genera of Genisteae form the subtribe Genistinae. Diversity of Genistinae is centred around the Mediterranean, with the three best-known genera being the brooms in Genista and Cytisus, and gorse in Ulex (Ulex and the closely related, possibly synonymous, genus Stauracanthus have been placed in a separate subtribe Ulicinae, but Ainouche et al., 2003, demonstrated that these genera fell within Genistinae). A number of other broom genera are recognised, but classifications may differ on which genera are recognised as including which species. The Genistinae also includes the tree genus Laburnum (Käss & Wink, 1997). Members of the Genistinae are characterised by adaptations for arid habitats such as very small or absent leaves and photosynthetic stems. Ulex species have the leaves modified into long spines. The species Ulex europaeus (commonly called simply "gorse") was introduced into New Zealand for use in hedges, a role which it apparently fulfils admirably in Britain. Unfortunately, the imported gorse plants found the New Zealand climate much more to their liking than that of their native Britain, and are currently one of New Zealand's most widespread and visible weed species. Yours truly has many unwelcome memories of hot summer days spent grubbing up gorse plants.

REFERENCES

Ainouche, A., R. J. Bayer, P. Cubas & M.-T. Misset. 2003. Phylogenetic relationships within tribe Genisteae (Papilionoideae) with special reference to genus Ulex. In Advances in Legume Systematics part 10, Higher Level Systematics (B. B. Klitgaard & A. Bruneau, eds.) pp. 239-252. Royal Botanic Gardens: Kew.

Käss, E., & M. Wink. 1997. Phylogenetic relationships in the Papilionoideae (family Leguminosae) based on nucleotide sequences of cpDNA (rbcL) and ncDNA (ITS 1 and 2). Molecular Phylogenetics and Evolution 8 (1): 65-88.