Yellow tulipwood Drypetes deplanchei, from CSIRO.

Belongs within: Fabidae.
Contains: Violineae, Peraceae, Rafflesiaceae, Euphorbiaceae, Phyllanthaceae, Picrodendraceae, Linaceae, Chrysobalanales, Elatinaceae, Malpighiaceae, Putranjivaceae, Erythroxylaceae, Rhizophoraceae, Ochnaceae, Clusiaceae, Calophyllaceae, Hypericum.

The Malpighiales is a large clade of flowering plants supported primarily by molecular analyses. Morphological synapomorphies for the clade are few, except for a toothed leaf lamina margin and dry stigma (Angiosperm Phylogeny Website). Members include the Humiriaceae, a primarily Neotropical group (except a single species in west Africa) of trees and shrubs with blackish-drying, short-petiolate leaves that often bear longitudinal lines down the abaxial surface. The Ixonanthaceae are a pantropical but poorly known group of trees with spirally arranged leaves.

A simple stream life
Published 15 July 2010

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.

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, photographed 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?

Malpighiales: a glorious mess of flowering plants
Published 26 November 2013
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.

Systematics of Malpighiales
<==Malpighiales [Euphorbiales, Hypericineae, Lineae, Ochnineae, Parietales]
    |  i. s.: GonystylaceaeT00
    |           |--Gonystylus bancanusT00, H03
    |           `--AetoxylonT00
    |  |  `--HumiriaceaeXR12
    |  |       |  i. s.: HumiriastrumCBH93
    |  |       |--Schistostemon retusumXR12
    |  |       `--+--Sacoglottis gabonensisXR12, FGN07
    |  |          `-+--VantaneaXR12
    |  |            `--Humiria balsamiferaXR12
    |  `--+--+--PeraceaeXR12
    |     |  `--+--RafflesiaceaeDL07
    |     |     `--EuphorbiaceaeXR12
    |     `--+--+--PhyllanthaceaeXR12
    |        |  `--PicrodendraceaeXR12
    |        `--+--LinaceaeXR12
    |           `--IxonanthaceaeXR12
    |                |  i. s.: PhyllocosmusT00
    |                |--Ixonanthes icosandraXR12, WM09
    |                `--+--OchthocosmusXR12
    |                   `--CyrillopsisXR12
    |  |  `--Balanops [Balanopaceae, Balanopales]XR12
    |  |       |--B. caledonicaXR12
    |  |       `--B. pachyphyllaXR12
    |  |--+--CentroplacaceaeXR12
    |  |  |    |--Centroplacus glaucinusXR12
    |  |  |    `--BhesaXR12
    |  |  `--+--ElatinaceaeXR12
    |  |     `--MalpighiaceaeXR12
    |  |--CaryocaraceaeXR12
    |  |    |--Anthodiscus peruanusXR12
    |  |    |--Retisyncolporites angularis Gonzalez-Guzman 1967CBH93
    |  |    `--CaryocarXR12
    |  |         |--C. brasilienseK06
    |  |         |--C. coriaceumK06
    |  |         `--C. villosumK06
    |  `--+--Lophopyxis [Lophopyxidaceae]XR12
    |     `--PutranjivaceaeXR12
       |  |    |--Ctenolophon englerianusXR12
       |  |    `--Ctenolophonidites costatusCBH93
       |  `--+--ErythroxylaceaeXR12
       |     `--RhizophoraceaeXR12
       |  |    |--Microdesmis casearifoliaXR12
       |  |    `--Galearia maingayiXR12
       |  `--IrvingiaceaeXR12
       |       |--DesbordesiaT00
       |       |--Irvingia malayanaXR12
       |       |--KlainedoxaXR12
       |       `--AllantospermumAPG16
          |  `--+--Medusagyne [Medusagynaceae]XR12
          |     |    `--M. oppositifoliaXR12
          |     `--QuiinaceaeXR12
          |          |--FroesiaXR12
          |          `--+--TourouliaXR12
          |             `--Quiina glazioviiXR12
             |  `--BonnetiaceaeXR12
             |       |--ArchytaeaXR12
             |       `--Ploiarium sessileXR12, J06
                `--+--Podostemaceae [Podostemales]XR12
                   |    |  i. s.: MarathrumXR12
                   |    |         Malaccotristicha australisLK14
                   |    |         Nitophyllites zaisanica Ilinskaja 1963CBH93
                   |    |--TristichoideaeT00
                   |    `--PodostemumXR12 [PodostemoideaeT00]
                   |         `--P. ceratophyllumXR12
                   `--Hypericaceae [Hypericoideae]XR12
                        |  `--CratoxylumXR12
                        |       |--C. formosumP88
                        |       `--C. polyanthumT-W89
                                |--V. ferrgineaXR12
                                `--V. guianensisK06

*Type species of generic name indicated


[APG16] Angiosperm Phylogeny Group. 2016. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society 181: 1–20.

[CBH93] Collinson, M. E., M. C. Boulter & P. L. Holmes. 1993. Magnoliophyta (‘Angiospermae’). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 809–841. Chapman & Hall: London.

[DL07] Davis, C. C., M. Latvis, D. L. Nickrent, K. J. Wurdack & D. A. Baum. 2007. Floral gigantism in Rafflesiaceae. Science 315: 1812.

[FGN07] Fontaine, B., O. Gargominy & E. Neubert. 2007. Land snail diversity of the savanna/forest mosaic in Lopé National Park, Gabon. Malacologia 49 (2): 313–338.

[H03] Heads, M. 2003. Ericaceae in Malesia: vicariance biogeography, terrane tectonics and ecology. Telopea 10 (1): 311–449.

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.

[J06] Johnstone, R. E. 2006. The birds of Gag Island, Western Papuan islands, Indonesia. Records of the Western Australian Museum 23 (2): 115–132.

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.

[K06] Kwiecinski, G. G. 2006. Phyllostomus discolor. Mammalian Species 801: 1–11.

[LK14] Lyons, M. N., G. J. Keighery, L. A. Gibson & T. Handasyde. 2014. Flora and vegetation communities of selected islands off the Kimberley coast of Western Australia. Records of the Western Australian Museum Supplement 81: 205–244.

[P88] Polunin, I. 1988. Plants and Flowers of Malaysia. Times Editions: Singapore.

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.

[T-W89] Tenison-Woods, J. E. 1889. On the vegetation of Malaysia. Proceedings of the Linnean Society of New South Wales, series 2, 4 (1): 9–106, pls 1–9.

[T00] Thorne, R. F. 2000. The classification and geography of the flowering plants: dicotyledons of the class Angiospermae (subclasses Magnoliidae, Ranunculidae, Caryophyllidae, Dilleniidae, Rosidae, Asteridae, and Lamiidae). The Botanical Review 66: 441–647.

[WM09] Wang, H., M. J. Moore, P. S. Soltis, C. D. Bell, S. F. Brockington, R. Alexandre, C. C. Davis, M. Latvis, S. R. Manchester & D. E. Soltis. 2009. Rosid radiation and the rapid rise of angiosperm-dominated forests. Proceedings of the National Academy of Sciences of the USA 106 (10): 3853–3858.

[XR12] Xi, Z., B. R. Ruhfel, H. Schaefer, A. M. Amorim, M. Sugumaran, 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 Sciences of the USA 109 (43): 17519–17524.

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