GENERAL ARTICLES Parasitic flowering plants on postal stamps: vehicles for learning Daniel L. Nickrent* and Ajit Vartak It is proposed that philately and the study of parasitic plants can be conflated for educational purposes. Of the 12 lineages of parasitic flowering plants, eight are currently represented on postal stamps. The most frequent genus seen on stamps is Rafflesia, closely followed by Viscum. These stamps convey messages about the history and importance of parasitic plants such as witchweed (Striga), sandalwood and mistletoes. Some of the parasitic plants are beautiful wildflowers such as Castilleja, Euphrasia and Pedicularis, whereas many mistletoes in Loranthaceae have flowers that rival orchids. Countries with rich parasite floras that currently do not have stamps featuring these plants should consider them as worthy subjects. Keywords: Botanical diversity, educational purpose, parasitic angiosperm, philately, plant blindness. ACCORDING to Raven1, ‘we need to find ways to pay more attention to plants’. To combat ‘plant blindness’ and increase the connections that humans feel for plants, activities such as gardening, participation in outdoor activities and anthropomorphization have been recommended2. One way to promote appreciation of plants that has not been widely discussed is the seemingly unlikely conflation of botany with another popular human endeavour: the collection and study of postage stamps (philately). It is proposed here that collecting stamps is an activity that can provide a gateway for both young and old to explore botanical diversity. Images of stamps featuring parasitic flowering plants (angiosperms) were first posted on the Parasitic Plant Connection website decades ago3. Since then, it has become increasingly evident that philately can be used as an educational tool. Philatelic organizations are international in nature with thousands of members who often specialize in particular themes. For the biological world, a rich inventory of stamps exists depicting animals4,5 and fungi6 as well many botanical themes. Indeed the journal Biophilately represents an international cooperative society dedicated to the study of biological postal stamps. Although the study of parasitic plants is a specialized botanical sub-discipline, research in this area spans a myriad of fields, including agriculture, forestry, ecology, physiology, anatomy, phytopathology, biotechnology, molecular biology, biochemistry, taxonomy and phylogenetics. Daniel Nickrent is in the Plant Biology Section, School of Integrative Plant Science, College of Agriculture and Life Science, Cornell University, Ithaca, NY 14853, USA. Ajit Vartak is in the Maharashtra Vruksh Samvardhini, Pune 411 001, India. *For correspondence. (e-mail: dn277@cornell.edu) 1538 Parasitic plants are referred to as heterotrophic, i.e. ‘different feeding’ because they obtain at least some of their water and nutrients from another vascular plant. The parasite attaches to the host plant through a structure called the haustorium that connects to the xylem and, in some cases, the phloem of the host. Parasites that obtain water and minerals from the host, but are green and retain photosynthetic activity are called hemiparasites. They attach to the xylem but not the phloem. In contrast, parasitic angiosperms that attach to both xylem and phloem and have lost the ability to photosynthesize are called holoparasites. They also have highly altered morphologies, often with leaves reduced to scales, or in some cases, where stems and leaves are missing altogether. Mycoheterotrophs are another type of heterotrophic angiosperms that rely on mycorrhizal fungi to obtain nutrients, but do not attach directly to tree roots. The question ‘how many angiosperm parasite lineages exist?’ was recently reviewed7 and it appears that this nutritional mode has evolved independently 12 times. Holoparasites exist in 10 of the 12 lineages, and seven of these are composed entirely of holoparasites. In 11 of the 12 orders, the parasitic members are present in one family, whereas in the sandalwood order (Santalales) about 20 families can be recognized. Finding the closest photosynthetic, non-parasitic relatives of the holoparasites has proven to be challenging, but molecular analyses have provided definitive answers. The purpose of this study is to highlight the parasitic angiosperms that occur on stamps and discuss their importance to human endeavours as well as their role in natural ecosystems. We also want to promote future production of additional parasitic plant stamps that are scientifically relevant, of economic or humanistic importance or simply beautiful. CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 GENERAL ARTICLES Parasitic plants: the good, bad and ugly The term ‘parasite’ often provokes a negative first impression, leading to the idea that these plants kill their hosts. That this is not true is supported by the fact that only about 25 of the 280 genera of parasitic angiosperms are pathogens that negatively impact plants cultivated by humans8. Two genera are particularly damaging to crops: Striga (witchweed) and Orobanche (broomrape). Witchweed derives its name from the observation that it ‘bewitches’ its host. Even before the parasite emerges from the soil, the host shows physical symptoms of parasitism, such as stunted growth and loss of reproduction. S. asiatica and S. hermonthica cause major crop damage in Subsaharan Africa, and have thus been the focus of intense research to mitigate these losses9. The broomrapes are problematic in the Middle East, Europe and Asia10. Some stem parasites (mistletoes) in Santalales can cause damage to fruit trees. The dwarf mistletoes (Arceuthobium spp.) are major pathogens of coniferous trees grown in agroforestry settings11. Human alteration of the forest ecosystem, such as planting monocultures, encourages the spread and damage caused by these native components of the flora. The above examples highlight some of the ‘bad’ players, but what about the ‘good’ parasitic plants? That expression may sound like a non-sequitur, but indeed the vast majority of parasitic angiosperms are benign and even integral components of their ecosystems12. Work in Australia showed that removal of mistletoes negatively affected the nesting behaviour of several bird species; thus they are keystone resources in these woodland habitats13. Similarly, root parasites such as Rhinanthus may lower host plant biomass, thus allowing increased diversity. At high density they enhance the number of invertebrates and affect herbivores, predators and detritivores14. Parasitic plants can even alter the physical environment around them, such as soil water and nutrients, atmospheric carbon dioxide and temperature15. Many parasitic plant species are rare and, being dependent upon their hosts, are particularly susceptible to population decline; thus they merit conservation and restoration efforts16. These organisms represent a large proportion of biodiversity and therefore their importance should be recognized17. Finally, what about ‘ugly’ parasites? Like troll dolls, whether any parasite species is ugly or beautiful is probably in the eye of the beholder. As shown below, some parasitic angiosperms are as beautiful as orchids and thus fully deserve showcasing on postal stamps. Parasitic plants on stamps Of the 12 lineages (orders) containing parasitic angiosperms, eight have been represented on postal stamps. Table 1 shows these groups and the full taxonomic names CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 of the species included. At least 95 different stamps show parasitic angiosperms representing 52 species in 29 genera. Indonesia has the highest number of parasitic plant stamps (seven) followed by Malaysia (six). These two countries achieved high numbers because Rafflesia is a popular plant to be represented on stamps. Like the rhinoceros and panda, Rafflesia is a ‘charismatic mega flora’ because its members have the largest flowers in existence. The next most popular subject is Viscum owing to its symbolism during the Christmas season (see below). Although this study focused on haustorial angiosperm parasites, we would be remiss to not mention the New Caledonian endemic Parasitaxus usta (Podocarpaceae), the only parasitic gymnosperm. Parasitaxus is a physiological chimera mixing haustorial parasitism with mycoheterotrophy18. This amazing plant was featured on a commemorative postal stamp by New Caledonia in 1989. Lamiales – Orobanchaceae One-third of all stamps bearing images of parasitic plants (10 genera, 23 species) show members of the broomrape family, Orobanchaceae (Figure 1). This is the largest family among parasitic angiosperms with 101 genera and over 2100 species8. S. asiatica is a major pathogen on maize, sorghum, rice and sugarcane19. In response to its accidental introduction into the United States in the 1950s, the US Department of Agriculture initiated a control programme. This species is shown on stamps from British Indian Ocean Territory (Figure 1 k) and Saudi Arabia. Striga elegans, a close relative of S. asiatica, was depicted on two stamps, one from Lesotho and another from Togo (Figure 1 l). Given their morphological similarity, it has been suggested that these species share a recent common ancestor. S. asiatica is agrestal, that is, it is only known from agricultural fields, whereas S. elegans is only known from native grasslands20. Thus human activities may have provided the conditions necessary for the evolution of this crop pathogen. Another major crop pest in Africa is S. hermonthica, which is shown parasitizing maize on a stamp issued in 1989 from Burkina Faso. In recent years the genus Orobanche has undergone taxonomic revision owing to DNA sequence data analysis. Orobanche contains 117 species, whereas its segregates are Phelipanche (62 species) and the New World Aphyllon (20 species). Two of these holoparasites are feature on stamps, Orobanche rapum-genistae (Figure 1 h) and Phelipanche purpurea (Figure 1 j). In contrast to the stamps with Striga, none of the ex-Orobanche crop pests such as Phelipanche aegyptiaca, P. cernua and P. ramosa is featured on stamps. A stamp showing Cistanche was issued in 1959 from French West Africa (eight former French colonies, dissolved in 1960), possibly the first parasitic flowering plant stamp. This species was also featured on a stamp from Bahrain in 1993 (Figure 1 d) as 1539 GENERAL ARTICLES Table 1. Taxon Lamiales, Orobanchaceae Alectra sessiliflora (Vahl) Kuntze Alectra vogelii Benth. Bellardia viscosa (L.) Fisch. and C.A. Mey Castilleja linariifolia Benth. Castilleja sp. Castilleja sp. Haustorial parasitic flowering plants shown on stamps Parasite type Political entity Year Comments hemi-, root hemi-, root hemi-, root South Africa Burkina Faso Baliwick of Jersey 1991 From Transkei (formerly independent) 1989 In group of four stamps with Orobanchaceae 2006 On stamp as Parentucellia viscosa Caruel (a synonym) hemi-, root hemi-, root hemi-, root USA (Wyoming) USA USA Cistanche phelypaea (L.) Cout. Cistanche phelypaea (L.) Cout. holo-, root holo-, root Bahrain Jordan Cistanche phelypaea (L.) Cout. holo-, root Oman Cistanche phelypaea (L.) Cout. Cistanche sp. holo-, root holo-, root Saudi Arabia French West Africa Cycnium tubulosum (L.f.) Engl. hemi-, root Kenya Euphrasia transmorrisonensis Hayata Harveya huttonii Harm. hemi-, root Taiwan 1982 1992 Stylized painting of paintbrush. Wildflower issue 1992 With Rufous Humming bird. In a series of five stamps with humming birds 1993 2003 On stamp as Cistanche tubulosa (Schenk) Wight (a synonym) 2013 On stamp as Cistanche tubulosa (Schenk) Wight (a synonym) 1990 1959 A former French administrative unit now comprising several African countries. Species may also have been on a stamp in 1943 1983 On stamp as Rhamphicarpa montana N.E.Br. (a synonym) 2011 holo-, root South Africa Harveya speciosa Bernh. Orobanche rapum genistae Thuill. holo-, root holo-, root Pedicularis apodochila Maxim. Pedicularis hirsuta L. Pedicularis hoermanniana K. Malý Pedicularis oederi Vahl hemi-, root hemi-, root hemi-, root South Africa Balliwick of Guernsey (Alderney) Japan Greenland Yugoslavia hemi-, root Norway Pedicularis sceptrum-carolinum L. Pedicularis siamensis Tsoong hemi-, root hemi-, root Germany Thailand Pedicularis sudetica Willd. Phelipanche purpurea (Jacq.) Soják Striga asiatica (L.) Kuntze hemi-, root holo-, root Striga asiatica (L.) Kuntze Striga aspera (Willd.) Benth. Striga elegans Benth. Striga elegans Benth. Striga gesnerioides (Willd.) Vatke Striga hermonthica (Delile) Benth. Striga hermonthica (Delile) Benth. hemi-, root hemi-, root hemi-, root hemi-, root hemi-, root hemi-, root hemi-, root Poland Balliwick of Guernsey (Alderney) British Indian Ocean Territory Saudi Arabia Burkina Faso Lesotho Togo Burkina Faso Burkina Faso Burkina Faso hemi-, root Santalales, Balanophoraceae Thonningia sanguinea Vahl holo-, root Thonningia sanguinea Vahl Santalales, Loranthaceae Agelanthus brunneus Tiegh. Agelanthus cf. musozensis (Rendle) Polhill and Wiens 1991 On stamp as Harveya pulchra Hilliard and B. L. Burtt (a synonym). From Transkei (formerly independent) 1991 From Transkei (formerly independent) 1994 Parasitic on Cytisus scoparius ssp. maritimus (Rouy) Heywood with Bombus lucorum L. 1986 An ambiguous name 1992 Highest latitude of any parasitic plant 1997 Former Yugoslavia now six republics. Known also from Italy, quite rare 2000 Endangered. See Kobiv & Nesteruk. (2001, Polish Bot. J., 46, 241–250) 1981 Endangered bog, marsh meadows and water plants 2001 Name ambiguous. Known only from Chiang Mai Province 2006 Endangered 2016 Shown as Yarrow Broomrape; formerly Orobanche purpurea Jacq. 2001 1990 1989 1985 2000 1989 In group of four Orobanchaceae stamps 1989 In group of four Orobanchaceae stamps 1963 On stamp as S. senegalensis Benth. (a synonym); as Republique de Haute-Volta 1960 On stamp as Belgisch (Belge) Congo holo-, root Democratic Republic of Congo Ivory Coast hemi-, stem Rwanda hemi-, stem Rwanda 1981 On stamp as Tapinanthus brunneus (Engl.) Danser (a synonym) 1975 On stamp as Tapinanthus prunifolius (E. Mey ex Harv.) Tiegh. (a synonym), now Agelanthus prunifolius (E. Mey. ex Harv.) Polhill and Wiens, but this species not reported from Rwanda 2013 A group of three stamps, with the mistletoe Tapinanthus belvisii (DC) Danser and the aquatic plant Ottelia ulvifolia (R.Br.) Rich (Contd) 1540 CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 GENERAL ARTICLES Table 1. (Contd) Taxon Parasite type Political entity Year Comments Amyema incarnatiflora (Elmer) Danser Amyema scandens (Tiegh.) Danser Decaisnina forsterianum (Schult) Barlow hemi-, stem Philippines hemi-, stem hemi-, stem New Caledonia Fiji Macrosolen melintangensis Miq. hemi-, stem Federated States of Micronesia Tapinanthus belvisii (DC) Danser hemi-, stem Ivory Coast Tapinanthus globiferus (A. Rich.) Tiegh. hemi-, stem Chad Santalales, Opiliaceae Opilia amentacea Roxb. hemi-, root Burkina Faso 1977 On stamp as Opilia celtidifolia Endl. exWalp. (a synonym); as Republic of Upper Volta Santalales, Santalaceae Santalum album L. hemi-, root Portugal Santalum album L. Santalum album L. hemi-, root hemi-, root India India hemi-, root hemi-, root hemi-, root hemi-, root Indonesia New Caledonia New Caledonia Pitcairn Islands 2015 East Timor (= Timor-Leste) gained independence from Portugal in 1975, later invaded by Indonesia, independent again in 1999 2006 Stylized tree representing sandalwood, with elephant 2019 Two stamps, one showing Sandalwood and jasmine perfumes 1995 Stamp shows trunk of sandalwood tree 1991 2004 1992 On stamp as S. hendersonense F. Br. P. Scott Commemorative expedition hemi-, root Fiji 1990 Trees series hemi-, stem USA hemi-, stem USA (Oklahoma) hemi-, stem hemi-, stem hemi-, stem hemi-, stem hemi-, stem hemi-, stem hemi-, stem Hungary Hungary Yugoslavia Switzerland Baliwick of Guernsey England Canada hemi-, stem hemi-, stem hemi-, stem hemi-, stem Switzerland Sweden Baliwick of Jersey Baliwick of Guernsey hemi-, stem hemi-, stem hemi-, stem Baliwick of Jersey England Netherlands 1964 Christmas stamp; group of four, also showing poinsettia, holly and spruce 1982 Oklahoma, USA with a Scissor-tailed Flycatcher, the state bird of OK 1963 New Year stamp showing mistletoe and calendar 1963 New Year stamp 1973 Former Yugoslavia now six republics 1974 Pro Juventute: Poisonous plants of the forest 1978 Christmas flowers series 1980 13.5 Pence Christmas stamp 1987 Viscum album L. or Phoradendron leucarpum (Raf.) Reveal and M. C. Johnst.? The berries are shown as both white and red 1997 Pro Juventute: Mistletoe, animals on and in the water 1998 Christmas 1998. Julpost 1999 Christmas 1999 2008 Photograph of plant in fruit. Christmas 2008 – Festive Foliage 2013 Frosts and nature 2014 Post and go winter greenery 2014 Kissing under the mistletoe hemi-, root hemi-, root hemi-, root hemi-, root Botswana Malawi Malawi Mozambique 1979 1993 2016 1998 holo-, root Indonesia 1992 The ‘rare flower’ (Puspa Langka in Indonesian) Santalum album L. Santalum album L. Santalum album L. Santalum insulare var. hendersonense (F.Br.) Fosberg and Sachet Santalum yasi Seem. Santalales, Viscaceae Phoradendron leucarpum (Raf.) Reveal and M. C. Johnst. Phoradendron leucarpum (Raf.) Reveal and M. C. Johnst. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Viscum album L. Santalales, Ximeniaceae Ximenia caffra Sond. Ximenia caffra Sond. Ximenia caffra Sond. Ximenia caffra Sond. Malpighiales, Rafflesiaceae Rafflesia arnoldii R.Br. 2007 In group with Rafflesia lagascae Blanco (as R. manillana Teschem.) 1978 Showing flowers emerging from epicortical runners. 1983 On stamp as Amylotheca insularum Danser (a synonym). Widely distributed in the south Pacific (see Barlow and Schodde 1993). 2015 On stamp as ‘Loranthaceae’. Photo is of Macrosolen melintangensis Miq. taken by Arthit Buarapa in the Phu Luang Wildlife Sanctuary in Thailand. No Macrosolen species occur in Micronesia 2013 Accurately representing this species that occur in the Ivory Coast. Fake versions of these stamps were made. 1975 The only species of Tapinanthus from Chad (Polhill and Wiens 1998). Also looks like T. dependens (Engl.) Danser (S of Chad) Painting of plant with flowers and fruit Painting of plant in fruit Photograph of the plant in fruit A stylized painting of the plant in fruit (Contd) CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 1541 GENERAL ARTICLES Table 1. (Contd) Taxon Parasite type Political entity Year Comments Rafflesia arnoldii R.Br. holo-, root Indonesia Rafflesia arnoldii R.Br. holo-, root Indonesia Rafflesia arnoldii R.Br. holo-, root Malaysia Rafflesia arnoldii R.Br. holo-, root Malaysia Rafflesia arnoldii R.Br. holo-, root Malaysia Rafflesia azlanii Latiff and M. Wong Rafflesia kerrii Meijer holo-, root Malaysia holo-, root Thailand Rafflesia lagascae Blanco holo-, root Philippines Rafflesia micropylora Meijer holo-, root Indonesia Rafflesia micropylora Meijer holo-, root Indonesia Rafflesia pricei Meijer Rafflesia pricei Meijer holo-, root holo-, root Rafflesia sp. Rafflesia sp. Rafflesia sp. Rafflesia sp. holo-, root holo-, root holo-, root holo-, root Brunei Darussalam Indonesia, Jakarta, St Maarten Caribbean Netherlands Malaysia Malaysia Philippines holo-, root holo-, root Brunei Darussalam Thailand Rafflesia does not occur in the Caribbean Netherlands Unidentified species of Rafflesia, with elephant World Youth Stamp Exhibition 2014 Painting is vague, resembles R. arnoldii R.Br. which does not occur in the Philippines 2000 Specific epithet misspelled ‘lowi’ 2006 International Letter Writing Week stamp hemi-, stem Marshall Islands 1985 Under the medicinal plants series holo-, root South Africa 1991 From Transkei (formerly independent) holo-, root Bahrain 1993 As the ‘Maltese Fungus’, under mushroom theme hemi-, root Federated States of Micronesia hemi-, stem Balliwick of Guernsey (Alderney) Rhizanthes lowii (Becc.) Harms Sapria poilanei Gagnep. Laurales, Lauraceae Cassytha filiformis L. Piperales, Hydnoraceae Hydnora africana Thunb. Saxifragales, Cynomoriaceae Cynomorium coccineum L. Zygophyllales, Krameriaceae Krameria lappacea (Dombey) Burdet and B.B. Simpson Solanales, Convolvulaceae Cuscuta epithymum (L.) L. well as stamps from Jordan, Oman and Saudi Arabia (Table 1). Except for Alectra, the remaining hemiparasitic plants shown on stamps are non-pathogenic species. Indeed, some of these are beautiful wildflowers that are admired and cherished, at least by a few. Wyoming adopted Castilleja linariifolia as its state wildflower in 1917, but not without objection by botanist Aven Nelson. He argued that only experts could distinguish among the more than 200 species of Indian paintbrush (the common name for Castilleja). He also asked, ‘who would want to plant a parasitic plant in a garden?’ Despite these objections, Wyoming retained paintbrush as its state flower and it was featured on a stamp in 1982 along with the western meadowlark (Figure 1 c). With about 600 species, Pedi1542 1993 The ‘rare flower’ (Puspa Langka). The common name in Indonesian is Padma Raksasa meaning ‘Giant Rafflesia’ 2008 Stamp shows a photograph of the species. Indonesia-Japan Joint Issue 1979 Misidentified as R. hasseltii Suring. Possibly R. arnoldii R.Br. or R. gadutensis Meijer 1979 Misidentified as R. hasseltii Suring. Possibly R. arnoldii R.Br. or R. gadutensis Meijer 2007 Unidentified but likely Rafflesia arnoldii R.Br., with hornbill 2013 Wonders of Malaysian Forests 2006 Group of 4 stamps, with Sapriapoilanei Gagnep. and two carnivorous plants, Nepenthes and Drosera 2007 Identified as R. manillana Teschem., this Rafflesia from Mt Makiling is now known as R. lagascae Blanco 1989 Identified only as Rafflesia on the stamp, but the morphology indicates R. micropylora Meijer 2002 Issued with another stamp showing Trimeresurus hageni (Lidth de Jeude) – a snake 2000 Flower series 2012 Misidentified as Rafflesia arnoldii R.Br. 2012 2004 2014 2019 ? On stamp as K. triandra Ruiz and Pav. (a synonym) 1994 Parasitic on Ulex europaeus L. with the Dartford Warbler – Sylvia undata (Boddaert) cularis (lousewort) is the most speciose genus in Orobanchaceae followed by Euphrasia (eyebright, Figure 1 f ) with 246 species8. Pedicularis has been chosen as a subject for stamps by seven different political entities. Greenland chose P. hirsuta which notably occurs at the highest latitude on earth for any parasitic flowering plant. Except for P. apodochila of Japan (Figure 1 i), the remaining five Pedicularis species are rare and/or endangered and require conservation efforts for their preservation. Santalales – six families Sandalwood (Santalales) has 179 genera and 2428 species8, of which 11 genera and 16 species are represented CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 GENERAL ARTICLES Figure 1. Stamps of parasites in Orobanchaceae. a, Alectra vogelii; b, Bellardia viscose; c, Castilleja linariifolia; d, Cistanche phelypaea; e, Cycnium tubulosum; f, Euphrasia transmorrisonensis; g, Harveya speciosa; h, Orobanche rapum-genistae; i, Pedicularis apodochila; j, Phelipanche purpurea; k, Striga asiatica; l, Striga elegans. on stamps. Six stamps commemorate the sandalwood of commerce, Santalum album. This root parasitic tree has been utilized for centuries because of its aromatic heartwood that contains the essential oil santalol, the source of odour in incense21. Various themes are depicted on these stamps, such as the woody trunk of this tree (Indonesia), an elephant harvesting sandalwood trees (India 2006), and perfume (India 2019; Figure 2 f ). Indeed, the Indian sandalwood stamps were infused with the fragrance of the plant, possibly the first and only time this has been done. The 150th anniversary of the arrival of Captain Robert Clark Morgan and his ‘santaliers’ in New Caledonia in 1841 has been commemorated on a 1991 stamp. The ‘boom and bust’ history of exploiting sandalwood in Hawaii is well documented; however, such commercial CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 operations were also taking place in other regions as early as the 16th century. The stamp from 2015 showing a woman bearing baskets with sandalwood commemorates the 500th anniversary since Portugal colonized East Timor. Other sandalwood species besides S. album were also harvested and two of these are shown on stamps: Santalum insulare var. hendersonense from Pitcairn Islands in 1992 and Santalum yasi from Fiji in 1990 (Figure 2 g). Three African countries (Botswana, Malawi and Mozambique) have all issued stamps with images of the root parasite Ximenia caffra (Figure 2 l) commonly known as sour plum for its edible fruits. In Africa, the leaves and roots are used medicinally to treat several diseases and ailments22. In a wild fruit series of stamps issued by Upper Volta (now Burkina Faso) in 1977, Opilia amentacea 1543 GENERAL ARTICLES Figure 2. Stamps of parasites in Santalales. Loranthaceae: a, Agelanthus brunneus; b, Agelanthus cf. musozensis; c, Amyema scandens; d, Amyema incarnatiflora; e, Tapinanthus belvisii. Santalaceae: f, Santalum album; g, Santalum yasi. Opiliaceae: h, Opilia amentacea. Balanophoraceae: i, Thonningia sanguinea. Viscaceae: j, Phoradendron leucarpum; k, Viscum album. Ximeniaceae: l, Ximenia caffra. was shown (as Opilia celtidifolia, a synonym; Figure 2 h). This root parasite is a woody vine that is widely distributed from tropical Africa to Australia. One holoparasite in Santalales has been illustrated on stamps: Thonningia sanguinea (Balanophoraceae), a plant utilized in traditional medicine. This family, with 14 genera and 42 species, contains some of the most unusual holoparasites in the angiosperms. The plants are highly modified to the point that some of them resemble fungi. The Democratic Republic of Congo (Kinshasa) issued its stamp with Thonningia in 1960 (Figure 2 i) followed by the Ivory Coast in 2013 that also showed a mistletoe (Tapinanthus belvisii). The mistletoe family Loranthaceae includes members with large, showy flowers. Thus they have often been chosen as subjects for stamps. Eight species are listed in Table 1; however, in some cases, the species were misidentified or the taxonomy has changed since the stamp was issued. A stamp from Rwanda shows Agelanthus brunneus (Figure 2 a) or possibly A. krausii that also 1544 occurs in that country. Another stamp from Rwanda shows a brush fire juxtaposed with a painting of a mistletoe labelled Tapinanthus prunifolius (Figure 2 b). According to Polhill and Wiens23, that species (now in Agelanthus) does not occur in Rwanda; thus the painting may depict A. muzozensis which is native to that country. A stamp issued in 1983 from Fiji shows the mistletoe Decaisnina forsteriana (as Amylotheca insularum), which is widely distributed in the South Pacific. Although not specifically identified, the stamp from the Federated States of Micronesia shows Macrosolen melintangensis, a species that occurs in Southeast Asia but not Micronesia. Stamps with correctly identified Loranthaceae include Amyema scandens from New Caledonia (Figure 2 c), A. incarnatiflora from the Philippines (Figure 2 d), Tapinanthus belvisii from Ivory Coast (Figure 2 e) and T. globiferus from Chad. The mistletoe family Viscaceae contains seven genera, two of which are featured on stamps: Phoradendron and Viscum. Although displaying these plants during the Christmas season originated in Europe, North Americans CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 GENERAL ARTICLES Figure 3. Stamps of holoparasites in Rafflesiaceae. a, Rafflesia arnoldii; b, Rafflesia azlanii; c, Rafflesia kerrii; d, Rafflesia lagascae; e, Rafflesia micropylora; f, Rafflesia pricei; g, Rhizanthes lowii; h, Sapria poilanei. continued the custom, substituting Phoradendron for Viscum. In 1964 the US Postal Service circulated a Christmas theme group of four stamps, one of which was P. leucarpum. In 1893, this mistletoe was chosen as the state floral emblem for the Oklahoma Territory in USA. Soon after Oklahoma became a state in 1907, controversy about the mistletoe ensued and the Indian blanket (Gaillardia pulchella, Asteraceae) became the state wildflower. That the mistletoe could not be a state flower or wildflower stems from the common misunderstanding that it is not a flowering plant. In 1982, Oklahoma issued a beautiful stamp featuring the scissor-tailed flycatcher (its state bird) and P. leucarpum whose seeds could be dispersed by this bird (Figure 2 j). The European mistletoe, Viscum album is a popular parasitic plant for stamps, again owing to its Christmas symbolism. Two stamps issued by Hungary in 1963 were apparently the first to show this species, in both cases associating it with good luck. In 1974, Switzerland issued a mistletoe stamp beautifully illustrating V. album in fruit as part of its ‘Poisonous Plants of the Forest’ series, highlighting the fact that mistletoe fruits and seeds contain viscotoxins. European political entities that have issued V. album stamps include England, the Bailiwicks of Guernsey and Jersey (Figure 2 k), Sweden, Yugoslavia and the Netherlands. The Netherlands also issued a stamp in 2014 depicting the Yuletide custom of kissing under the mistletoe. Malpighiales – Rafflesiaceae The ‘queen of parasites’ is Rafflesia, some species of which have flowers over 1 m in diameter, the largest among all angiosperms4. The plant lacks stems, leaves and roots, CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 and exists within the host vine (Tetrastigma) as a filamentous endophyte. Its presence is only revealed when the flower buds emerge and later open. It is no wonder that these unusual and record-breaking plants have been the subject of stamps in Thailand, Malaysia, Brunei, Indonesia and the Philippines. Rafflesia arnoldii is one of three national flowers of Indonesia. Species misidentifications have occurred frequently with Rafflesia stamps. In some cases, a generalized image is shown with just the genus Rafflesia indicated. In other cases the Rafflesia illustration suggests a species, such as in Malaysia (2007) where R. arnoldii is shown with a hornbill bird. Similarly, the stamp issued by Indonesia in 1989 clearly shows R. micropylora from Sumatra (Figure 3 e). In 1979, Malaysia issued a series of Rafflesia stamps, each individualized for one of its 13 states. While, the name on the stamp was R. hasseltii, the image showed either R. arnoldii or R. gadutensis (two similar species). Indonesia erred in the opposite direction regarding its 2012 stamps. The grouping of four stamps each showed a quarter of a Rafflesia flower, identified as R. arnoldii when in fact it was R. pricei from Kalimantan (Figure 3 f ). Fortunately, R. arnoldii was properly illustrated and identified on other stamps from Indonesia in 1992 (Figure 3 a), 1993 and 2008. Other correct identifications include R. azlanii (Malaysia, 2013; Figure 3 b), R. kerii (Thailand 2006; Figure 3 c), R. micropylora (Indonesia, 2002) and R. pricei (Brunei Darussalam, 2000). At the time when the Philippines Rafflesia stamp was issued (2007), this species was known as R. manillana, but later taxonomic work24 named it as R. lagascae (Figure 3 d). Both of the other genera in Rafflesiaceae have also been featured on stamps. Rhizanthes lowii, whose bizarre flower is fringed with worm-like appendages, was showcased on the 2000 1545 GENERAL ARTICLES Figure 4. Parasites in various families. Lauraceae: a, Cassytha filiformis. Hydnoraceae: b, Hydnora africana. Cynomoriaceae: c, Cynomorium coccineum. Krameriaceae: d, Krameria lappacea. Convolvulaceae: e, Cuscuta epithymum. Representative members of parasite families not yet present on stamps. f, Pilostyles thurberi (Apodanthaceae); g, Cytinus ruber (Cytinaceae); h, Mitrastema yamamotoi (Mitrastemonaceae); i, Pholisma culiacana (Lennoaceae). stamp from Brunei Darussalam (Figure 3 g). Finally, an accurate and strikingly beautiful painting of Sapria poilanei was issued by Thailand in 2006 (Figure 3 h). Laurales – Lauraceae Cassytha, popularly known as love vine, is a genus of about 20 species, most of which occur in Australia. One species, C. filiformis, is pantropical, often reaching remote islands where it forms dense masses of stems that indiscriminately parasitize many different plant species. Under its Medicinal Plant Series, the Marshall Islands issued a stamp in 1985 that well illustrated C. filiformis (Figure 4 a). Tea made from this species is considered an aphrodisiac in the Bahamas. Piperales – Hydnoraceae Hydnoraceae includes Hydnora of the Old World and Prosopanche of the New World. Hydnora has been described as ‘the strangest plant in the world’25 and its vegetative and floral morphology bear this out. These holoparasites form angular pilot roots and haustorial roots 1546 that attach to host roots. No stems or leaves are formed; only a fleshy flower with a brown, rough exterior and a pink or orange smooth interior, the part of the flower visited by pollinating beetles. In its Parasitic Plant Series in 1991, the Republic of Transkei (now part of South Africa) issued a stamp (Figure 4 b) showing the flower of Hydnora africana with its Euphorbia host plants in the background. Saxifragales – Cynomoriaceae Cynomorium coccineum (Cynomoriaceae) has been used for thousands of years by ancient people for food, medicine and dyeing. Arabs call the plant ‘tarthuth’; Bedouins ate the interior portions of fresh young stems, prepared infusions of older stems to treat colic or stomach ulcers, or dried and pulverized the plant for use as a condiment with meat dishes26. Following the Crusades, the Knights Hospitaller from Jerusalem relocated to Malta where they continued the medicinal use of Cynomorium that they learned from the Muslims. The site called Fungus Rock where the ‘Maltese mushroom’ grew was thereafter vigorously guarded. Despite its appearance, Cynomorium is certainly not a mushroom as its position within the CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 GENERAL ARTICLES angiosperm phylogeny was confirmed using DNA sequencing27. Under a mushroom theme, however, Cynomorium was featured on a 1993 stamp by Bahrain (Figure 4 c). Zygophyllales – Krameriaceae At the end of the alphabet for angiosperm orders is Zygophyllales, containing two families: Zygophyllaceae and Krameriaceae. The former includes lignum vitae (Guaiacum) notable for having one of the densest woods in the world, and creosote bush (Larrea tridentata) which produces ring clones over 11,000 years old. Krameriaceae contains the genus Krameria with about 23 species commonly called rhatany. The flowers of Krameria have showy sepals and some petals are modified into glands. The fruit is globose and covered with spines. The Federated States of Micronesia issued a group of ten stamps of plants used in traditional medicine, none of which was native to Micronesia. One of these was K. lappacea (Figure 4 d) (synonym K. triandra) that constitutes the true rhatany of commerce28. This plant has been used by Andean people to stanch blood flow, clean their teeth and in the treatment of combat diarrhoea, mouth ulcers, etc. Solanales – Convolvulaceae Within the large family Convolvulaceae, which includes morning glory vines, a lineage of parasitic plants exists that is commonly called dodder. The genus Cuscuta contains over 200 species of vines which may be hemiparasitic or holoparasitic8. Although the vast majority of species do not harm the crop plants, some such as C. campestris can attack diverse crops, particularly legumes29. The Bailiwick of Guernsey is the only political entity that has shown a dodder species on a stamp (Figure 4 e). In 1994, it issued a stamp with a beautiful painting of C. epithymum parasitic on Ulex europaeus (Fabaceae) along with the Dartford Warbler (Sylvia undata). As the name implies, this dodder usually uses thyme (Thymus) as its host plant, but other hosts can also be parasitized. Parasitic plants not on stamps Four orders and families of holoparasites are not currently represented on stamps (Figure 4 f–i). Three of these families were once considered to be related to Rafflesiaceae, but molecular phylogenetic work showed that they were all in different orders30. The first is Apodanthaceae (Cucurbitales) which includes the genus Pilostyles (Figure 4 f ). These holoparasites have tiny flowers that emerge from the branches of legume host trees and shrubs. The genus is widely distributed with species in North and South America, Africa, the Middle East and Australia. The next ex-Rafflesiaceae family is Cytinaceae (Malvales) which CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021 includes the genus Cytinus (Figure 4 g) in the Old World and Bdallophytum in the New World. The third exRafflesiaceae family is Mitrastemonaceae (Ericales) which includes only the genus Mitrastema (Figure 4 h) with two disjunct species, M. yamamotoi in East Asia and M. matudae in Mexico. Who could have guessed that these three lineages were related respectively, to cucumber, okra and blueberries. The order Boraginales contains the family Lennoaceae with just two genera, Lennoa and Pholisma (Figure 4 i), distributed in the western US and Mexico. The inflorescences of these holoparasites emerge from the desert sand and display a tight cluster of tubular flowers. Pholisma sonorae, an endangered plant species in the southwestern US, is called ‘sand food’ because native Americans consumed it. Parasitic plants from all four of these lineages would make fascinating and informative stamps that highlight the unique biodiversity of their respective regions. In conclusion, approximately 46 political units have issued parasitic plant stamps, with the highest numbers coming from Indonesia (seven) and Malaysia (six). Interestingly, the African country of Burkina Faso also has six. The small territory, Baliwick of Guernsey has five stamps. Large regions that have rich parasite floras, such as Central Asia, South America and Australia, have no parasitic plant stamps. We hope that this article will encourage more representation of these incredible organisms on stamps. 1. Raven, P. H., Plants make our existence possible. Plants, People, Planet, 2021, 3, 2–6. 2. Balding, M. and Williams, K. J. H., Plant blindness and the implications for plant conservation. Conserv. Biol., 2016, 30, 1192– 1199. 3. Nickrent, D. L., The Parasitic Plant Connection; http://www. parasiticplants.siu.edu/ 4. 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L., Blarer, A., Qiu, Y.-L., Vidal-Russell, R. and Anderson, F. E., Phylogenetic inference in Rafflesiales: the influence of rate heterogeneity and horizontal gene transfer. BMC Evol. Biol., 2004, 4, 40. Received 10 July 2021; revised accepted 15 November 2021 doi: 10.18520/cs/v121/i12/1538-1548 CURRENT SCIENCE, VOL. 121, NO. 12, 25 DECEMBER 2021