Cassytha is a genus of some two dozen species of obligatelyparasiticvines in the family Lauraceae. Superficially, and in some aspects of their ecology, they closely resemble plants in the unrelated genus Cuscuta, the dodders.[1] When fruit and flowers are absent in the field, the physical resemblance is so close that few people without technical training can discern the difference. In this respect and in their ecology the two genera present a spectacular example of convergent evolution.[2] Nonetheless, Nickrent comments that "Cassytha is uneqivocally assigned to Lauraceae based on (both) morphological and molecular data."[3][4] In its divergence from habits typical of the Lauraceae, Cassytha also presents examples of mosaic evolution[5]
Several species of Cassytha are regarded as pests in various regions, though as a rule they are not as serious a problem as the true dodders. Some even yield a welcome harvest of fruit, or are valued for their perceived medicinal or aphrodisiac properties, partly because, like many members of the Lauraceae, some are fragrant when bruised.[6][7] Their stems make useful strings for construction of thatched roofs and certain styles of lei and the like.[8]
Common names and confusion
Probably the most useful common names for Cassytha species are laurel dodder or dodder laurel, because they look like dodder and are fragrant members of the laurel family, Lauraceae. The name love vine has merit because some species, in particular C. filiformis, are regarded as aphrodisiacs in the Caribbean region.[9][10] In practice, the confusion between the various species of Cassytha and Cuscuta is so unavoidable that their common names are more or less interchangeable. Practically all the common names for dodder accordingly are widely applied in error to Cassytha as well, but as a matter of convenience in Florida at least, where members of both groups of plants are present as agricultural pests, a publication of the Department of Agriculture adopts the names woe vine for Cassytha and dodder for Cuscuta.[11]
Context and distribution
Though the Lauraceae constitute a large family, with thousands of species in tens of genera, Cassytha is its only known parasitic genus,[12] and its climbing habit also is atypical of the family; most Lauraceae are woody shrubs or trees. The genus at one time was assigned its own family, Cassythaceae, but currently agreement on its inclusion into the Lauraceae is general.[2][13]
As currently defined, Cassytha has a wide distribution for a genus of so few species. Most are native to Australia (including temperate regions, where they are the only native members of the family), but a few are indigenous to Africa, southern Asia, various islands, and regions in the Americas. Some species seem to have been spread inadvertently by humans and probably by birds as well, and now occur on several continents. C. filiformis, for example, grows in Hawaii (where it is said to be indigenous),[14] the Australasian realm, northern South America, Central America, southern Florida, Japan,[15] and South Africa.[16] It also appears to have been transported to many major islands, and now is effectively pantropical.
Botanical details
The genus is cited as Cassytha L., Sp. Pl. 35 (1753), which means that Carl Linnaeus formally described it in 1753 in his monumental work, Species Plantarum. Otto Stapf updated the work in Flora Capensis in 1912.[17]
Cassytha is unusual among Lauraceae in at least two respects: they are scandentherbaceous plants, and they are obligate parasites. Their stems are thread-like or wiry, and like most twining species, they twine round the host clockwise as seen from the source of growth.[18] The vines generally turn yellowish once they have established themselves on a productive host because they then reduce or stop their production of chlorophyll.[12]Cassytha species are stem parasites, adhering to their hosts by uniseriatehaustoria that generally are small and oblong. Their leaves are without stipules, alternate, simple, and easily overlooked, being minute and scale-like.
Various species of Cassytha bear flowers in racemes, spikes, or heads. Depending on the species, the flowers are sessile or pedicellate. The individual flowers are hermaphroditic and bracteolate, each being attended by a bract and two smaller bracteoles. In general, the flowers are small, so much so that in many species they are inconspicuous.[19]
The perianth has six tepals, the three outer tepals smaller than the inner. The 12 stamens are in four whorls. The receptacle of the fertilised fruit gradually envelops the ovary, becoming the fleshy part of the ripe fruit, which often retains the dried remnants of the perianth at its tip. In effect, the resulting fruit structure is a tiny drupe. The endocarp is bony and plays an important part in the reproductive process, both in protecting the seed while the fruit is eaten and in inhibiting germination until the endocarp decays, thereby permitting long-lived soil seed banks to accumulate.[20][21]
List of accepted Cassytha species.[22] - Unresolved species marked with *
Some species of the cactus genus Rhipsalis once were assigned to Cassytha in an error arising from the resemblance in habitus. An unfortunate consequence has been that the homonymCassythaMill. (1768) is often mentioned as a synonym of the genus Rhipsalis, although this perception is incorrect, since the generic name Cassytha had already been applied to a completely different genus in a different plant family.
The morphology and ecology of Cassytha are so atypical of the family Lauraceae, they have been the subject of molecular genetic research to confirm their taxonomic relationships. Though special aspects to their phylogeny certainly are under debate, their assignment to the Lauraceae generally is regarded as undoubtedly correct.[2]
Reproduction and ecology
Cassytha fruits are ecologically valuable to some fruit-eating birds. The birds either regurgitate the seeds or pass them through their gut. Mammals, for example Australian macropods, also transport the seeds in their gut.[14] The bony endocarp that protects the seed in its passage through animal gut also prevents the seed from immediate germination even if conditions are favourable. Instead, the seeds survive on or in the ground till decay weakens the endocarp sufficiently to permit moisture to enter and germination to begin. This process is not deterministic, so some of the seeds might remain inactive in the soil seed bank for many years before they germinate at unpredictable intervals. Accordingly, once soil is infested with large numbers of seeds, eradication of the population generally requires considerable time. On germination, the seedlings behave as aggressive parasites; they twist about till they find a host, and those that fail to locate hosts soon die, typically in months.[14]
Seedlings and actively growing shoots are green at first. Once their haustoria are fully established on a suitable host, the plants lose most of their chlorophyll and generally become yellowish or orange, and the Cassytha plant abandons its connections with its root, which soon dies.
Cassytha species are perennials; although they attack practically whatever host plants they encounter, including suitable annuals. They seem to show some preference for woody perennial hosts. Consequently, they often find themselves on seasonally dormant host species. When that happens and the supplies from the host largely dry up, the stems of most species of Cassytha turn green until the host once again becomes productive. This suggests that such species are at least marginally photosynthetic, but do not invest resources unnecessarily when photosynthesis is not required.[12]Cassytha species do produce some of their own nutrients while green, so their chlorophyll production is both actual and functional.[11]
Technically, Cassytha could be regarded as hemiparasitic rather than holoparasitic, but their own autotrophic contributions are plainly limited to what it takes to tide over temporary shortages. When all the hosts of a plant die, so does the parasite, so whatever the details of their biology, Cassytha species certainly are unconditionally obligately parasitic. No doubt their lack of a persistent root system dooms any Cassytha plants whose hosts supply insufficient water and mineral nutrients.
The effects of Cassytha on host plants varies. They are not very selective and they parasitise hosts from many plant families, often overwhelming a host so drastically as to kill it. Even when host plants survive, a heavy infestation commonly causes drastic reduction in vigour and reproductive capacity. Accordingly, some species of Cassytha have been examined as potential weed control agents, and others are regarded as agricultural pests in their own right.[23]
Although Cassytha species are vigorous parasites, they are less aggressive, and correspondingly less serious agricultural pests, than Cuscuta species; Cuscuta species generally grow several times faster, produce more seed, and germinate more rapidly, though their seeds are less persistent in the soil.[11]
Among their various ecological effects, Cassytha species act as vectors, though not exclusive vectors, for various plant diseases. They may pass various fungi, Agrobacterium species, viruses, and other pathogens to host plants, or from one host plant to another.[11]
Classification
Due to its herbaceous and parasitic habit, distinguishing the genus from all other Lauraceae,[24]Cassytha has historically presented a problem for classification. Most early systems of classification based on morphological characters divided the Lauraceae into two subfamilies: Cassythoideae and Lauroideae;[25][26][27][28] however, more recent molecular data disputes this division.
Based on the trnK intron, a common phylogenetic marker for classifying angiosperms,[29] Rohwer and Rudolph (2005)[30] created a phylogeny of the Lauraceae. They nested Cassytha within the family, more closely related to Caryodaphnopsis, and placed both as sister to the tribe Cryptocaryeae, one of the three tribes of the Lauroideae.[28] More recently, embryological evidence, specifically the development of the anther tapetum, substantiated the close relationship of the two genera, but nested them within the tribe Cryptocareae.[31] Ultimately, the exact phylogeny of Cassytha is still in dispute; however, its non-basal placement within the Lauraceae is undoubtedly accurate.
Uses
Cassytha is best known for its parasitic habit, and the various species are neither prominent as crop plants, nor as beneficial plants. Species within the genus do have minor uses in rural communities globally. For example, the fruit of various species are eaten, both by birds and by humans, and C. melantha in particular has been documented as a wild-harvested Australian indigenous food. The flesh of fruit is very sticky and chewy, and has a taste reminiscent of feijoa.[32] In the Caribbean region, C. filiformis is one of the plant species known as love vine, because it has a reputation as an aphrodisiac.[9]
Plants in the genus contain low concentrations of several alkaloids that have not yet been shown to be of great value, but do have biochemical properties worthy of investigation.[33][34] Together with the fragrant essential oils in some species,[6] the alkaloids might be relevant to the wide application of Cassytha species in folk medicine and in traditional teas.[35]
Though they are not of commercial interest as fibre crops, suitably prepared vines from some Cassytha species are of value in rural communities as a source of cord. They may be used for binding bundles of materials such as thatch, or for stringing decorative festoons.[8]
Related Research Articles
Cuscuta, commonly known as dodder or amarbel, is a genus of over 201 species of yellow, orange, or red parasitic plants. Formerly treated as the only genus in the family Cuscutaceae, it now is accepted as belonging in the morning glory family, Convolvulaceae, on the basis of the work of the Angiosperm Phylogeny Group. The genus is found throughout the temperate and tropical regions of the world, with the greatest species diversity in subtropical and tropical regions; the genus becomes rare in cool temperate climates, with only four species native to northern Europe.
Convolvulaceae, commonly called the bindweeds or morning glories, is a family of about 60 genera and more than 1,650 species. These species are primarily herbaceous vines, but also include trees, shrubs and herbs. The tubers of several species are edible, the best known of which is the sweet potato.
Lauraceae, or the laurels, is a plant family that includes the true laurel and its closest relatives. This family comprises about 2850 known species in about 45 genera worldwide. They are dicotyledons, and occur mainly in warm temperate and tropical regions, especially Southeast Asia and South America. Many are aromatic evergreen trees or shrubs, but some, such as Sassafras, are deciduous, or include both deciduous and evergreen trees and shrubs, especially in tropical and temperate climates. The genus Cassytha is unique in the Lauraceae in that its members are parasitic vines. Most laurels are highly poisonous.
Menispermaceae is a family of flowering plants. The alkaloid tubocurarine, a neuromuscular blocker and the active ingredient in the 'tube curare' form of the dart poison curare, is derived from the South American liana Chondrodendron tomentosum. Several other South American genera belonging to the family have been used to prepare the 'pot' and 'calabash' forms of curare. The family contains 78 genera with some 440 species, which are distributed throughout low-lying tropical areas with some species present in temperate and arid regions.
A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the haustorium, which penetrates the host plant, connecting them to the host vasculature – either the xylem, phloem, or both. For example, plants like Striga or Rhinanthus connect only to the xylem, via xylem bridges (xylem-feeding). Alternately, plants like Cuscuta and some members of Orobanche connect to both the xylem and phloem of the host. This provides them with the ability to extract resources from the host. These resources can include water, nitrogen, carbon and/or sugars. Parasitic plants are classified depending on the location where the parasitic plant latches onto the host, the amount of nutrients it requires, and their photosynthetic capability. Some parasitic plants can locate their host plants by detecting volatile chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plants in approximately 20 families of flowering plants are known.
Cuscuta californica is a species of dodder known by the common names chaparral dodder and California dodder. This is an annual parasitic plant that may resemble fine strands of spaghetti or twine strewn across other species in its habitat. A mature plant of this species may fulfill all of its food and water needs from the host plant, but they rarely kill their host. It is native to western United States and Baja California in Mexico.
Cuscuta epithymum is a parasitic plant assigned to the family Cuscutaceae or Convolvulaceae, depending on the taxonomy. It is red-pigmented, not being photosynthetically active. It has a filiform habit, like a group of yarns. Its leaves are very small, like flakes. Its flowers, disposed in little glomerules, have a white corolla, with the androecium welded to the corolla.
Cuscuta approximata is a species of dodder known by the common name alfalfa dodder. It is native to Eurasia and Africa, but it is also found in North America, where it is an introduced species and uncommon noxious weed. It is a parasitic vine which climbs other plants and takes nutrition directly from them via a haustorium. The dodder resembles a pile of light yellow to orange-red straw wrapped tightly around its host plant. It is mostly stem; the leaves are reduced to scales on the stem's surface, since they are not needed for photosynthesis while the dodder is obtaining nutrients from its host. It bears clusters of tiny yellowish bell-shaped flowers which are only about 3 millimeters wide. The dodder reproduces by seed, with each plant capable of producing over 10,000 seeds at once. This plant is a weed of alfalfa, clover, and tomatoes, as well as other crop plants and native flora. This species is sometimes treated as a subspecies of Cuscuta epithymum.
Cuscuta campestris, with the common names field dodder, golden dodder, large-seeded alfalfa dodder, yellow dodder and prairie dodder, is a parasitic plant which belongs to the family Convolvulaceae. It was formerly classified in the family Cuscutaceae.
Cassytha melantha is a parasitic vine. Common names include coarse dodder-laurel and large dodder-laurel. The fruits are about 10–15 millimetres (0.39–0.59 in) in diameter and are green, drying to black. These are edible and are harvested in the wild.
Cuscuta japonica, commonly known as Japanese dodder, is a parasitic vine. It has been listed by the State of California as a noxious weed. It has a range of effects on its host and has repeatedly been introduced to the United States of America. C. japonica looks very similar to other vines, making it difficult to distinguish.
Cassytha pubescens is a native Australian hemiparasitic vine species, in the Laurel family. Common names for the species include devils twine, dodder-laurel, spilled devil's twine, snotty gobble or downy dodder-laurel. It is a widespread and common species in south eastern Australia. The species was first formally described in 1810 by the Scottish botanist Robert Brown in Prodromus Flora Novae Hollandiae et Insulae Van Diemen. Leaves are reduced to scales and photosynthesis is achieved through chlorophyll contained in the plants stems. Stems are between 0.5mm and 1.5mm in diameter and the haustoria are between 2 and 3 mm long.
Cassytha glabella, commonly known as the slender devil's twine, is a common twining plant of the Laurel family, found in many of the moister parts of Australia. A hemi-parasitic climber. The specific epithet glabella is from Latin, referring to the lack of hairs. The fruit are sweet and mucousy to taste. The Devil's Twine and Cassytha melantha are similar, but with thicker hairier stems.
Cuscuta pacifica is a species of dodder. Its common name is goldenthread.
Cassytha filiformis or love-vine is an orangish, wiry, parasitic vine in the family Lauraceae. It is found in coastal forests of warm tropical regions worldwide including the Americas, Indomalaya, Australasia, Polynesia and tropical Africa.
Cuscuta sandwichiana is a parasitic vine and the only member of the genus Cuscuta that is endemic to the Hawaiian Islands. It parasitizes a wide variety of indigenous, endemic and introduced plants on all of the main Hawaiian islands. It grows in coastal areas with sandy soils from sea level elevation to 975 feet. The indigenous kaunaʻoa pehuCassytha filiformis is a similar-looking species with the same parasitic nature. It is an unrelated plant in the Laurel family Lauraceae which can be distinguished by it larger, coarser yellowish-green stems.
Cuscuta reflexa, the giant dodder or ulan ulan, is one of 100-170 species in the genus Cuscuta, belonging to the Morning Glory Family (Convolvulaceae), and is common in the Indian subcontinent and the Greater Himalayas and as far south as Malaysia and Indonesia. This parasitic plant species is a leafless twined sprawling thin vine that grows over a host plant, including large trees with garlands hanging down from the canopy as much as 10 metres (33 ft). Flowers are small, bell shaped and white in colour with yellow filaments. Fruits and seeds are produced from the flower.
Cuscuta compacta, the compact dodder, is a parasitic plant that specializes on woody plants. This species is distributed across the Eastern and Midwestern USA, Eastern Canada, and Mexico.
Cuscuta australis, commonly known as Australian dodder, is a herb in the family Convolvulaceae.
Cuscuta gronovii is a yellow vine that grows as a parasite off other plants. It is a dicot.
References
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↑ Weber, J. Z.:A taxonomic revision of Cassytha (Lauraceae) in Australia. Journal of Adelaide Botanic Gardens 3:187-262, 1981
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