Tuatara: Volume 6, Issue 3, December 1957
Plant Pirates — Some New Zealand Parasitic Plants
Some New Zealand Parasitic Plants
Parasitic plants are those which grow on other living plants, and derive part or the whole of their food supply from their hosts by organic union with them. They should be distinguished from Epiphytes which grow upon other plants but have no organic union with them, e.g. Asplenium flaccidum, the fish-tail fern, is an epiphyte which often grows in the accumulated humus of a tree fork but does not develop any contact with the living tissues of the tree. The parasitic method of nutrition is made possible by special structures called haustoria, by means of which the parasite is closely attached to and penetrates the host tissue. These haustoria invade the vascular system of the host, and divert food materials and water from the host xylem and phloem into the parasite. The parasitic plants described here are all obligate, i.e. they can live only parasitically, and may or may not be deleterious to their hosts. They can be classified either (a) as Holoparasites which have little or no chlorophyll and are wholly dependent on their hosts, or (b) as Hemiparasites which possess chlorophyll and are capable of photosynthesis, but depend on their hosts for water and mineral salts.
The parasitic habit is found in widely different families of the Dicotyledons and in one family of Monocotyledons, so there can be little doubt that it has evolved separately in these various lines of descent. Altogether about 1,400 species of parasitic Angiosperms are known, occurring chiefly in the following groups: Archichlamydeae, Santalales, Loranthaceae (the Mistletoes): 30 genera, 520 species, common in tropics; the New Zealand genera are Loranthus, Elytranthe, Tupeia and Korthalsella, all with endemic species. (The little-known Phrygilanthus possibly also occurs in New Zealand, but is not described in this account.) Balanophoraceae: 15 genera, 40 species, tuberous root parasites, tropical. The New Zealand representative is the monotypic Dactylanthus taylori. Santalacea: 26 genera, 250 species, mostly tropical root parasites, no parasitic forms known from New Zealand. Aristolochiales, Rafflesiaceae: 7 genera, 22 species, tropical root parasites, none from New Zealand. Hydnoraceae: 2 genera, 7 species, South American and African root parasites, none from New Zealand. Ranales, Lauraceae: Cassytha is a parasitic herb member of this tropical and subtropical family of trees and shrubs. New Zealand has one species page 88 of the total of 15. Sympetalae, Tubiflorae, Orobanchaceae: 12 genera, 140 species, cosmopolitan root parasites. One species of Orobranche has been naturalised in New Zealand. The Scrophulariaceae includes a tribe, the Rhinantheae, with 11 genera and 350 species of root parasites. The Convolvulaceae includes 10 species of the parasitic Cuscuta. Several of these have been introduced into New Zealand, though there is possibly a distinct native species. There are also a number of monocotyledonous root parasites belonging to the family Orchidaceae, the New Zealand representatives of which belong in the genus Gastrodium, but are not dealt with here.
The New Zealand parasitic flora includes at least 10 genera, from six different families. Some of these families are exclusively parasitic, e.g. Loranthaceae; in others there are some genera with the ordinary autotrophic method of nutrition and some with parasitic methods, e.g. Lauraceae. The percentage of parasites in the total flora is very small, but as a group the parasites are extremely interesting because of their morphological and physiological adaptations.
The following key can be used to identify the main New Zealand plant parasites to the generic level. The noteworthy features of each of these genera are then discussed, together with a brief account of the species they include. For full descriptions of the species, reference should be made to Cheeseman's ‘Manual of the New Zealand Flora’.
Key to Genera of New Zealand Parasitic Plants
|1||Holoparasites, devoid of chlorophyll or nearly so, brownish root parasites or yellow leafless twining stems.||2|
|Hemiparasites, possessing some chlorophyll, twiggy shrubs with green leaves or jointed succulent stems; or green-yellow twining stems.||4|
|2||Parasitic on shoots, plant body a yellow, leafless twining stem.||Cuscuta (Fig. B)|
|Parasitic on roots, plant body not twining.||— 3|
|3||Plant body erect, a purplish-brown succulent aerial stem with sessile flowers; attachment to host subterranean by haustorium.||Orobanche (Fig. E)|
|Plant body a spherical brown warty subterranean rhizome, giving off brown scale-covered inflorescences which project above ground, attachment region causing modification to host root.||Dactylanthus (Fig. C)|
|4||Plant body wiry, filiform, twining, pale green-yellow.||Cassytha (Fig. A)|
|Plant body bearing green leaves or having succulent stems.||— 5|
|5||Attachment to host at one point only, main root forming a single haustorium.||— 6|
|Attachment to host at many points, branching lateral root forming many sucker contacts.||— 7|
|6||Plant leafless, but having green jointed succulent stems, either flattened or rounded.||Korthalsella (Figs. G, H, I and J)|
|Plant with thin green leaves, a twiggy shrub, flowers usually unisexual.||Tupeia (Figs. K, L, and M)page 89|
|7||Leaves large, 1-1 ½-3 in. long, flowers minute, greenish, ovary 1-celled.||Loranthus (Fig. N)|
|Leaves usually smaller, 1 ½-3 in. long, flowers brightly coloured, ovary 2 or more celled.||Elytranthe (Fig. F)|
Cuscuta (Dodder) (Fig. B)
The Dodders are parasitic flowering plants of the family Convolvulaceae. The seed germinates in the ground and sends up a reddish or purplish, filamentous, leafless stem, the free end of which, in the course of its circling growth movement, may contact a suitable host. The thin wire-like stem then entwines the host, developing minute haustoria or sucker pads on the side of the stem in contact with the host plant. The haustoria arise from the internal tissues of the parasite and penetrate the vascular tissues of the host, absorbing nutriment directly from the phloem and xylem and diverting it into the tissues of the parasite. At first the peg-like processes consists of undifferentiated cells but later those adjacent to the xylem become tracheids and those in contact with the phloem develop phloem-like elements. Once the parasite is thus attached, its seedling-root dies. The Dodder is then entirely dependent on its host for its water and mineral salt supply, and, since it has little or no chlorophyll and cannot synthesise its own food, manufactured organic substances must be derived, pirate fashion.
There are several species of Dodder which have been introduced into New Zealand, and Cheeseman records one native species, Cuscuta densiflora, which is closely relately to, if not identical with, those introduced. The introduction of these parasitic plants has serious economic implications because they attack and may eventually kill such leguminous crop plants as lucerne and clover. The Dodders deplete their food supply and impoverish the crop plants sufficiently to cause death. The thread-like stems are produced so rapidly that the host plant often appears to be smothered in a mass of intertwining threads on which fascicles of flowers may develop later in great profusion. Sometimes the parasite develops branches which attach by suckers to adjacent host plants. If left unchecked the pest can thus spread through and destroy an entire crop. The Glover Dodder, C. epithymum var. trifolii, is the most destructive species and has been designated ‘Devil's guts’.
Dactylanthus taylori (Fig. C)
This is one of the most curious of our native parasitic plants being a root parasite of certain trees and shrubs in scattered localities in the North Island forests. It belongs to the Balanophoraceae, a family of tropical root parasites, and is known only from New Zealand. The plant consists of a brown, rounded, tuberous rhizome 1-12 cm. in diameter, covered with hard, wart-like tubercles, and it is affixed to the modified termination of the host root. The plant is mostly subterranean, the brown, scaly inflorescences, 2-6 in. high, being the only portions projecting above the leaf mould on the forest floor. The flowers are unisexual and closely packed together in heads surrounded by overlapping brown scales. They have a sweet, heavy perfume which in many cases has led to their detection.page 90
In the autumn the terminal scales of the inflorescence bud open to expose the capitulum of spadices. It is presumed that pollination is brought about by insects which are attracted by the flowers' scent.
In New Zealand homes the so-called ‘Wooden Roses’ of Dactylanthus have become collectors' pieces. It was the common practice of bushmen when they discovered the rhizomes of the plant to cut off the parasitised host root, break off any flowering stems, and then boil the host root with its terminal parasite in water until the whole of the parasite together with the bark of the root could be removed. The modified terminal portion of the host root is then seen to be hollowed and fluted and to resemble a finely chiselled ‘wooden rose’. (Fig. D.)
Dactylanthus is a complete parasite, lacking chlorophyll and deriving its food supply entirely from the host plant root system. The specialised layer of cells at the junction of parasite and host acts in an haustorial fashion. These cells are contiguous to the phloem and xylem of the host and derive food material for the parasite. Dactylanthus has no true haustorial organs; it is also interesting in being relatively long-lived and having stout and bulky tissues. For a detailed account of the anatomy and reproduction, see Moore (1940).
Cassytha paniculata (Devil's Twine) (Fig. A)
This is a leafless twining parasite similar in habit to Cuscuta and, like it, deriving most of its food from the host through penetrating cushion-like haustoria in contact with the xylem of the host. It grows on living shrubs and trees, and has pale, yellow-green, wiry stems which are several feet long and much interwoven. Very small scales replace the leaves. It is a common parasite on Manuka hosts in the gumlands of the far north of New Zealand, where it smothers the trees with its yellowish wiry stems. Unlike Dodder, Cassytha may have a small amount of chlorophyll, and is therefore capable of synthesising a little of its own food.
Cassytha occupies an isolated position in the Lauraceae, a family of otherwise woody plants, including Tawa and Tairaire. The only New Zealand species, C. paniculata, also occurs in Australia.
Orobanche minor (the Broom Rape) (Fig. E)
Fig. A, Cassytha paniculata on Manuka. Fig. B, Cuscuta. Fig. C, root infected with Dactylanthus taylori, flowering specimen (after Cheeseman). Fig. D, ‘Wooden Roses’; host root after removal of parasitic Dactylanthus. Fig. E, Orobanche minor on root of clover. (All figures half natural size.)
Korthalsella (Pygmy Mistletoes) (Figs. G, H, I and J)
New Zealand has three species of Pygmy Mistletoes which were first put into the Mistletoe genus Viscum by Hooker. Now, however, the three closely allied species are placed in the separate genus Korthalsella. They are small, tufted, succulent plants, 2-5 in. high, leafless and with conspicuously jointed greenish stems, and belong to the family Loranthaceae.
The jointed stems are terete in K. salicornioides (Fig. H) and closely resemble the round succulent stems of the glasswort (Salicornia) of tidal salt-marshes, hence the specific name. This species is found as a parasite on Manuka. In K. Lindsayi (Figs. I and J) and K. clavata (Fig. G) the internodes are flattened. In the former the internodes are broadly obovate and in the latter they are more attenuated.
The greenish flowers are minute, borne with one male and four female flowers grouped together, two groups forming a whorl at a node. The seeds are small and sticky. Germination of the seeds may take place on a suitable host or sometimes on the parent plant, resulting in a form of cannibalism. Attachment to the host is achieved by the seedling haustorium penetrating the host stem and spreading out in a club-shaped head in contact with the vascular supply of the host stele. Carbohydrates, water and mineral salts are derived from the host by the parasite which, however, is able to supply some of its own manufactured food since it possesses some chlorophyll.
Fig. F, Elytranthe tetrapetala. Fig. G, Korthalsella clavata (after Kirk). Fig. H, Korthalsella salicornioides. Fig. I. Korthalsella lindsayi. Fig. J, Korthalsella lindsayi with fruit. Fig. K, female flowers of Tupeia antarctica. Fig. L, male flowers of Tupeia antarctica. Fig. M, fruits of Tupeia antarctica. Fig. N, Loranthus micranthus showing haustoria and fruits. (All figures half natural size.)
Tupeia antarctica (Figs. K, L and M)
This is an endemic parasitic shrub with thin, narrow, pale-green leaves, oval to oblong lanceolate in shape, which are usually arranged in opposite pairs on pale grey stems. It is a member of the family Loranthaceae. Its yellowish green flowers are dioecious, or occasionally hermaphrodite, and are arranged in small axillary and terminal bundles. The seeds are ovoid and sticky, and are white or pinkish. The attachment to the host is by an haustorium at a single point. This parasite is found on a variety of hosts, although the most usual is the endemic monotypic Carpodetus serratus (Putaputaweta). T. antarctica has been recorded as a double parasite on both Elytranthe and Loranthus, and may hybridise with Loranthus.
The life history of the plant is described by Smart, 1952.
Loranthus micranthus (Fig. N)
The New Zealand representative of this large tropical genus of parasitic shrubs (of the family Loranthaceae) is characterised by having small greenish flowers whereas other Loranthus species often have highly coloured flowers. It parasitizes Coprosma, Melicope, Leptospermum, and also introduced Acacia, Rhododendron, Poplar, Plum, Apple, Hawthorn. It contains chlorophyll in its tissues and is able to manufacture its own food to a limited degree, but is dependent on the host for water and mineral salts. Its yellow viscid berries are dispersed by birds. The seeds are left on branches where they germinate, each giving rise to a long root-like organ which follows the course of the host branch, attaching itself into the host tissues at intervals by suckers. The young foliage is bronze coloured at first, but later becomes green.
Loranthus micranthus, the common New Zealand Loranthus, is found throughout New Zealand but tends to be localised.
The seedling development and haustorial development is described in Menzies (1954).
Elytranthe (New Zealand Mistletoes)
Cheeseman describes four endemic species of parasitic green shrubs belonging to this genus, which is a member of the Loranthaceae. The plants parasitize Nothofagus, forming large bushes in the tops of the trees. Elytranthe colensoi, which bears abundant racemes of bright-scarlet flowers 1 ½-2 in. long, can make a spectacular display in the flowering season amidst the dull foliage of Nothofagus. It can grow on other hosts. E. tetrapetala (Fig. F), the Pirirangi, also bears red flowers, but they are produced singly or in small groups. It grows on Nothofagus and also Quintinia in the North Island. A yellow-orange flowered species, E. flavida, grows on native beech from East Cape southwards. A less common species with duller red flowers, E. adamsii, grows on Great Barrier Island and the Coromandel area. Its hosts are Coprosma, Suttonia and Melicope.
Cheeseman, T. F., 1925.— Manual of N.Z. Flora.
Kirk, T., 1887.— On the Naturalized Dodders and Broomrapes of New Zealand. Trans. N.Z. Inst. XX: 182-186.
Kirk, T., 1891.— On a New Mistletoe. Trans. N.Z. Inst. XXIV: 429-431, 1 plate.
Laing and Blackwell.— Plants of New Zealand.
McLuckie, J., and McKEE, H. S., 1954.— Australian and New Zealand Botany. Chap. 34: Heterotrophic Angiosperms.
Menzies, Barbara, 1954.— Seedling development and haustorial system of Loranthus micranthus. Phytomorphology Vol. 4.
Moore, L. B., 1940.— Dactylanthus taylori - the Structure and Life History of the Root Parasite. N.Z. Jour. Sci. and Tech. XXI 4B: 206B-224B.
Smart, Cynthia, 1952.-The Life History of Tupeia. Trans. Roy. Soc. N.Z. 79: 459-466.
Stevenson, G. B., 1934.-Korthalsella, the Life History of the N.Z. species of the parasitic genus. Trans. Roy. Soc. N.Z. 64: 175-190, 8 pls.
Thomson, W. A., 1949.-A Natural Hybrid of Loranthus micranthus and Tupeia antarctica. Trans. Roy. Soc. N.Z. 77: p. 208.