Viscum crassulae

Eckl. & Zeyh.

Family : Viscaceae
Common names : Spekboom mistletoe, Spekboom-voëlent

 Viscum crassulae in fruit

Viscum crassulae is a rounded, succulent shrublet growing as a parasite on branches of Portulacaria afra (spekboom). It is easily grown by placing its sticky seed on living branches of spekboom.

Description
Viscum crassulae is a much-branched, stiff and brittle, evergreen, hemi-parasitic, rounded shrublet up to about 30 cm in diameter. It grows on branches of Portulacaria afra (spekboom) and occasionally on arborescent (with a tree-like habit) species of Euphorbia.

The point of attachment (haustorium) to the host plant is slightly swollen. The lower main branch of the spekboom mistletoe is about 15 mm in diameter (in cultivation up to 30 mm) and brownish-green. The young branches are succulent, round (terete), green, minutely papillate and about 5 mm in diameter. The stems are slightly articulated at the prominent nodes which are about 5–10 mm apart.

Viscum crassulae (green) showing attachment to host (brown branches)

Its leaves are almost without petioles (subsessile), fleshy, and in opposite pairs (decussate), glossy green to dull green at first, sometimes becoming yellowish-green. They are egg-shaped (ovate), rounded to obovate (ovate with broadest part above) 8–10 mm long and 7–8 mm broad; the base is wedge-shaped (cuneate) and the tip is rounded, but sometimes slightly pointed. The lower surface is convex and the upper surface slightly hollow (concave). The leaves on the older branches are shed.

The inflorescence consists of a modified dichasium (a type of inflorescence in which the oldest flower is at the tip, with the younger flowers borne below it) in which the bracts are fused, forming a cup-like structure (bracteal cup) subtending the flowers. The minute flowers are unisexual and solitary or in clusters of up to four. Calyx and corolla are fused, forming 3 or 4 floral segments. Male flowers have as many stamens as floral segments and female flowers have a 1-locular inferior ovary.

The fruit is a single-seeded, ovoid, stalkless (acaulescent) berry 5–6(–8) x 4 mm, bright orange to reddish with a persistent style. The seed is imbedded in viscin (sticky, sweet-tasting substance).

The colourful berries are eaten by fruit-eating birds, and the sticky seed is deposited on the branches of the host plant when they wipe their beaks. Germination is rapid (within 3 weeks). The initial hypocotyl (portion between root and seedling leaves) grows away from light (negatively phototropic) towards the branch surface of its host where penetration takes place and contact with the host's conductive tissue is established (Visser 1981).

Flowering period: During winter and spring (occasionally to summer) and the berries ripen during spring and summer (October and November).

Conservation status
The plant is locally common in the Eastern Cape. It was therefore not necessary to include it in the Red Data Book (Raimondo et al . 2009).

Distribution and habitat
The plant is widespread on suitable succulent host plants ( Portulacaria afra and arborescent Euphorbia species) in Albany Thickets of the Eastern Cape (Mucina & Rutherford 2006). Its distribution falls within the AlbanyCentre of Floristic Endemism (Van Wyk & Smith 2001). It is especially common in Spekboomveld ( Portulacaria afra dominant) in the Addo district. Viscum crassulae has been recorded from the lower Kouga River (near the Kouga Dam) and Hankey, west of Port Elizabeth in the southwest, inland to the Graaff-Reinet, Jansenville and Cookhouse districts and eastwards to the lower Great Fish River and the Sundays River.

 Spekboom in Addo Elephant Park

Derivation of name and historical aspects.
The generic name Viscum is the Latin name for the plant (pertaining to the birdlime made from the berries). The English term “viscous” is also derived from the name (Jackson 1990). The specific epithet “crassulae” (meaning ‘of Crassula', or ‘belonging to Crassula') was chosen by Ecklon & Zeyher who collected the spekboom mistletoe on what they considered to be an arborescent Crassula (very likely the kerkei, Crassula ovata ). However, research by R.A. Dyer (1935) shows that this mistletoe grows mainly on Portulacaria afra and occasionally on Euphorbia grandidens. The epithet crassulae may therefore be due to an incorrect identification by Ecklon & Zeyher (1837). Viscum crassulae superficially resembles the succulent genus Crassula (family Crassulaceae).

Viscum crassulae is one of 17 southern African Viscum species, all hemi-parasitic on various indigenous trees and shrubs. Viscum minimum is another unique relative which is an endoparasite, living within the living tissue of Euphorbia polygona. The only time it becomes visible is during flowering, and especially fruiting time. This species is also popular among succulent Euphorbia growers.

Viscum is a genus of about 100 species, widely distributed in the Old World (Africa, Madagascar and Asian tropics) and is also found in temperate zones of Europe and Asia (Polhill & Wiens 1998). The family Viscaceae (worldwide 7 genera and 450 species) is closely related to the Loranthus family (Loranthaceae) represented in South Africa by 11 genera and 37 species (Germishuizen 2000). Unlike Viscum species (all with tiny inconspicuous flowers about 2 mm in diameter), the Loranthaceae have conspicuous, often colourful, decorative flowers usually pollinated by birds (Visser 1981).

The depicted plants of Viscum crassulae were collected in Gamtoos Thicket in the Kouga Valley just below the Kouga Dam in October 1996. The author was accompanied by horticulturists Roger Clark, Juergen Damgaard (Denmark) and Alan Tait (Pretoria). Fresh berries were gathered from plants growing on spekboom ( Portulacaria afra ). The plants are difficult to detect as they resemble their host.

Growing in spekboom in habitat

Ecology
Pollination of the small inconspicuous flowers is done by insects. In habitat the seeds are dispersed by frugivorous birds.

In all the locations visited, Viscum crassulae did not seem to have a negative effect on its host plants. In fact, it never dominated its  host, and less than 5 % of the population were infected. Should the hemi-parasite kills its host, it would be suicidal for the parasite as well. The host provides water and nutrients for its growth. The Viscum also has green leaves (chlorophyll) and traps carbon for its growth. It is thus only partially dependent on its host.

Uses and cultural aspects .
No cultural uses have been recorded for Viscum crassulae.

  Growing Viscum crassulae

The spekboom mistletoe was successfully established at Kirstenbosch in 1996. Seed was removed from its skin and placed on various spekboom plants ( Portulacaria afra ) in the Kirstenbosch nursery, the Botanical Society Conservatory and in the main garden. The seed is imbedded in sticky viscin which attaches the seed to the stems. It was placed on a living branch of a suitable living Portulacaria afra or Euphorbia shrub. Germination occurred within 3 weeks. The initial hypocotyl (portion between root and seedling leaves) soon appeared curving back onto the Portulacaria stem. The hypocotyl attached itself to the stem. The seedling leaves soon appeared and the point of attachment (haustorium) gradually thickened.

Seed germinating on stem of spekboom at Kirstenbosch

The plants were initially slow-growing, reaching adulthood only in 2003 when they started flowering and fruiting. Sticky seed was then placed on the large-leaved form of Portulacaria afra (Wylies Poort, Limpopo Province) in the Conservatory. These grew much faster than the smaller specimens in the nursery houses or those planted in the garden. Measuring them in March 2006, the main branch had grown to a diameter of 30 mm r (green, mottled grey), and the plant was 450 mm high (from the attachment to the tip of the main branch). Viscum crassulae is thus easily cultivated on specimens of Portulacaria afra, forming a striking combination when in fruit.

Viscum crassulae thrives in cultivation, especially in Albany Thicket Gardens (see Van Jaarsveld 2010). It makes an attractive and interesting plant on cultivated spekboom. The fruit also attracts frugivorous birds. It grows slowly to medium fast (under ideal conditions) and should thrive in warm temperate to subtropical gardens all over the world. The plant requires suitable host plants such as Portulacaria afra, Euphorbia grandidens or E. tetragona. The plant can also be grown on Portulacaria as an indoor pot plant. Juergen Damgaard also successfully established the plant in their botanical gardens in Denmark from where it was distributed to other parts of Europe.

Growing on P.armiana at Kirstenbosch

Seed of this species was also successfully transferred to the whipstick spekboom ( Portulacaria armiana) , and the related Ceraria fruticulosa , both now grown in the Botanical Society Conservatory at Kirstenbosch.

References and suggested reading

  • DRÈGE, J.F. 1843. Zwei pflanzengeographische Documente. Flora 2, Suppl. Regensburg.
  • DYER, R.A. 1935. Two rare parasites on succulent species of Euphorbia . International Euphorbia Society, Vol. 1; October 1935, No. 4.
  • ECKLON, C.F. & ZEYHER, K.L.P. 1837. Enumeratio plantarum africae australis extratropicae , Part 3. Perthes & Besser, Hamburg.
  • GERMISHUIZEN, G. 2000. In O.A.Leistner, Seed plants of southern Africa: families and genera. Strelitzia 10: 560, 561. National Botanical Institute, Pretoria.
  • GERMISHUIZEN, G., VAN JAARSVELD, E.J. & CONDY, G. Flowering Plants of Africa 60: 58–62 (2007).
  • HARVEY, W.H. 1862. Viscum . In W.H. Harvey & O.W. Sonder, Flora Capensis 2: 580. Reeve, Kent.
  • JACKSON, W.P.U. 1990. Origins and meanings of names of South African plant genera . Eco Lab., Botany Department, University of Cape Town, Cape Town.
  • MUCINA, L. & RUTHERFORD, M.C. (eds) 2006. The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19. South African National Biodiversity Institute, Pretoria.
  • POLHILL, R. & WIENS, D. 1998. Mistletoes of Africa . The Royal Botanic Gardens, Kew.
  • RAIMONDO, D., VON STADEN, L., FODEN, W.,VICTOR, J.E., HELME, N.A., TURNER, R.C., KAMUNDI, D.A. & MANYAMA, P.A. (eds) 2009. Red List of South African plants 2009. Strelitzia 25. South African National Biodiversity Institute, Pretoria.
  • SPRAGUE, T.A. 1915. Loranthaceae. In W.T. Thiselton-Dyer, Flora Capensis 5, 2: 121–135. Reeve, London.
  • VAN JAARSVELD, E.J. 1010. Waterwise gardening in South Africa and Namibia . Struik, Cape Town.
  • VAN WYK, A.E. & SMITH, G.F. 2001. Regions of floristic endemism in southern Africa. A review with emphasis on succulents. Umdaus Press, Hatfield, Pretoria.
  • VISSER, J. 1981. South African parasitic flowering plants , Juta, Cape Town, Johannesburg.
  • WIENS, D. & TÖLKEN, H. 1979. Viscaceae. Flora of southern Africa 10,1: 43–56.

 

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Kirstenbosch NBG

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