Publication: Schumannia 3: 188 (2001 publ. 2002)
Description
Usually stay solitary, mature plants are easily recognizable by their cephalium, that have a covering of wool and orange or red bristles, The immature plant looks like a smallish barrel cactus, and there is nothing in its appearance that would suggest a melocactus. Melocactus arcuatispinus has long amber-brown arched spines.
Stem: Strongly longitudinally ribbed globose or spheroid.
Flowers: Small inconspicuous purple, self pollinating flowers grow at the top of the cephalium.
Spines: Very rather stout and arched, yellow, brown, or reddish, overlaid with gray except at the dark tips.
Fruits: White clavate.
Origin and Habitat
Northeastern Brazil ( caatinga of Pernambuco State).
Habitat: Grows just sit in narrow cracks of solid rook rows in dry areas where rainwater quickly disappears.
Ecology: The fruit of this plant sometimes remain embedded inside the cephalium bristles, with seeds waiting, perhaps, for the parent plant to die and provide a bit of organic matter for them to germinate in their surprisingly inhospitable environments.
Cultivation and Propagation
These cacti are not the easiest things to grow and aren’t plants for beginners. Melocactus arcuatispinus (zehentneri) grows from April to October, it can’t endure long stretches of total dryness, and also too much water will rot it, as its weak root systems tends to be inefficient at sucking up water from wet soil. Nonetheless, again as a result of their tropical origins, they need a fair amount of water, but allow the soil to dry quite a bit before watering again. Melocactus rests from October to April but can’t stand cold, or even fairly cool temperatures, so is indispensable to keep it above 8-12°C at all times, severe damage or death occurring at temperatures that the great majority of cacti wouldn’t mind in the least and prefer more frequent water in winter than other cacti, say once a month. Do not feed in winter.
The root system is weak and generally resents being repotted and can take a long time to re-establish. The soil mix should be very quick draining, prefers very bright light, not as much as the most arid growing cacti, but plenty nonetheless.
Propagation: Exclusively by Seeds. Sow in February-march in a light, sandy, porous soil. Cover germinating tray with glass to prevent seed from drying out. Germination is most successful at a temperature of 18 to 22° C.
Notes
The most remarkable part of a Melocactus is its cephalium a bristle-coated structure on the summit of the plant, only Melocactus, and the similar genus Discocactus possesses this type of permanent, apical, hatlike appendage. It’s only when a Melocactus reaches maturity that the cephalium begins to grow. Cylindrical, with a diameter distinctly smaller than that of the plant body below, the cephalium will keep growing for the rest of the plant’s life, but the body of the plant stops growing the moment the cephalium starts to form. As the plants age the cephalium doesn’t increase in circumference it will steadily grow taller.
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Bibliography
Major references and further lectures
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Stuart Max Walters “The European garden flora. 3.[Angiospermae], Dicotyledons. [Casuarinaceae to Aristolochiaceae]” Cambridge University Press, 1989
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Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
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David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. “The New Cactus Lexicon” dh books, 2006
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Urs Eggli, Leonard E. Newton “Etymological Dictionary of Succulent Plant Names” Birkhäuser 2004
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Taylor, N. P. 1991. “The genus Melocactus (Cactaceae) in Central and South America.” Bradleya 9: 1–80.
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Nigel Taylor, Nigel P. Taylor, Daniela C. Zappi, “Cacti of Eastern Brazil” Royal Botanic Gardens, Kew, 15/gen/2004
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Edgar Lamb, Brian Lamb “The Illustrated Reference on Cacti & Other Succulents” Volume 5 Blandford Press, 1978
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E Haustein “Der Kosmos Kakteenfuehrer (the Kosmos Cactus Guide)” Balogh Scientific Books 01 December 1998
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Clive Innes, Charles Glass “Cacti” Portland House, 1991
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Machado, M., Taylor, N.P., Braun, P. & Zappi, D. 2013. Melocactus zehntneri. In: “IUCN 2013. IUCN Red List of Threatened Species.” Version 2013.2. <www.iucnredlist.org>. Downloaded on 04 December 2013.