ALOCASIA DECUMBENS
ORIGINAL DESCRIPTION:
“Above ground stem is short, 6 cm in diameter at its thickest point. Tight internodal spacing. The leaves point upwards at first, and after anthesis the petioles curve. Petioles are about 40 cm tall, and smooth, even at the top, and wider at the bottom; base is approximately 5,5 cm in diameter, large and stately, gradually attenuated at the bottom of the base of the trunk. Leaf is ovate- or oblong-cordate with a sharp-pointed peak, 30+ cm. long, 22-25 cm. wide, dull whitish-green in color with thin veins and slightly discolored margins; lobes are approximately a fifth of the leaf, rounded, and very briefly connate (at most 1 cm.). Sinus is wide with triangular edges, and becomes parallel towards the top of the lobes, with an interlobar distance of 10 cm; 5-6 lateral veins, which form an angle of about 60° with the central vein, 1-3 back veins up to 17 cm. long; secondary veins angled at less than 90° from the lateral veins and arch, parralel to teach other towards the marginal vein.
The peduncle is erect, and about 1 cm. in diameter, and 40 cm long. The spathe is light green at first, ovoid tubular, very tight, 4 cm long anteriorly and 3.5 cm long posteriorly, 2.5 cm in diameter; after anthesis the spathe is about 12 cm long, 4-4.5 cm wide, with winding curved edges, and becomes almost parallel to the ground. Sessile spadix, about 13,5 cm long, with the female zone 1.1-1.2 cm long and 1.4 cm thick; the sterile mid zone is about 3 cm long and 0.7 cm thick ; the male zone is about 10 cm long and 1.2 cm thick; the appendix is cone shaped, ivory in color, and 5.5 cm long and 1.4 cm thick, synandrous, irregularly winding.
Tetragonal pistils, crowded, with green back; oblong ovary, which extend above, 12 ovules inserted on the 3-4 parietal placentas. Middle sterile part (synandrodium) is white, and elongated, slightly lower than the center of the spadix. The male zone (synandria) is hexagonal, usually 5-lobed, and is white-green at first, then buttery-yellow. Vertical protruding theca, almond colored. The scent of the inflorescence at anthesis is insignificant”
Described in 1939 by Samuel Buchet from a living cutting sent to Paris by Léopold Michel Cadière (often cited as R.P. Cadière, where R.P. stands for Révérend Père, a French honorary title for a reverend) in May 1938, which flowered there in March and April 1939⁴
Bulletin du Muséum national d'histoire naturelle (France) ser.2:t.11 (1939)
SYNONYMS: N/A
DISTRIBUTION: Vietnam | Endemic to Lạng Sơn Province (NE Vietnam), known only from the limestone karst around Hữu Liên Nature Reserve, Hữu Lũng District.⁴
The Cửa Tùng written on the type sheet, in present-day Quảng Trị Province (Central Vietnam), is where Cadière lived and kept a garden of rare plants after 1918, and where the plant was presumably grown after it was brought from Lạng Sơn. It is not a wild locality.⁴
CLIMATE: Temperate (dry winter, hot summer) in NE Vietnam, under the monsoon: at Hữu Liên the winters are dry and cool⁴
Humidity is moderately high throughout the year, ranging from 75% to 85%
Temperature across the northern half of Vietnam can vary, generally staying within the range of 50°F/10°C to 86°F/30°C throughout the year; the annual average at Hữu Liên is 72–73°F/22–23°C⁴
Annual rainfall at Hữu Liên is about 1,400–1,500 millimeters,⁴ most of it in the monsoon months (Jun–Sep), with an average of 30 millimeters monthly in the dry season.
SOIL: Areas where Alocasia decumbens has been observed are predominantly limestone, with interbedded sandstone, siltstone and clay shale. At Hữu Liên it grows directly on rough or broken limestone of the Bắc Sơn Formation, on surfaces from horizontal to vertical, in shade or in the open among shrubs and vines in disturbed forest.⁴
SPECIES DESCRIPTION:
Lithophytic, rhizomatous herb to 60 cm tall, rooted on rough or broken limestone; at flowering the leaves spread outward and bend down to the ground.⁴ Stems cylindrical, more or less horizontal and rooting where they touch the rock, 30–45 cm long and 3–6 cm in diameter, dark brown, ringed with prominent leaf scars and clothed in brown fibers left by old petioles; internodes 5–10 mm. Occasional constrictions along the stem probably mark slower growth in dry seasons or dry years, and shoots from the nodes build loose clumps of connected stems. In cross-section, a dull white rind about 5 mm thick surrounds pale brown to pale yellow tissue.⁴
Leaves 2–4, occasionally up to 6, together. Blade peltate, 25–48 cm long and 18–31 cm wide, glossy dark green and not waxy above, glossy gray-green beneath; anterior lobe broadly triangular to ovate, 24–35 cm long, narrowing abruptly to a slender tip 1.5–2 cm long; posterior lobes angular to semi-elliptic, 8–14 cm long, with rounded or blunt tips, held well apart and never overlapping, the sinus opening in a wide triangle. Four or five pairs of lateral veins leave the midrib at 60–70°, prominent beneath. Petiole slender and hard, 40–65 cm long and 2–2.5 cm thick, green to dark gray-green and dusted with a chalky white wax; sheath narrow, its wing about 1 cm wide, running a third to half the length of the petiole.⁴
Alocasia odora, common across northern Vietnam and also wild at Hữu Liên, shares its above-ground stem, narrowly peltate blade and glaucous green spathe. Alocasia decumbens is far smaller (to 60 cm, against 3 m) and has a rough, dark brown stem where Alocasia odora has a smooth, paler one; slender, hard, waxy petioles with a narrow sheath rather than thick, tender ones with a sheath wing 2–4 cm wide; fewer lateral veins (4–6 pairs rather than 6–10 or more); and posterior lobes set far apart rather than close together or overlapping. In flower, its sterile zone is longer than the male zone, and its stigma is a disc with a central hollow rather than three- or four-lobed.⁴
INFLORESCENCE:
Inflorescence solitary, on a peduncle about 22 cm long and 5–6 mm thick, medium to dark green, sheathed by a dark green, slightly waxy cataphyll. Spathe about 12 cm long, dark green to green and waxy outside. The lower tube is oblong-ovoid, about 4 cm long and 1.8 cm across, narrowed into the peduncle at the base and pinched around the sterile zone of the spadix; the limb opens wide at anthesis, then folds into a boat shape, 8.5 cm long and 4.5 cm wide, turning yellow with age.⁴
Spadix shorter than the spathe, almost sessile. Female zone cylindrical, 1.5 cm long and about 1 cm thick, dark green; sterile zone about 2.7 cm long, ivory white, made of much-reduced, elongated sterile male flowers and constricted two-thirds of the way up; male zone about 2.4 cm long, yellow; appendix conical and blunt, about 3 cm long, dull white, its surface finely wrinkled. Ovaries globose to three- or four-angled, dark green, each with 3–4 basal, orthotropous ovules and a nearly stalkless, disc-shaped stigma with a central hollow. Synandria about 3 mm tall, five- to six-sided and slightly concave on top, with 3–4 pairs of slender anthers opening by pores at the apex.⁴
Recorded in flower in the wild at Hữu Liên on December 22nd, 2007, and in cultivation in Hanoi on October 6th, 2013; the main flowering season is probably autumn, when rain is scarce.⁴
VARIEGATED FORMS: N/A
STORY:
STORY TITLE: The plant with a misread address
In May 1938, Father Léopold Cadière (1869–1955), a French missionary in Vietnam, sent a living cutting of an unfamiliar Alocasia to Paris. It flowered there in March and April 1939, and Samuel Buchet described it that year as Alocasia decumbens, from the Paris plant and without an illustration.¹,⁴ The herbarium sheet made from it, a leaf and an inflorescence held in the Paris herbarium and cited as the lectotype, gives its locality as Cửa Tùng, in present-day Quảng Trị Province. That is not where the plant grew wild. It is where Cadière lived after his retirement in 1918, and where he kept a garden of rare plants.⁴
Buchet, citing Cadière, gave the wild source as limestone ground at "Taish-Moi," near Lạng Sơn in Upper Tonkin, at 100 to 300 meters.¹ No Taish-Moi appears on a modern map. Van Du Nguyen and Peter Matthews suggest a misreading of handwriting, an "n" read as "is," for Thanh-Moi, a place shown on old French maps near what is now Hữu Liên, at the edge of a limestone massif on the route to the old city of Lạng Sơn.⁴
It was at Hữu Liên that Nguyen and Matthews found the species again, and their collections may be the first since Cadière prepared his cutting. A single population of about a hundred plants is scattered for roughly a kilometer along the steep, rocky shore of a small lake and swamp enclosed within the limestone, at about 225 meters, with a few more plants 2 to 3 kilometers to the west. The terrain and altitude match Cadière's account closely, and the old and new sites probably lie in the same massif, which leaves the species with a very small range. Nguyen, who has surveyed aroids widely in the karst of northern Vietnam, has not seen it anywhere else.⁴
The name records what Buchet saw when the Paris plant flowered: the leaf sheaths softened, the petioles arched toward the ground, and the blades lay pressed flat around the upright inflorescence in a wide rosette. A potted plant in Hanoi did the same.¹,⁴ Nguyen and Matthews suggest that the collapse may be a controlled wilting, reallocating water from the rest of the plant to keep the inflorescence, and later the fruit, displayed upright on dry, fast-draining karst. Peter Boyce has seen petioles recurve at flowering in the Bornean Alocasia princeps complex, though not only there, and the rock-dwelling habit, dark leaves with prominent posterior lobes and orthotropous ovules point the same way: toward a possible link between this Vietnamese endemic and a group otherwise confined to Borneo.⁴
The rough terrain, reachable only on foot, may be what protects the population from heavy harvesting; local people occasionally collect the rhizomes to feed household pigs. Its pollination, fruiting and seed dispersal had not been studied when the population was described, and the likeliest pollinators are Colocasiomyia flies, which visit Alocasia and Colocasia across Southeast Asia. Hữu Liên is a designated Gene Management Zone for wild relatives of crops, Colocasia among them, and Nguyen and Matthews recommend continued study and protection of Alocasia decumbens there.⁴