Wednesday, January 9, 2019

[Botany • 2018] Xylacanthus laotica (Acanthaceae, Acanthoideae) • A New Genus and Species from Laos

Xylacanthus laotica Aver. et K. S. Nguyen

in Averyanov, Nguyen & Maisak, 2018.

 A new monotype genus Xylacanthus Aver. et K. S. Nguyen with one species, X. laotica Aver. et K. S. Nguyen (Acanthaceae, Acanthoideae, Ruellieae), is described and illustrated. Described genus has arborous living form rather unusual among in Acanthaceae. This small deciduous tree was observed as a main co-dominant of primary deciduous xerophytic karst scrub on extra dry tops of remnant limestone mountains in Pon Xay district of Luang Prabang province in central part of northern Laos. New genus has rather isolated taxonomic position for its floral morphology, xerophytic living form and obligatory deciduous xerophytic character. The distribution of this plant is limited by limestone karstic areas of central part of northern Laos in the limits of Luang Prabang province. 

Keywords: Acanthaceae, karstic flora, Laos, new genus, plant endemism, plant taxonomy, Xylacanthus

Fig. 2. Xylacanthus laotica. A – Flowering and fruiting stem with removed bud scales, young leaves, floral bracts and flowers. B – Bud scales. C – Young leaves. D. Floral bracts. E – Flower. F – Calyx with young fruit. G – Dissected and flattened perianth tube with removed style. H – Intact anther, side view. I – Anther with removed hairs, half side view. J – Sagittal section of calyx and gynoecium. K – Ovary and style. L – Calyx and capsule. M – Sagittal section of the calyx and ripe capsule. N – Individual calyx lobe, adaxial surface. O – Ripe open capsule, side view. P – Capsule valve with ripening seeds, half side view. Q – Ripening seed with falcate retinaculum. R – Ripe seed, frontal and side view, and view from the base. All drawn from the type – LA-VN 2050 by L. Averyanov and T. Maisak.

Fig. 1. Xylacanthus laotica. A – Natural habitat of species in Phou Hua Ben Toc Mountain (Luang Prabang province, Pon Xay district, between Houay Man and Nam Bo villages, LA-VN 2130). B – Monodominant deciduous scrub on mountain summit formed by S. arborescens (LA-VN 2130). C – Individual tree of the species on mountain top (LAVN 2130). D, E – Flowering branches of canopy (LA-VN 2130). F-I, K – Flowering and fruiting shoots (Type – LAVN 2050). J, L – Flower, frontal and side view (LA-VN 2157). M – Stamens (LA-VN 2157). N – Hairiness of anther connective and filament (LA-VN 2157).
 Photos of L. Averyanov and Khang Sinh Nguyen.

 Xylacanthus Aver. et K. S. Nguyen, gen. nov. 
Type: X. laotica Aver. et K. S. Nguyen 
Monotype genus.

Description: Deciduous monoecious tree to 5 m tall flowering and fruiting before leaf formation; shoots isophyllous; leaves in terminal rosette, opposite, decussate, pilose; flowers solitary, axillary, sessile; floral bracts 0–2, lanceolate, villose, persistent; calyx 5-lobed, villose, with many cystoliths; corolla dark blue, cylindric, hairy, 5-lobed, lobes rounded, recurved, forming almost actinomorphic limb; stamens 4, didynamous, monadelphous, all fertile, pairwise distally connivent, filaments glandular hairy, at base with many long hairs; anthers basifixed, 2-thecous, thecae with prominent recurved, sterile, acuminate base; connective with many long hairs; ovary erect, narrowly ovoid, setose, 2-locular; style filiform, comose; stigma 2-cleft; capsule ellipsoid, shortly hairy, 6–8-seeded; seeds lenticular, pilose with appressed mucilaginous hairs. 

Etymology: Generic name refers to an arboreous plant habit. 

Note: The discovered plant superficially somewhat resembles species of Strobilanthes Blume, large widespread genus well presented in tropical southeast Asia and in countries of Indochinese Peninsular as well (Benoist, 1935; Pham Hoang Ho, 2000; Deng et al., 2006, 2011; Newman et al., 2007). However, it strikingly differs from all known Strobilanthes species in its unusual arborous plant habit, spurred anthers, filaments and anther connective densely hairy with long white hairs, as well as in many (more than 4)-seeded capsule. Some of these characters, as well as similar pollen grain structure, remind representative of the genus Echinacanthus Nees. However, the only true Echinacanthus species (E. attenuatus Nees) is a lowland slender herb of the Nepal-Darjeeling area, which is totally different in habit from our plant being also geographically separated. It is highly probable that discovered plant represents new undescribed genus endemic for limestone areas of central Laos. Like its expected relatives, it may be placed into subtribe Strobilanthinae (Acanthaceae, subfam. Acanthoideae, tribe Ruellieae) in the family classification proposed by A. Takhtajan (2009). Newly described genus includes one species described below. 

Xylacanthus laotica Aver. et K. S. Nguyen, sp. nov.

Etymology The species epithet refers to the country of its origin.

Distribution Northern Laos, Luang Prabang province, Pon Xay district (Houay Man, Nam Bo and Bane Phou Souong villages). Endemic. 

Note: This remarkable plant strikingly differs from almost all known representatives of Acanthaceae family in its arborous living form, xerophytic deciduous character, as well as flowering and fruiting before leaves formation. Oldest observed trees have age at least 2–3 decades and stands in obvious contrast with herbaceous and semi-woody commonly pliestesial Acanthaceae species commonly adopted to humid conditions of shady wet evergreen forests. Like many deciduous xerophytes of extra dry exposed karstic fields, described species has fairly short period of vegetation forming leaves only during rainy season from middle April till September. Tree remains completely leafless during dry rainless winter. Described monotype genus represents typical element of ancient strictly endemic xerophytic limestone flora historically adapted to extra dry summits of karstic limestone formations of northern Laos. This curious, highly specialized flora includes many convergent xerophytic derivates from different families. Unfortunately, it remains still weakly studied. Some other remarkable plants described recently from extra dry karstic limestone fields of northern Laos (Averyanov, Nguyen, 2012; Pimenov et al., 2016) are also similarly woody deciduous xerophytes, e. g. Begonia viscosa (Begoniaceae) and Xyloselinum laoticum (Apiaceae). Like X. laotica, these species also strikingly differ from its herbaceous mesophytic congeners.

 L. V. Averyanov, K. S. Nguyen and T. V. Maisak. 2018. Xylacanthus laotica (Acanthaceae, Acanthoideae), A New Genus and Species from Laos. Turczaninowia. 21(2); 101–110 DOI: 10.14258/turczaninowia.21.2.11

Аннотация. Новый монотипный род Xylacanthus Aver. et K. S. Nguyen с единственным видом, X. laotica Aver. et K. S. Nguyen (Acanthaceae, Acanthoideae, Ruellieae), описан в качестве нового для науки. Описание сопровождается цветными фотографиями и черно-белыми рисунками. Описываемое растение имеет древесную жизненную форму, что крайне редко встречается в семействе Acanthaceae. Открытый вид представляет из себя маленькое листопадное деревцо, являющееся главным доминантом первичной ксерофильной листопадной кустарниковой растительности, покрывающей исключительно сухие карстовые обитания на вершинах известняковых столовых останцев района Пхонсай провинции Луангпхабанг в центральной части северного Лаоса. Новый род занимает изолированное таксономическое положение по морфологии цветка, ксерофитному древесному характеру жизненной формы и облигатной листопадности. Распространение этого растения ограничено карстовыми реликтовыми скальными известняками центральной части северного Лаоса в пределах провинции Луангпхабанг. 
Ключевые слова: карстовые флоры, Лаос, новый род, таксономия растений, эндемизм растений, Acanthaceae, Xylacanthus
Л. В. Аверьянов, К. С. Нгуен, Т. В. Майсак. 2018. Xylacanthus laotica (Acanthaceae, Acanthoideae) – новый род и вид из Лаоса.  Turczaninowia. 21(2); 101–110 DOI: 10.14258/turczaninowia.21.2.11