Arjona megapotamica

Arjona megapotamica is a small, herbaceous, perennial root hemiparasite. A large number of unbranched stems, 15–20(–30) cm in height, sprout from a small woody central base, called a 'xylopodium'. These stems are 1–1.5 cm in diameter, sulcate (grooved in transect profile) and glabrous (hairless). The linear leaves are alternately distributed throughout the length of the stem. The leaves are simple, sessile, coloured green, glabrous, with the leaf blade 2–3 cm long and 1–1.5 cm wide. The leaves are tipped in a point, and have 1 to 3 prominent veins. The stems are topped by a terminal inflorescence, shaped as a spiked raceme. It is about 2 cm in length and densely packed with flowers. The 0.6–2 cm long, linear bracts, are tipped in sharp points and villous along their margins. The flowers are sessile. They have two linear, villous bracteoles 2–3 mm in length. The perianth is tubular, hypocrateriform, yellow, 10–12 mm in length, a few trichomes dispersed internally, with five lobes. The five stamens have dorsifixed anthers, and are oblong. The ovary is pilose, about 2 mm in length, and with pendulous ovules, a filiform, glabrous style, and a slightly 3-lobulate stigma. The ovoid fruit is ca. 3–4 mm in length with a truncated end and 10 ribs. The fruit, when eventually removed from or having fallen off the infructescence, leave an off-white scar at the base of the bract. …

Arjona megapotamica is endemic to southeastern Brazil. The species formerly occurred in the state of Rio Grande do Sul on granite hills which surround the coastal city of Porto Alegre. These hills have now been extensively developed, and the species was officially declared locally extinct in Rio Grande do Sul in 2014 (Decreto Estadual RS 52.109). It also occurs in the eastern part of the Santa Catarina Plateau in Santa Catarina state according to Mattos, where it presumably may still be found.

It is a root hemiparasite growing in cool montane grasslands. On Morro da Polícia, a 286 m high granite hill, it grew in a shrubby montane grassland. In Santa Catarina it has been collected at 1,670 m altitude in the Serra Geral (Urubici municipality), and 950 m in Lages municipality.