Genera Included: Arjona Cav., Quinchamalium Molina, and Schoepfia Schreber.
Habit: trees, shrubs and herbaceous perennials.
Parasitism: root hemiparasites; confirmed for Arjona (Kuijt 1969), Quinchamalium (Ruiz and Roig 1958) and Schoepfia (Piehl 1973, Werth et al. 1979).
Stem: For Schoepfia, distal shoots sympodially branched where the terminal bud either aborts or is converted to an inflorescence. For Arjona sections Euarjona and Psilarjona, the foliar shoots arise from an underground branched rhizome. The rhizome can extend vertically into the soil several centimeters and may form (from cataphylls) horizontal stolons. In A. tuberosa the rhizomes form edible tubers. In Arjona section Xylarjona, the plants are small shrubs with the basal portion of the aerial shoots forming a xylopodium. Quinchamalium species are mostly herbaceous perennials, often forming a rosette aspect with several, mostly unbranched stems arising from a fusiform root. Basal stems may be semidecumbent or prostrate.
Leaves: In Schoepfia, alternate, estipulate, petiolate, subcoriaceous, entire, elliptic to ovate. In Arjona, leaves are alternate, sessile, linear or lanceolate, small and thick or rigid, often pointed or even spine tipped (A. ruscifolia). The leaves in Quinchamalium are linear or subulate, glabrous, acuminate with a prickle tip.
Leaf anatomy: The following information applies only to Schoepfia. See Baas et al. (1982). Primary venation pinnate, secondary brachydodromous, tertiary reticulate, quaternary reticulate; leaf dendritic hairs absent but present as sparse, thick-walled unicellular hairs on midrib and petiole; epidermal and guard cells not lignified; paracytic to parallelocytic stomata present; cyclocytic and anisocytic stomata in S. obovata; stomata with narrow lumina; cuticular ledge of stomata inconspicuous; hypodermis mostly absent (except S. flexuosa); schizogenous cavities in leaf absent; laticiferous channel in leaf absent; druses absent in epidermal cells; silicified walls of epidermal cells present; mesophyll sclereids absent; petiole and midrib sclerenchymatous fibers absent; midrib, basal petiole, and distal petiole vascularization a simple bundle; unilacunar; leaf cystoliths absent; silica bodies in ray cells absent.
Stem anatomy: The following information applies only to Schoepfia. See Reed (1955). Less than 60% of vessels solitary; vessel member length below 900 µm; perforation plates (mature wood) simple; intervessel pits alternate; vascular tracheids present; ground tissue of libriform fibers; axial parenchyma abundant; axial parenchyma strand length less than 7 cells; all ray cells procumbent (homogeneous); ray size short (below 1000 µm); wood cystoliths absent.
Inflorescence: For Schoepfia, axillary cymose clusters, umbels or spikes. In cymose inflorescences the individual units are composed of mono- or dichasia. For Arjona and Quinchamalium, terminal spikes. Bracts and bracteoles present, in Schoepfia and Quinchamalium, these bracts fused into a lobed, cup-shaped epicalyx. The 4-toothed epicalyx in Quinchamalium persist on the mature fruit forming an irregularly lobed cupule. The floral anatomy study by Johri and Agarwal (1972) showed that the epicalyx differentiates before the floral primordia, thus supporting its interpretation as bracts, not a calyx.
Plant Sex: flowers bisexual.
Flowers: Pedicels absent or very short. Heterostyly has been
documented in all genera: Arjona
(Skottsberg 1940), Quinchamalium
(Riveros et al. 1987), and Schoepfia
(Tomlinson 1980 and
Fruit: a nut-like achene crowned by the disk and remains of the corolla and style in Arjona. The fruit in Quinchamalium is a pseudocarp, lacking a pericarp because "the endosperm is directly surrounded by the sclerotic bracts" (Johri and Agarwal 1965). In Schoepfia, a fleshy ovoid drupe.
Seed: single inside fleshy endosperm in Arjona and Quinchamalium, solitary and nearly filling the crustaceous endocarp in Schoepfia; embryo minute.
Chromosomes: Chromosome numbers have been reported for Schoepfia fragrans (Mehra, 1976): n = 12 and Arjona patagonica (Moore, 1981): n = 14.
Alternate Family Names: Arjonaceae Tiegh.