Psychotria subundulata Benth. var minor M.-Arg., Mart. Fl. Bras. 6(5): 303. 1881, not Psychotria minor Turrill, Jour. Linn. Soc. Bot. 43: 27. 1915.
Type. Inter Sao Joao et Santa Anna, Estado Para, Brazil, Burchell 9227.
Müller-Argoviensis associated this taxon with P. subundulata Benth., with which it shows no close relationship. It appears to be more closely allied to such species as P. bostrychothyrsus Sandw, and P. paniculata (Aubl.) Raeusch. in the irregular branching of the lateral axes of the inflorescence. From both of these taxa, it differs in having all the flowers on pedicellate-like stalks or branchlets and in the uniformly narrower inflorescence. In its numerous lateral axes of the inflorescence P. argoviensis approaches P. bostrychothyrsus, but differs in the more irregularly branched axes which lack the sessile flower in the center of the fork, and in having all the flowers on pedicellate-like stalks. It further differs from P. bostrychothyrsus in the puberulent rachis, axes, calyx, and hypanthium.
From P. paniculata it may be distinguished by the pedicellate-like stalks of the flowers throughout the inflorescence, by the more conspicuously developed calyx lobes, and by the narrower inflorescence with more numerous and shorter lateral axes. From both P. paniculata and P. bostrychothyrsus it further differs in the shorter petioles, shorter leaf blades, and linear or narrowly triangular-subulate instead of broadly triangular or oblong-ovate stipular teeth.
Some specimens of P. bostrychothyrsus with reduced inflorescences and shortened lateral axes, as exemplified by B. Maguire, Steyermark & C. K. Maguire 46923, Steyermark 89425, Trujillo 3512, Leprieur 138, Pires, Rodrigues & Irvine 50251, and Irwin, Egler & Westra 47350, resemble P. argoviensis, but have the central flower of a fork sessile, broadly triangular or oblong-ovate stipular teeth, and glabrous rachis, and axes of the inflorescence.