Alternanthera tenella Colla and Amaranthus blitum Linnaeus are weeds that occur in crops and uncultivated areas in the Maringá region of Paraná. In this study, a morphoanatomical analysis of fruit development and the pericarp of A. tenella and A. blitum was made in order to contribute structural information for species identification, fruit classification, and ecological investigations. Flowers and fruits were collected at the campus of the State University of Maringá, Paraná, fixed in glutaraldehyde, sectioned with a rotary microtome and stained with Toluidine blue. The bracteoles and perigone are persistent in the fruits and are structurally different in both species. The achene of A. tenella maintains the same number of cell layers as the ovary, and these layers collapse during the mature phase, except for the inner mesocarp, where U thickened cell walls and crystals remain. The utricle of A. blitum is similar to the ovary in number of cell layers and has aerenchyma when it is ripe. Mature seeds are exotestal, with a collapsed mesotesta and endotesta, and curved embryo. In this study, some structural characters of the perigone and of the fruits were found that could potentially be used to separate the species; unlike the seeds, which were very similar.
achene; perigone; utricle; weed
