Maturation of the Embryo, Endosperm, and Fruit of the Dwarf MistletoeArceuthobium americanum(Viscaceae)

Abstract

The seed of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. lacks an integument‐derived seed coat, takes two growing seasons to mature within the fruit, and is explosively dispersed. Embryo, endosperm, and fruit tissue (pericarp) development occurring from the end of the first growing season to dispersal was investigated with LM and EM. At the end of the first season, the embryo is globular and, over the second season, develops a uniseriate protoderm, two vestigial cotyledons, a prominent radicular apex, procambium, and ground meristem. Throughout development, the embryo remains enveloped by an endosperm containing chloroplasts; an endospermic cap resides above a radicle that lacks a root cap. Before discharge, radicle elongation compresses cap cells. Pericarp differentiates into endocarp, mesocarp, and exocarp at the end of the first season. Tannins accumulate over winter and, over the second season, fill the endocarp and lower mesocarp, forming a “pseudo–seed coat.” Upper mesocarp deve...