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

The seed of the parasitic angiospermArceuthobium americanumNutt. 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 develops into a complex tissue composed of elongated viscin cells, which elongate via vacuoles and secrete a pectic mucilage via Golgi bodies, as well as isodiametric, hydrophobic vesicular cells. Only viscin cells are discharged; vesicular cells remain with exocarp. Polyembryony occurred at a rate of 1%.