Embryo root cap cells adhere to the megagametophyte and sheathe the radicle of white spruce (Picea glauca [Moench.] Voss.) seeds following germination

In all seeds of the Pinaceae, after germination is complete, the radicle is ensheathed in a translucent tissue of elongated cells that it eventually penetrates. When germinated seeds of white spruce (Picea glauca [Moench.] Voss.) are bisected and the embryo is removed, these elongated cells adhere to the micropylar end of the megagametophyte. These same cells are present at the chalazal end of the megagametophyte in seeds with inverted embryos. The cells adhering to the micropylar end of the megagametophyte after radicle protrusion are histochemically similar to the cells of the embryo root cap but not to those of the radicle, megagametophyte, or nucellus. The elongated cells are diploid, as determined by fluorographic intensity after DNA staining with 4',6-diamidino-2-phenylindole dihydrochloride and flow cytometry, indicating embryonic origin. They are not part of the megagametophyte in mature ungerminated seeds, in seeds that have not completed germination, or in seeds that have had their testae removed in liquid nitrogen and the micropylar end of the nucellus and megagametophyte excised surgically prior to imbibition. Excised embryos grown on Murashige and Skoog minimum organics media supplemented with 6% w/v sucrose grow and produce elongated cells that sheath the radicle. We conclude that the elongated cells ensheathing the radicle are derived from the embryonic root cap.