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 radic le is ensheathed in a translucent tissue of elongated cells that it ev entually penetrates. When germinated seeds of white spruce (Picea glau ca [Moench.] Voss.) are bisected and the embryo is removed, these elon gated 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 si milar to the cells of the embryo root cap hut not to those of the radi cle, megagametophyte, or nucellus. The elongated cells are diploid, as determined by fluorographic intensity after DNA staining with 4',6-di amidino-2-phenylindole dihydrochloride and flow cytometry, indicating embryonic origin. They are not part of the megagametophyte in mature u ngerminated seeds, in seeds that have not completed germination, or in seeds that have had their testae removed in liquid nitrogen and the m icropylar end of the nucellus and megagametophyte excised surgically p rior to imbibition. Excised embryos grown on Murashige and Skoog minim um organics media supplemented with 6% w/v sucrose grow and produce el ongated cells that sheath the radicle. We conclude that the elongated cells ensheathing the radicle are derived from the embryonic root cap.