FORMATION OF HAPLOID EMBRYOIDS OF LARIX DECIDUA: EARLY EMBRYOGENESIS

Callus initiation of sections of megagametophytes of Larix decidua occurs just below the cut surface and is followed by the formation of one or more long protruding cells. The long cells then divide transversely at their tips to yield small cytoplasmically-dense terminal cells. The latter divide, forming loose aggregates of dense cells, microcalli, which develop long cells that radiate in all directions and divide terminally to produce aggregates of small cells. This long/short cell alternation is repeated a few times. Eventually the aggregates divide in a polarized manner producing files of long cells predominantly in one direction. These loosely bundled long cells form a suspensor-like structure. The meristematic small cells continue dividing forming a mass of embryonal cells. This early embryoid eventually turns green and produces both a root and shoot. Haploid embryoids are also derived from long cells in which karyokinesis but not cytokinesis occurs, resulting in a four-nucleate coenocyte. The nuclei migrate to one pole and become surrounded by cell walls. The cells thus formed are the originators of a new embryoid.