Dimensional model of zygotic Douglas-fir embryo development

A dimensional model of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) zygotic embryo development was constructed from polarization of the embryo into a stele promeristem and root cap in early July until seed-shed in mid-September. Total embryo length was found to be a useful variable for integrating organ and meristem development. Apical meristems formed and had completed development just before cotyledon initiation when the embryo was ca. 800-1200 mu m in length (<25% of final length). A dramatic transition from root cap growth to a rapid phase of hypocotyl and cotyledon development occurred following cotyledon initiation at ca. 1300 mu m total length. This indicates that the early and late stage of embryo development should be defined on whether histogenesis is complete and cotyledons have been initiated. Cell division continued in the root cap, hypocotyl, and cotyledons during the late stage. A distinct phase of cell enlargement in the absence of cell division was not observed. Instead, hypocotyl and cotyledon cells increased in size gradually during the late stage, while root cap cells did not exhibit any cell enlargement. Completion of dimensional development occurred in a root cap, hypocotyl, and cotyledon sequence based on when organs reached mature length ranges and when cell division ceased.