Sexual reproduction in western red cedar (Thuja plicata)

Abstract

Pollen cones and seed cones ended dormancy in mid-Feb., microsporogenesis occurred in late Feb., and pollination occurred for about 1 wk in early March. Pollen was shed at the 2-celled stage. Pollination drops were exuded from only a few ovules at a time. Pollen contacting the pollination drop was rapidly taken in. The pollination drop was withdrawn into the micropyle which was later sealed by enlargement of cells lining the micropylar canal. Megasporogenesis occurred in late Feb. but female gametophytes did not mature and fertilization did not occur until late May. An archegonial complex formed containing 7-9 archegonia, of which several usually were fertilized. Proembryo development varied depending upon the size and shape of the archegonia. Usually, a 12-celled, 3-tiered proembryo formed by mid-June. Cleavage polyembryony was not observed. Embryos were mature by mid-Aug. and most seed was shed in Sept. and Oct. The potential seed set was only 16 seeds per cone and filled seed averaged only 2.6 per cone. Most potential seed was lost because of early ovule abortion from unknown causes, insect damage, or low temperatures at or shortly after pollination. Some potential seeds were lost because the ovules were not pollinated or the embryos aborted. These seeds were soft but nearly normal appearing and contained spongy female gametophyte tissue. Methods of maximizing seed production are suggested.