Embryo development, megagametophyte storage product accumulation, and seedefficiency in Taxus brevifolia

Taxus brevifolia Nutt. has a reduced ovulate structure that consists of a s ingle ovule in a leaf axil instead of a compound ovulate strobilus. Taxus b revifolia on southern Vancouver Island, British Columbia, were studied over three seasons. Proembryos occurred from mid-May to mid-June. They underwen t four free nuclear divisions forming 16 nuclei before cellularization. Ear ly embryos were present from mid-May to mid-August. Simple polyembryony was observed up to the massive embryo stage, and differential growth of the em bryonal cells was interpreted as incomplete cleavage polyembryony. Mid-embr yos were present from mid-June to late August and had a distinct protoderm and focal zone. Late embryos were visible from mid-July onwards. Carbohydra tes began accumulating at the early embryo stage, whereas proteins and lipi ds accumulated in the late embryo stage. The presence of a red aril corresp onded to increased amounts of lipid in the megagametophyte cells. Individua l seeds matured from July until November. The seed efficiency ranged from 0 to 16% and averaged 5%. Prezygotic loss was the most common fate of ovules , followed by postzygotic loss. Possible causes of this poor seed efficienc y are poor pollination success, insect damage, or light limitation.