A phenological and cytological study of pollen development in western hemlock (Tsuga heterophylla)

Abstract

Pollen-cone development, which was divided into nine phenological stages, was compared with pollen cytology in eight field-grown western hemlock (Tsuga heterophylla (Raf.) Sarg.) clones in 1983 and three container-grown clones in 1984. Phenology proved to be an accurate indicator of cytology, independent of collection dates and rate of development. The breaking of dormancy, the resumption of development, and meiosis occurred during stage 1 (quiescent bud). During stage 2 (swollen pollen-cone bud) the tetrad of microspores separated. The exine wall was completely developed during stage 3 (bud burst). The one-celled microspores began to expand as a result of accumulation of reserves during stage 4, when the pollen cones were one-quarter to one-half emerged through the bud scales. The first cell division occurred during stage 5, when the cones were greater than half emerged. The second and third cell divisions occurred during stage 6, after the cones had completely emerged through the bud scales. The fourth and final cell division, resulting in mature five-celled pollen grains occurred during stage 7 (stalk elongation). Mature pollen consisted of two prothallial cells and a stalk, a body, and a tube cell. Pollen shed occurred at stage 8. The empty cones (stage 9) remained on the trees until the following winter. Some practical implications of relating pollen-cone phenology to cytology are discussed.