Sexual reproduction of mountain hemlock (Tsuga mertensiana)
Meiosis of pollen mother cells begins in October of the year in which cones are initiated. They reach pachytene then become dormant until the next March. Meiosis is complete and the winged pollen mature by mid-June. Meiosis of the megaspore mother cell occurs in May, forming a linear tetrad of megaspores. The female gametophyte undergoes free nuclear division at pollination in mid-June. No pollination drop is present; rather, the pollen adheres to the sticky, splayed edge of the micropyle, where it germinates and pollen tubes grow toward the nucellus. The nucellus elongates into the micropylar canal, forming a nucellar beak, which makes contact with the pollen tubes. Several pollen tubes penetrate the nucellus.At the time of fertilization early in August, each ovule contains two to four aichegonia each having two to four neck cells in one tier. Pollen tubes penetrate the neck cells and two male gametes are formed. The ventral canal cell breaks down and fusion occurs in the center of the archegonium. Four free nuclei form and migrate to the base of the archegonium. cell walls form, and a 16-celled proembryo develops. Both simple and cleavage polyembryony occur. Rosette cells divide but do not form complex embryos. The embryo and seed are mature in October and the cones dry and open during October and November. Mature cones averaged 70 seeds, of which 46% were filled.Reproduction in mountain hemlock (Tsuga mertensiana (Bong.) Carr.) is similar to that in other species of Tsuga except for the presence of winged pollen. Any attempt to place the species in the genus Picea or place it as a hybrid midway between Picea and Tsuga is unfounded based on all of the more-conservative reproductive and embryological characteristics.