Floral phenology and morphology of black cottonwood,Populus trichocarpa (Salicaceae)

Seasonal changes in the development of reproductive structures are documented for Populus trichocarpa Torr. and Gray. Buds were collected and studied from several trees for a 2-yr period, but to maintain a congruous phenology, representative structures from only one male and one female tree are presented. Collected tissues were fixed, dehydrated, embedded in wax, sectioned, and differentially stained. The development of reproductive meristems begins early in the spring, before leaves emerge. However, the anatomy of male and female flowers is virtually indistinguishable until late spring. The structures of the gynoecium develop in about 2 wk, then continue to enlarge through the summer and autumn until the trees become dormant. The unilocular ovary consists of usually three, but sometimes four carples. Stamen development begins in the center of a disk-shaped meristematic region and proceeds centrifugally. Megasporogenesis and microsporogenesis take place late in the winter, approximately 2 wk before anthesis. Dissection of mature flowers revealed 30–50 seeds per capsule on female trees and 40–60 stamens on male trees. When compared to most other Populus species, P. trichocarpa has a relatively large number of reproductive structures.