Morphology and Development of Gametophytes and Embryo in Unisexual and Anomalous Bisexual Flowers of Quaking Aspen

Abstract

Several populations of dioecious quaking aspen (Populus tremuloides Michx.) and four trees with bisexual (perfect) flowers were studied. Flower development in all catkins was acropetal, initiated in winter buds, and completed prior to anthesis. Bisexual flowers were protandrous with one-five stamens. Anther wall development in both staminate, and bisexual flowers conformed to the Dicotyledonous type. The tapetum was of the secretory type and tapetal cells remained uninucleate throughout development. Microsporogenesis was simultaneous, forming tetrahedral tetrads. Pollen was spherical and of variable sizes of which 93 and 86 percent were fertile among staminate and bisexual flowers, respectively. Pollen was shed as single binucleate grains. In both pistillate and bisexual flowers, ovules were anatropous, female gametophyte development conformed to the Polygonum type, endosperm was nuclear, and embryo development was of the Asterad type. The innermost layer of the single integument was sclerified following fertilization, and the hairs associated with seeds were of funicular origin. In five percent of the bisexual flowers, the fertilized ovules either degenerated or were arrested at the zygote stage. In 10 percent of the flowers, only one ovule in the ovary developed while the remaining ovules degenerated. Other abnormalities in the bisexual flowers included degeneration of one to several stamens.