Flowering of Conifers

Some people find it hard to believe that plants must flower in order to set seed. In many woody plants the flower isn't very attractive. For some, the flowers are barely distinguishable from the leaves. Flowers of the conifers (pine, spruce, fir, and other cone-bearing woody plants) are called strobili, which means small cones. They do not have a calyx, corolla, stamens, or pistils as many flowers do. The strobili consist of a central axis with distinctively shaped scales and bracts. Male and female cones are separate and, in most cases, both are present on the same tree. The male cone bears two pollen sacs on the lower surface of each scale. These cones are often brightly colored in shades of yellow, purple, or red. When mature they release an abundance of yellowish pollen grains and then disintegrate. The female cone bears two inverted ovules on the upper surface of each scale. The ovate cones are sometimes colorful. These small cones develop into the hard woody cones containing varying numbers of naked seeds or the fleshy fruits of juniper, yew, and ginkgo. In many of the pines and in Douglas-fir, female flowers are most numerous in the upper crown of the tree.

Seed initiation is possible only after successful union of pollen within the female ovules. This involves some reasonable synchronization between pollen dispersal and flower receptivity. For conifers, pollen is dispersed by wind over a period of several days. Of course, pollen dispersal depends on humidity, temperature, and wind conditions. Under prolonged conditions of high humidity, shedding of wind-borne pollen may be inhibited. Unidirectional wind during pollen dispersal may result in one-sided pollination. When female cones are receptive, scales spread apart to allow pollen grains to come in contact with the ovules. Fertilization occurs and seed development and maturation continues. Depending upon the conifer genus, cones will ripen during the current growing season or the next.

This article originally appeared in the June 9, 1993 issue, p. 86.