Polar willow (Salix polaris) is like all other willows, dioecious, meaning having separate male and female individuals. The top pictures show a female flower (and individual) with bright red styles and split stigmas, whereas the individual on the bottom picture only produces male flowers with red stamens carrying yellow pollen. This pollen will need to disperse to another plant, a female plant, where it can fertilize an egg and become a seed.
Being dioecious is a rare strategy amongst plants (just about 6% of the world’s plant species). Most plants are monoecious, with both male and female flowers on the same plant. The benefit of this most common strategy is that you can self-pollinate if necessary. On the other hand, too much self-pollination can become a problem as well since it stops the exchange of genetic material between individuals and may for example hinder evolutionary adaptation.
In Norway there are several dioecious plants that are quite common, such as goat willow (Salix caprea), aspen (Populus tremula), cloud berry (Rubus chamaemorus), catsfoot (Antennaria dioica) and red campion (Silene dioica). Have you for example gotten upset when, after scouting in mid summer and finding several cloud berry plants in bloom, you return in autumn only to find a mire but no fruits? Since cloud berry have a clonal growth form it is not unusual that the flowers you see flowering in summer are all from one or just a very few individuals. If this happens to be a male individual, well then no wonder there are no berries.
This post is part of a series of shorter plant stories.
Korpelainen, H., K. Antonius-Klemola, and G. Werlemark. “Clonal structure of Rubus chamaemorus populations: comparison of different molecular methods.” Plant Ecology 143.1 (1999): 123-128.
Renner, Susanne S., and Robert E. Ricklefs. “Dioecy and its correlates in the flowering plants.” American journal of botany(1995): 596-606.