Floral dimorphism activity
Illustration of a short and long-styled primula veris. Darwin, The Different Forms of Flowers, 15.
Research assistants with the Darwin Correspondence Project joined the teaching team for a new Freshman Seminar at Harvard College called “Getting to Know Darwin.” The class was composed of first year undergraduates and was taught by Ned Friedman, Director of the Arnold Arboretum at Harvard University and Professor in the Department of Organismic and Evolutionary Biology. Here is a link to a feature about the course.
To prepare for this week’s experiment, the class read chapter 1 of Charles Darwin’s 1877 The Different Forms of Flowers on Plants of the Same Species. This chapter discusses the Primula veris as an example of heterostyled dimorphic plants. Like most of Darwin’s work, his examination of dimorphic flowers provided further evidence of his theory of evolution by natural selection.
To engage firsthand with Darwin’s work on dimorphic plants, the class used microscopes to examine Primula vulgaris and Primula veris flowers. By microscopically examining these flowers, the class was able to readily observe the two sizes of style and pollen within these plants. To observe the different sizes of the pollen, your students should scrape pollen from a long-styled flower, mix it with water, and put it on a slide. They should do the same with pollen from a short-styled flower. They should then view the two sets of pollen on the same level of magnification. Having done this, the students should be able to appreciate the fact that the pollen from the short-style is notably larger than the pollen from the long-style. Darwin himself found that “the grains distended with water from the short-styled flowers were about .038 mm…in diameter, whilst those from the long-styled were about .0254 mm.”[1] Darwin suggested that the difference in pollen sizes between the short and long-styled flowers is evolutionarily advantageous because it ensures that the pollen from a long-styled plant is best suited to fertilize a short-styled plant and vice versa. Darwin believed that this “intercrossing of distinct plants” is critical to the “height, vigour, and fertility” of their offspring.[2]