In 1877 Darwin published a book that included a series of smaller studies on botanical subjects. Titled The different forms of flowers on plants of the same species, it consisted primarily of papers on heterostyled flowers (flowers which have styles of differing lengths) which had been previously published with the Linnaean Society. In his autobiography Darwin commented on the joy this work gave him: "no little discovery of mine ever gave me so much pleasure as the making out of the meaning of heterostyled flowers. The results of crossing such flowers in an illegitimate manner, I believe to be very important as bearing on the sterility of hybrids; although these results have been noticed by only a few persons." These botanical studies also gave Darwin an opportunity to collaborate with two of his closest scientific colleagues and friends.
The materials in this teaching module highlight Darwin’s relationships with two of his closest scientific colleagues and personal friends; Asa Gray and Joseph Dalton Hooker. The letters interweave family news, reflections on professional life and experimental results. Included in the resources is a supplementary reading by Jim Endersby that reflects on the role of sympathy in Darwin's work and in his friendship with J. D. Hooker. Notice how Darwin utilises not only his own observations but also those of his trusted friends and their networks. Darwin’s findings on floral dimorphism were eventually published in 1877, but these experiments and observations were conducted fifteen years previous to the publication of that work, highlighting Darwin’s ability to carry out themes and investigations for decades at a time.
You can download the entire packet of reading materials here: Floral Dimorphism Source Pack
Darwin, C. R. 1877. The different forms of flowers on plants of the same species, London: John Murray. Chapter 1: Heterostyled Dimorphic Plants: Primulaceae
Letter 3468- Darwin to JD Hooker, 7 March 1862
Darwin wishes he could sympathize with Asa Gray’s politics. He mentions his work on floral dimorphism and the appearance of his new Orchid book.
Letter 3515- Daniel Oliver to Darwin, 23 April 1862
Daniel Oliver, an assistant under Joseph Hooker at the Royal Botnic Gardens at Kew, writes to Darwin with an update on a set of experiments carried out at Darwin’s request. (Oliver would later become a professor of botany at University College, London). Oliver was unable to find anything distinctly dimorphic in the Oxalis.
Letter 3757- Joseph Dalton Hooker to Darwin, 12 October 1862
J. D. Hooker writes to Darwin to report on different anthers in the same flower in the genus Cassia. He also mentions the ongoing work of his assistant Daniel Oliver. He reflects on the difference between his scientific abilities and Darwin’s and notes what he perceives to be the dullness of his work in comparison to Darwin’s. Remember, however, that Hooker and Darwin were very close and Darwin held Hooker’s work in high esteem.
Letter 4053- Darwin to Asa Gray, 20 March 1863
Darwin discusses dimorphic plants with Gray. He also mentions his regret that Charles Lyell did not come out more forcefully on the subject of species in Lyell’s latest book. Darwin also comments that his ill health has slowed his work.
Endersby, Jim. "Sympathetic Science: Charles Darwin, Joseph Booker, and the Passions of Victorian Naturalists." Victorian Studies, Vol. 51, No.2, Special Issue: Darwin and the Evolution of Victorian Studies (Winter 2009), pp.299-320. (JSTOR access required)
1. What tone does Darwin use in his letters to Asa Gray? Is it similar to his way of writing to Hooker? Why do you think these relationships were an important part of Darwin's science?
2. Why do you think that Darwin's work with heterostyled plants was so satisfying to him? Why do you think Darwin worked so often with botanical subjects in his later years?
3. Were you surprised to find that Darwin included news of his family, his personal health, and his botanic work all in one letter? Why or why not?
An example of course activity performed at Harvard University:
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.” 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.
 Charles Darwin, The Different Forms of Flowers on Plants of the Same Species (London: John Murray, 1877), 16.
 Ibid., 30.