The Different Forms of Flowers on Plants of the Same Species by Charles Darwin [PDF]

Information: The Different Forms of Flowers on Plants of the Same Species

• Author: Charles Darwin
• Publication Date: 1877
• Main Characters:
  • Heterostyly: The condition where a plant species produces two (dimorphic) or three (trimorphic) distinct flower forms, differing in the relative lengths of stamens and pistils. This promotes cross-pollination between forms.
  • Legitimate vs. Illegitimate Pollination: Legitimate crosses occur between different flower forms and produce higher seed yields. Illegitimate crosses occur within the same form and result in reduced fertility, similar to the effects of inbreeding.
  • Dimorphic Plants: Species like primroses (Primula) and flax (Linum) that produce two flower forms: long-styled (with a tall pistil and short stamens) and short-styled (with a short pistil and tall stamens).
  • Trimorphic Plants: Species like purple loosestrife (Lythrum salicaria) that produce three distinct flower forms, creating eighteen possible pollination combinations, only six of which are fully legitimate.
  • Cleistogamic Flowers: Flowers that never open and are obligately self-pollinating. Darwin discusses these as a contrasting strategy to heterostyly, showing the spectrum of reproductive approaches within flowering plants.
  • Natural Selection and Outbreeding: Darwin's overarching argument: heterostyly evolved because cross-pollination produces stronger, more fertile offspring, giving outbreeding species a selective advantage over self-pollinating ones.
• Brief Summary: Darwin examines why certain plant species produce two or three structurally different flower forms, a condition he calls heterostyly. Through extensive crossing experiments with primroses (Primula), flax (Linum), and purple loosestrife (Lythrum salicaria), he shows that pollination between different forms (legitimate crosses) produces significantly more seeds than pollination within the same form (illegitimate crosses). The book also covers cleistogamic flowers, which never open and self-pollinate by default. Darwin argues that heterostyly evolved as a mechanism to promote outbreeding, reinforcing his broader theory of natural selection. The work includes detailed fertility tables and statistical comparisons that make it one of his most data-driven publications.
• Thematic Analysis: The central theme is the relationship between flower structure and reproductive strategy. Darwin shows that the arrangement of stamens and pistils in heterostyled plants is not random but serves a specific function: ensuring pollen is carried between different forms by insects. This connects to his larger argument that variation within species is shaped by natural selection. A secondary theme is the cost of inbreeding. By comparing the fertility of legitimate and illegitimate crosses across generations, Darwin builds a case that self-fertilization produces weaker offspring. The book also touches on the spectrum between outbreeding and self-pollination through its analysis of cleistogamic flowers, species that maintain both open (cross-pollinating) and closed (self-pollinating) flowers on the same plant.
• Historical Context: Published on July 9, 1877, by John Murray in London, this book arrived eighteen years after On the Origin of Species and drew on five papers Darwin had published between 1861 and 1868. By the 1870s, Darwin was focused on botanical research that provided concrete, experimental evidence for natural selection. The study of heterostyly was part of this effort. Darwin built on earlier observations by scientists like Christian Konrad Sprengel and Friedrich Hildebrand, adopting Hildebrand's term "heterostyly" in place of his own earlier "dimorphism." The book was well received in scientific circles and printed 1,250 copies in its first edition. A second edition appeared in 1880. It remains a foundational text in plant reproductive biology.