Country diary: the killer role of pollen in the hawkweed’s life cycle
Old limestone quarries are fantastic places for nature, and here at Ketton the Wildlife Trust reserve fills the space left by stone extracted in the 1930s. Next door there is light industry, on what would have been the quarry floor, and then beyond that the vast modern excavations and tall cement works that dominate the landscape.
The weather is balmy as we clamber on the eroding slopes, picking out little clasped heads of brachiopods – clam-like animals, some smooth, others grooved with zigzag jaws. They lived here 169m years ago in Jurassic seas. Scattered over the steep stony ground are little clumps of distinctive leaves, pointed at the tip, with a broadly rounded base and bearing a few widely spaced small, shallow peaks along their white-hair-fringed margins – and dramatically splattered with reddishblack, sharply edged blotches, creating cartographic patterns of lakes or islands. Several clumps each sprout a single stem stretching up to 60cm high, some branching into two to six heads, each bearing a bright little dandelion flower or an aniseed-ball-sized clock of dark seeds and white whiskers.
These plants are so distinctive that their identity must surely be easily revealed, but this is where the story takes a turn, for they are hawkweeds (Hieracium sp) and they don’t follow the rules. Whereas most plants require the transfer of pollen to produce fertile seeds, hawkweeds all reproduce by apomixis; they clone, every seed almost genetically identical to its parent. Exchanging genetic material is fundamental to maintaining discrete and identifiable species – groups of organisms that evolve together. Cloners are just divergent: every single seed is on its own evolutionary pathway. Botanists classify British hawkweeds into about 340 “species”, of which 176 are dangerously rare. Huge misidentification pitfalls mean that identifying them can only be attempted in early summer, when there is fresh leaf growth, by the brave, naive or adept.
Pliny the Elder believed that hawks (hierax in ancient Greek) sprinkled sap from hawkweeds into their equally yellow eyes to dispel dimness. But the plant’s true hawkish behaviour was only recently discovered: when the pollen that they produce is transferred to other plants, it kills their embryos, stopping them from producing seeds that would compete for space with the hawkweed’s cloned offspring.