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Common Rockrose

Helianthemum nummularium

  • Written also: Common Rock-rose
  • Family: Rockrose Family – Cistaceae
  • Growing form: Perennial dwarf shrub.
  • Height: 10–20 cm (4–8 in.). Stem procumbent–ascending, sometimes creeping, base woody.
  • Flower: Corolla regular (actinomorphic), golden yellow, 15–30 mm (0.6–1.2 in.) broad; petals 5, base sometimes with orange spots. Sepals 5, of which 3 large and 2 small, larger ones densely stellate-haired. Stamens many. Pistil a fused carpel. Inflorescence a 1–10-flowered cyme, flowers with subtending bracts.
  • Leaves: Opposite, short-stalked, stipulate, stipules longer than stems. Blade elliptic–lanceolate–narrowly ovate, with entire margins, 1-veined, top sparsely haired and green, underside densely haired and grey, edge revolute.
  • Fruit: Ovoid, hairy, unilocular capsule, opening by 3 valves.
  • Habitat: Dry slopes, river banks, meadows, pastures, banks, gravels. Calciphile.
  • Flowering time: June–July.
  • Endangerment: Near threatened, protected in all of Finland except the Åland Islands.

Common rockrose in Finland is a relic from the warm period that followed the Ice Age, so it is an archaeophyte. In Finland it is most common in meadows on the main Åland Island. Its habitat stretches to the south-western Finnish archipelago, where there are separate stands as far as Korppoo and Hiittinen. Around the Archipelago Sea it is very rare, however, and in danger of disappearing completely as its habitats become overgrown. It has apparently moved with people in ancient times to the old forts where it still grows today inland. Each stand may be descended from plants that were transplanted as ornamentals.

Common rockrose has adapted to very hot and dry habitats. The drier it is, the more of a dwarf shrub it becomes. It continues to assimilate in warm weather too despite evaporation because it is exceptionally efficient at taking in water. Despite the modest size of its aerial parts, common rockrose’s roots can reach up to 50 cm (20 in.) down into the ground, and this is further complemented by an abundant surface rootstock. The leaves’ revolute margins and densely felt-haired undersides help the plant save water and withstand drought. In extreme circumstances common rockrose can shed some leaves and reduce surface-area water loss.

Common rockrose’s inflorescence has only one flower open at a time and is quite short-lived. The plant only opens its flowers in fine weather and always directs them away from the sun. The flower doesn’t secrete nectar but is rather visited by pollen-gathering insects. Its stamens are sensitive to movement and react to dampness. As the pollinating insect brushes the anther the stamens drop pollen onto its fur and start to turn to the side, and this inflection makes it easier for the pistil in the middle of the flower to touch the next insect guest, and thus enhance its chances of receiving pollen from a neighbouring flower on its stigma.

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