The Role of Exchangeable Calcium in Saxicolous Calcicole and Calcifuge Mosses

S ummary Species of bryophyte restricted to calcareous rocks in the British Isles consistently exhibit concentrations of calcium 16 to 17 times greater than in species found on non‐calcareous rocks. In both groups of plants, this calcium is predominantly in an exchangeable form. Experiments in which the exchange sites were saturated with Ca 2+ ions indicate that the calcicoles have Ca 2+ exchange capacities three or four times those of the calcifuges. Several possible explanations for this difference are discussed. One hypothesis, that a high affinity for Ca 2+ ions corresponds with a high calcium requirement for permeability control, has been tested experimentally. The results, although far from conclusive, suggest that calcicole mosses may require enriched Ca 2+ concentrations in their tissues to maintain the integrity of their cell membranes.