Distribution of Epiphytic Bacteria on Freshwater Plants

(1) The spatial distribution of epiphytic bacteria on seven species of common aquatic plants has been determined on intact leaves by direct examination. (2) The significance of the observed differences in the bacterial population density within and between leaves, between plants on the same date, and between dates has been tested. (3) The proportion of the total variance due to the variance between leaves ranged widely (0.04-0-82). (4) Two species (Nasturtium officinale and Veronica beccabunga) had significantly larger populations on the abaxial surfaces of their submerged leaves than on the adaxial surfaces of the same leaves. This difference may be due to the unilateral release of hydroxyl ions during bicarbonate utilization. (5) The older the leaf the greater was the bacterial density. (6) The aerial parts of floating and emergent plants had substantially lower bacterial densities than the submerged parts. (7) No significant seasonal differences were discernible between leaves of the same age, but significant short-term changes were detected on Ranunculus penicillatus var. calcareus.