Investigation of adaptive characters in the rheophytic fern Osmunda lancea by quantitative analyses
Rheophytes had been defined that “rheophytes are plant species which are in nature confined to the beds of swift-running streams and rivers and grow there up to flood-level but not beyond the reach of regularly occurring flash floods.” by van Steenis(1981,1987). Rheophytes have adaptive characters to endure the water pressure during floods. Many rheophytes have narrower leaves or leaflets, which are considered as adaptive characters to reduce the water pressure and have been well studied. Other adaptive characters, however, have not been investigated sufficiently. In this study, characters of petiole and root were focused. As for petiole, Imaichi and Kato (1992) reported that petiole of a rheophytic species Osmunda lancea is thinner than that of its dryland ally O. japonica. But the plant size of O. lancea is smaller than that of O. japonica. Therefore, in this study, it was hypothesized that O. lancea has relatively thicker petiole than O. japonica does when their plant size are taken into a consideration. The characters of rhizome in genus Osmunda have been well studied because of abundant fossil records. It was also hypothesized that O. lancea has relatively larger number of roots or longer roots than O. japonica does. In order to verify the first hypothesis, leaf samples were collected at the localities near Iruma river, Hannou City, Saitama Pref. and near Sakai river, Uenohara City, Yamanashi Pref. and were used as materials for analyses. After measuring diameter of petiole, samples were pressed between sheets of newspaper and were put in a drier. And the following characters were measured: leaf length, petiole length, length of the lowest pinna, leaf areas, number of ultimate pinnule veins, dry weight of rachis, dry weight of pinna, and pinnule thickness. As the result, diameter of petiole was larger in O. lancea than that in O. japonica when their leaf length was taken into a consideration, and our hypothesis was supported. In addition, dry weight of leaf per unit area of O. lancea was heavier than that of O. japonica. Therefore, it was thought that O. lancea has tough petioles and leaves to endure the water pressure during floods. In order to verify the second hypothesis, experiments in an incubator were carried out because it was impossible to collect the whole rhizomes with roots of O. lancea in wild populations. Spores of O. japonica and O. lancea were sown on agar culture medium in petri dishes, and kept in the incubator. In two months, spores were germinated and grown to be mature prothalia, so they were fertilized by watering. After the fertilization, prothalia were arranged at random on new medium. Prothalia were replaced one more time onto new medium after two months, until that time, sporophytes grew, and in additional two months, those cultured plants were ready for analyses. So, photographs of leaves and rhizomes with roots were taken. Then, shoot and rhizome with roots of each indivisual were separated, and their dry-weight was measured. As the result, difference in root/shoot ratio between young sporophytes of O. japonica and those of O. lancea was not detected. Our hypothesis was denied. It was, however, observed in our experiment that O. japonica grew more rapidly than O. lancea did. So, it is possible that the condition of incubator was more favorable for O. japonica. It is also possible that O. japonica may have some species-specific traits related to its higher growth rate, and the experiments should be repeated under different conditions. As a future direction, I would like to examine whether thickness of petiole is positively related to the viability in habitats of rheophytes and observe cross sections of pinnules of O. lancea and O. japonica.
首都大学東京, 2016-03-25, 修士（理学）