Abstract
Anatomical survey was carried out on narrow, (ob)lanceolate leaves(stenophylls) of about 30 rheophytic ferns and several related dryland species. The mesophyll cells of stenophylls are less-expanded, and then their intercellular spaces are smaller than those of related dryland species. Epidermal cells are also less-lobed in rheophytes. To know the origin of the rheophytic stenophylly, ontogeny of narrow pinnules of Japanese rheophyte, Osmunda lancea, was compared with that of broad pinnules of a related dryland species, 0. japonica. It was clarified that the mesophyll and epidermal cells of 0. lancea remain smaller in size because of weakened cell expansion, which leads to reduction of growth period of pinnule. The stenophylly of 0. lancea is believed to have arisen by heterochronic evolution, in particular, progenesis.
