Contributions to the morphology of the fern genus Oleandra

Abstract

The rhizome of Oleandra pistillaris and 0. wallichii is elongated (erect in the former, creeping in the latter), cylindrical, clothed with peltate, non-glandular, hairy paleae and bearing leaves in small clusters separated by long leafless portions. The ground tissue of the rhizome is composed of thick-walled parenchyma, and there is a peripheral sclerenchymatous sheath as well as a few scattered slender sclerenchyma strands in the pith close to the vascular cylinder. The vascular cylinder is dissected into a loose reticulum of slender meristeles by crowded, spirally arranged leaf gaps. In the 'leafless' portions of the rhizome these leaf gaps are associated with a solitary vestigial leaf trace: where well-developed leaves occur, each gap is associated with two or three leaf trace bundles. In 0. pistillaris, well developed leaves are in whorls at regular intervals; in 0. wallichii they are in small well-separated groups, each group consisting of two to six leaves in two irregular rows on the dorsal surface (i.e. leaves all along the ventral surface are suppressed). Leaves in both the species are simple with an entire, free-veined lamina and with an articulated stipe, the articulation being towards (but not at) the base and formed of an abscission pad. Branches of the rhizome are borne in pairs and each branch trace is a solitary cylindrical vascular strand, unaccompanied by any branch gap in the stelar cylinder. Sporangia are in circular sori placed in a row close to the midrib and protected by reniform indusia. The sporangial stalk bears glandular hairs, and is two cells thick except at the capsule base, where a short third row is developed secondarily during sporangial development as a protrusion from a lateral wall cell of the capsule base. The spores are of the bilateral type provided with a loose wrinkled perine: spinose excrescences occur over the exine as well as the perine. The prothallus is of the cordate type, bearing unicellular papillate hairs profusely on the surfaces and margin. It develops from a uniseriate germ-filament which develops into a prothallial plate by longitudinal divisions in the anterior cells. The terminal cell of the germ filament ends in a hair either prior to or immediately after the initiation of plate formation. A meristematic cell is formed early, either as a daughter cell of the terminal cell or of the penultimate cell. On the basis of morphological comparisons it is suggested that Oleandra is probably allied to the Aspidioid rather than to the Dennstaedtioid or Lindsaeoid groups of ferns.