Studies in Pteridaceae. V. Contributions to the morphology of some species of the maidenhair ferns

Abstract

Morphology of 24 species of Adiantum is described. The rhizome in the genus is paleate and erect, semi-erect or creeping. Paleae are basally attached, and devoid of glandular hairs. They originate as uniseriate hairs which broaden by divisions of the intercalary cells. The stelar cylinder of the rhizome is commonly solenostelic (dictyostelic in some species) and in the simplest cases possessing small leaf gaps and scattered branch gaps. The leaf trace is either a single gutter-shaped strand or a pair of strap-shaped strands which later fuse together into a gutter-shaped vascular strand. In many species the leaf and branch traces are associated, the former originating adaxial to the latter. The branch trace is usually a single gutter-shaped strand, but in some species may be split longitudinally into two. The foliar epidermis has characteristic, elongated, thick-walled cells over the veins. Sporangia occur superficially near the apices of free-ending veinlets towards the leaf margin, the extreme apex of the veins being devoid of sporangia. The entire fertile area of the leaf lamina is infolded. The spores are trilete and with the exine more or less smooth. The prothallus is cordate and naked, developing from a four to six cells long germ filament, through the activity of a wedge-shaped apical meristematic cell which is replaced later by a multicellular meristem. Sex organs are of the common type in advanced ferns. The opercular cell of the antheridium may occasionally be divided. Wing cells of the mature prothalli are usually uniformly thin walled, but in some species there are collenchyma-like thickenings on the corners. The early juvenile leaves are broadly cuneate, with a single vein dichotomizing once or twice. Progression to the adult leaf form is described.