Adiantum

Adiantum (Division Pteropsida, Order Filicales), the 'maidenhair fern' (also called 'walking fern'), grows across the tropics and temperate zones; Indian species include A. capillus-veneris, A. pedatum, A. caudatum and A. venustum. The sporophyte is differentiated into a rhizome, roots and leaves.

The rhizome is a perennial, dichotomously branched, subterranean structure — creeping in A. capillus-veneris, erect in A. caudatum — covered in persistent leaf bases and hairy outgrowths called ramenta. Roots are adventitious, arising from the rhizome. The leaves (fronds) are pinnately compound (unipinnate in A. caudatum, bipinnate in A. capillus-veneris), young fronds are coiled in circinate vernation, the petiole is long, black and shiny, venation is free and dichotomous fanning out through the lamina, and the fronds carry marginal sori covered by a false indusium.

Internally, the root shows an epidermis with root hairs, a cortex split into an outer wide parenchymatous zone and an inner narrow sclerenchymatous zone, and a simple stele with a central diarch xylem core flanked by phloem. The rhizome shows a cuticle-covered epidermis, a two-to-three-layered sclerenchymatous hypodermis, parenchymatous ground tissue, and a stele that starts as an amphiphloic siphonostele in young rhizomes but matures into a solenostele or dictyostele (broken into several meristeles) in older ones. The petiole shows an epidermis, a sclerenchymatous hypodermis for mechanical strength, extensive ground tissue, and one large horseshoe-shaped stele with a central xylem core surrounded by phloem. The pinnule shows upper and lower epidermis, stomata confined to the lower surface, undifferentiated mesophyll (no separate palisade/spongy layers), and a vascular bundle wrapped in a sclerenchymatous bundle sheath. …