B. Papp szerk.: Studia Botanica Hungarica 38. 2007 (Budapest, 2007)

Bercu, Rodica: On the anatomy of the endangered plant species Asplenium lepidum (Aspleniaceae)

ON THE ANATOMY OF ASPLENIUM LEPIDUM 127 The stele consists of xylem and phloem and is surrounded by a one-layered pericycle. The protoxylem vessels (5 for each bundle) are in an exarch position and face the pericycle. The phloem sieve cells, lack companion cells, are located on either side of the xylem string (Fig. IB). The xylem elements (2 for each bundle) are joined together toward the centre by their metaxylem vessels (2 for each bundle). This attributes to the root a diarch structure (Fig. 1A, B). Cross sections of the petiole base up to 2 cm reveal that the epidermis consists of a single layer of thick-walled cells (Fig. 2A). Below the epidermis is the cortex consisting of 3-4 layers of large parenchyma cells, followed by a single layer of regularly arranged cells that is endodermis (Fig. 2A). Remarkable is the presence of a large sclerenchyma mass placed in the adaxial portion of the stele with mechanical role, which towards the rachis tip is absent (Fig. 2B). Characteristic to the Aspleniaceae petiole structure the stele is monofascicular (OGURA 1972, BlR 1957, BERCU 1998, 2006), composed of two centrally located xylem strings, surrounded by phloem. That attributes to the vascular bundle a hadrocentric structure. The two xylem curved strings configuration reveals that initially two rhizome vascular bundles, called by OGURA ( 1938) meristeles, join together to form a foliar trace (Fig. 2B). The stele is surrounded by pericycle composed of a single layer of cells, locally in two or even three rows, sometimes called "special pericycle" (ANDREI 1978). Transversal sections of the petiole and rachis, cut from the base (2 cm) up to 4 cm exhibit almost the same structure, but the monostele xylem strings appear in an X-shaped arrangement, characteristic to the Aspleniaceae species (OGURA 1972, BlR 1957, BERCU 1998, 2006). The phloem surrounds the xylem elements (Fig. 3A, B). Cross sections of the rachis from 4 cm up to 6 cm, exhibit the same succession of tissues, but the xylem vessels form a T-shape arrangement (Fig. 4A, B), surrounded by phloem and endodermis, the latter with parenchymatous sheath value (Fig. 4B) (BERCU 2004, 2005). The small vascular bundle, derived by marginal breaking up, from the rachis stele, to form the pinnulae main vein. Remarkable is the reduced number of the cortex layers of cells and vascular elements. A typical sclerenchymatous hypodermis is present. Transversal sections of the rachis tip (6.5 cm from the petiole base) show a blade structural organisation with the same conformation of the stele vascular elements.