Histo-anatomical observations on some eastern Mediterranean wild orchids

Anatomical and histological observations have been carried out on the stem (inflorescence axis) and leaf structure of twelve species of Orchis, Ophrys and Barlia sampled on the East Mediterranean islands of Crete and Rhodes. This study represents the preliminary phase of a wider morphological analysis regarding European and Mediterranean wild orchids, carried out to verify orchid relationships and some taxonomic interpretations. Relatively few studies, concerning histological and anatomical analysis, have been conduced on vegetative organs of European terrestrial orchids: Camus and Camus, (1928); Solereder and Meyer, (1930); Borsos, (1980); Del Prete et al., (1991); Stern, (1997); Del Prete and Miceli, (1999); the more recent studies nevertheless have provided a useful contribution to knowledge of orchid relationships.The leaves appeared bi-facial in all examined species: the vascular bundles were in a single row in the middle of the mesophyll, the xylem oriented to adaxial surface; stomata were found to be abaxial and superficial; epidermal cells were approximately isodiametric in transversal section and the adaxial noticeably greater than abaxial cells. With regards to mesophyll structure two different groups were distinguishable: Ophrys heldreichi x O. reinholdii, O. tenthredinifera, O. iricolor, Orchis italica, O. quadripunctata and O. sancta had a compact, homogeneous mesophyll. In Ophrys cretica ssp. cretica, O. cretica ssp. karpathensis, O. episcopalis, O. heldreichi, O. spruneri and Barlia robertiana instead, there was a loose mesophyll structure; in some of these species the mesophyll was found to be differentiated into a palisade and a spongy tissue.Transversal sections through the stems showed a mechanical sheath separating the cortex and ground tissue in all species. Observation under fluorescent light made it possible to identify the degree of lignification of the cell walls, and class it as a sclerenchymatous ring. Barlia robertiana presented a vascular system characterized by many collateral vascular bundles, scattered in the peripheral area of the ground parenchyma. In Orchis and Ophrys species observed instead, the vascular bundles were neatly arranged in a circle at the periphery of the ground tissue: in O. quadripunctata and O. italica the vascular bundle were so peripheral that they made contact with the sclerenchymatous ring, as reported for Orchis mascula in a previous study (Del Prete and Miceli, 1999). In all the Ophrys species examined, but also in Orchis sancta, the vascular bundles were arranged below the sclerenchymatous ring but with a portion of ground tissue interposed. This evidence could support the suggestion, made by Strack et al., (1989), that Orchis sancta and the related species O. coriophora, O. fragrans and O. martrinii be placed in a separate genus, Anteriorchis.