Vegetative anatomy of subtribe Orchidinae (Orchidaceae)

Abstract

Leaves in Orchidinae are essentially glabrous; anticlinal walls of foliar epidermal cells are basically straight-sided to curvilinear, and cells are fundamentally polygonal on both surfaces; adaxial cells are larger than abaxial cells. Stomata are anomocytic and usually only abaxial and superficial; substomatal chambers are small to moderate; outer and inner guard cell ledges are mostly small. There is no hypodermis nor are there fibre bundles. Mesophyll is homogeneous, chlorenchyma cells are thin-walled, and intercellular spaces numerous. Crystalliferous idioblasts abound. Vascular bundles are collateral, organized in a single series, and lack associated sclerenchyma. Bundle sheath cells are thin-walled and chlorophyllous. Stems are glabrous; stomata are frequent in one species, lacking in others. Cortical cells are oval to circular, thick-walled, and interspersed with triangular intercellular spaces. Ground-tissue cells are circular, and triangular intercellular spaces are present. Vascular bundles are collateral and scattered throughout the ground-tissue or are absent from the central ground-tissue. Epidermis in absorbing roots is one-layered and non-velamentous. Exodermal cells are thin-walled and dead cell walls bear tenuous scalariform bars; some species lack an exodermis. Outer cortical cells are polygonal and lack intercellular spaces; middle layer cortical cells are rounded with triangular intercellular spaces; inner layer cells are polygonal and lack intercellular spaces. Endodermis and pericycle are thin-walled and one-layered. Vascular cylinder is mostly 7–9-arch with xylem and phloem components alternating regularly; vascular tissue is embedded in parenchyma; pith cells are parenchymatous, polygonal, thin-walled and lack intercellular spaces. Root tubers generally bear a velamen of variable thickness; bulbous-based unicellular hairs frequently form a dense mat; exodermal cells are thin-walled; dead cells have scalariform bars, passage cells are sparse. Ground-tissue consists of rounded water-storage and assimilatory cells interspersed with triangular or quadrangular intercellular spaces; peripheral cells are polygonal lacking intercellular spaces. Vascular tissue consists of monarch to pentarch meristeles distributed thoughout the ground-tissue each surrounded by a uniseriate endodermis of thin-walled cells. Thin roots ofPlatantheraexhibit a typical central cylinder surrounded by a homogeneous cortex uninterrupted by meristeles; thicker roots show a central vascular cylinder and cortex in which meristeles are also present; in globoid root tubers there is no central cylinder, and the ground-tissue is replete with scattered meristeles. Because the central vascular cylinder inPlatantheragives rise to branches (meristeles), these represent components of a single vascular system and are not separate stelar entities as implied by the use of the term «polystele».