Vegetative anatomy of subtribe Orchidinae (Orchidaceae)

Leaves in Orchidinae are essentially glabrous; anticlinal walls of foliar epidermal cells arc basically straight‐sided to curvilinear, and cells arc fundamentally polygonal on both surfaces; adaxial cells are larger than abaxial cells. Stomata arc 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, chlorcnchyma cells arc 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 arc 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 arc collateral and scattered throughout the ground‐tissue or are absent from the central ground‐tissue. Epidermis in absorbing roots is one‐layered and non‐velamcntous. Exodcrmal 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 xylcm 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 arc polygonal lacking intercellular spaces. Vascular tissue consists of monarch to pentarch meristeles distributed thoughout the ground‐tissue each surrounded by a uniscriale endodermis of thin‐walled cells. Thin roots of Plalanthera exhibit 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 in Platanthera gives 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’.