Epidendroideae

Epidendroideae stands as the largest subfamily within the vast Orchidaceae family, representing a significant portion of orchid diversity with over 15,000 species organized into 576 genera. This subfamily is primarily composed of tropical epiphytes, plants that grow on other trees or plants without being parasitic, often featuring characteristic pseudobulbs which serve as storage organs.

Habitat and Adaptation

The natural habitat for most epidendroid orchids is within tropical and subtropical rainforests, environments that provide the high humidity necessary for their survival. They are found in various positions on host trees, from forks to thin branches, adapting their growth to access optimal light conditions. While predominantly epiphytic, the subfamily also includes terrestrial species, such as those in the genus Epipactis, and some myco-heterotrophs, which obtain nutrients from mycorrhizal fungi.

Epiphytic orchids have developed adaptations to survive periods of drought, despite their lack of direct access to soil moisture. Many exhibit Crassulacean Acid Metabolism (CAM), a photosynthetic pathway that allows them to absorb carbon dioxide at night and store it, minimizing water loss during the day. They also possess absorptive tissues capable of quickly taking up available water. However, high night-time temperatures and low humidity can impede CAM, leading to stress, reduced growth, and even carbon loss, thereby limiting the range of habitats a species can inhabit.

Floral Characteristics

Epidendroideae orchids are typically monandrous, meaning they possess a single fertile anther. This anther is characteristically incumbent, or strongly convex, and suberect, often forming a right angle with the column axis or pointing backward. The pollinia, masses of pollen, are usually hard and may be accompanied by a caudicle and viscidium. The stigma is typically entire or three-lobed, featuring a prominent beak, and the apical part of the middle stigma lobe forms a stipe, which attaches to the pollinium. The ovary is unilocular, and the leaves are arranged distichously or spirally on thickened stems.

Taxonomy and Classification

The subfamily was first published by Vincenz Franz Kosteletzky in 1831, with Epidendrum L. designated as the type genus. The classification of Epidendroideae is complex and has been divided into two major clades: the 'higher epidendroids' (also referred to as vandoids) and the 'lower epidendroids'. These groups are further subdivided into numerous tribes, such as Cymbidieae, Vandeae, Arethuseae, and Epidendreae, among others. This taxonomic framework is dynamic and subject to frequent revisions as new morphological and genetic data become available.