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Monocotyledons pp 1299-1304 | Cite as

Orchidaceae

  • U. EggliEmail author
Reference work entry
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Part of the Illustrated Handbook of Succulent Plants book series (SUCCPLANTS)

Abstract

A diagnostic description of the family is given with special emphasis on the occurrence of succulence. This is followed by information on the ordinal placement, a selection of important literature, and information on the geographical distribution. A short discussion of the family’s position in the angiosperm phylogeny is supplemened by a summary of its past and present classification in a phylogenetic context. The succulent features present amongst the species of the family are shortly summarized, as is its general economical importance.

Including Apostasiaceae Lindley.

Including Cypripediaceae Lindley.

Including Limodoraceae Horaninow.

Including Liparidaceae Vines.

Including Neottiaceae Horaninow.

Including Neuwiediaceae Reveal & Hoogland.

Including Ophrydaceae Vines.

Including Pycnanthaceae Ravenna.

Including Vanillaceae Lindley.

Perennial terrestrial or predominantly epiphytic herbs, rarely shrubs or lianas or aquatics (Habenaria repens), rarely saprophytic and without chlorophyll or completely subterranean; R often tuberous in terrestrial taxa, with multi-layered velamen in epiphytic taxa; stems virtually absent, or forming corms or rhizomes, or conspicuous and often distinctly swollen to form pseudobulbs, often rooting at the nodes; L entire, alternate and spirally arranged (sometimes distichous, rarely opposite or whorled), often fleshy, sometimes scale-like or completely absent (“root orchids” with green roots for photosynthesis); Inf lateral racemes or panicles, or flowers solitary; Fl mostly bisexual (rarely unisexual and plants dioecious), 3-merous, usually zygomorphic, often resupinate; Per of 3 + 3 usually petaloid tepals, one of the inner tepals (labellum) usually much larger and/or differently coloured; St 1 (−3), mostly united with the style to form a gynostegium; pollen normally united into complex pollinia (rarely pollen dustlike, Apostasioideae); Ov of 3 united Ca, inferior, normally 1-locular (3-locular in Apostasioideae); Fr capsules opening with 3 (or 6) longitudinal slits but remaining closed at either end; Se very numerous, dust-like with undeveloped minute embryo.

Order: Asparagales

Important Literature: Pridgeon (1992: illustrated encyclopedia); Bechtel & al. (1992: synopsis cultivated taxa); Dressler (1993: traditional classification); Cameron & al. (1999: molecular phylogeny); Pridgeon & al. (1999: synopsis Apostasioideae, Cypripedioideae); Pridgeon & al. (2001: synopsis Orchidoideae part 1); Chase & al. (2003: molecular phylogeny); Pridgeon & al. (2003: synopsis Orchidoideae part 2, Vanilloideae); Pridgeon & al. (2005: synopsis Epidendroideae part 1); Cameron (2006: molecular phylogeny); Pridgeon & al. (2009: synopsis Epidendroideae part 2); Górniak & al. (2010: molecular phylogeny); Gustafsson & al. (2010: evolution & diversification); Pridgeon & al. (2014: synopsis Epidendroideae part 3); Stern (2014: anatomy); Chase & al. (2015: updated classification).

Distribution: Worldwide but concentrated in the tropics and subtropics.

With 736 genera (Chase & al. 2015) and (depending on the source consulted) 17,500 to 35,000 species (Pires & al. 2006), the orchids are the largest plant family. Orchids show a number of special developments. Like in the ApocynaceaeAsclepiadoideae, the pollen is united into complex pollinia in most orchids. Unlike the pollinia of the asclepiads with their clasping mechanism of the translator, orchid pollinia have a sticky plate (viscidium) at the basal end of the caudicle and are glued on the body of the pollinator. The minute orchid seeds in nature normally germinate only in the presence of certain mycorrhizal fungi (but can be germinated on appropriate laboratory media in cultivation). Mycorrhiza are probably universally present throughout the family also in adult plants.

Classification: The family has been unambiguously found to be monophyletic, and to represent the basal sister to the remaining families of the Asparagales by all recent studies (Bogler & al. 2006; Givnish & al. 2006; Graham & al. 2006; Pires & al. 2006; Gustafsson & al. 2010; Seberg & al. 2012).

The infrafamilial classification of this vast family is a challenge in view of the species number, but also because of the traditional reliance on characters of floral morphology. Floral morphology has been found to be evolutionarily much more labile than accepted in the past (Carlsward & al. 2003; Kocyan & al. 2008), and floral convergencies due to shared pollination syndromes are frequent.

The correct placement of many genera has been elusive for long (Dressler 1993), but DNA-based studies in the past 20 years have added considerably to our knowledge. They have provided a firm classificatory backbone, but have also shown that many large genera need to be restudied carefully with regard of their morphology-based circumscription and infrageneric classification (e.g. Dendrobium, Adams (2011)).

The epiphytic habit so common in the family has evolved several times in parallel (Chase & al. 2003) and the more basal clades are all terrestrial.

A classification into five subfamilies was first proposed by Cameron & al. (1999), who found that Vanilla and related genera should be recognized as separate subfamily Vanilloideae, and that Spiranthoideae, Neottioideae and Vandoideae are imbedded in other clades. This backbone classification (Fig. 1) was corroborated by several subsequent studies (Chase & al. 2003; Cameron 2006; Górniak & al. 2010).
Fig. 1

Summary phylogeny of Orchidaceae (26,400 species total, data from Chase & al. 2015 and Govaerts 2014+). The monospecific clades Codonorchideae (Orchidoideae) and Thaieae (Epidendroideae) are omitted. 1 = Apostasioideae, 2 = Cypripedioideae, V = Vanilloideae; Are = Arethuseae, Col = Collabieae, Cran = Cranichideae, Diur = Diurideae, Podo = Podochileae. (Copyright: U. Eggli)

The Apostasioideae (L spiral, plicate; Fl sometimes resupinate, indistinctly irregular; St 2–3, pollen never in pollinia; sometimes treated as family of their own, Apostasiaceae) represent the basal clade, and they are sister to the rest of the family.

The Cypripedioideae (L spiral or distichous, sometimes plicate; Fl resupinate, labellum slipper-shaped; St 2 (of the inner whorl) + 1 staminode (of the outer whorl); pollen rarely in true pollinia) is another small group that is sometimes regarded as a separate family Cypripediaceae.

The classification of the largest subfamily, Epidendroideae, is still in a state of some flux: Chase & al. (2003) recognize the 3 tribes Epidendreae (confined to the New World), Vandeae and Cymbidieae, while Cameron (2006) accepts a larger number of molecularly well-supported tribes. Górniak & al. (2010) recognize 31 clades (tribes and subtribes) for the subfamily. The most recent classification is that of Chase & al. (2015), which recognizes 16 tribes and a total of 37 clades for Epidendroideae.

Succulence: Numerous orchids occur in semi-arid climates or as epiphytes in semi-arid niches, and can be considered to be truly succulent. Leaf succulence predominates, esp. amongst epiphytic taxa, but stem and root succulence also occurs. While the roots esp. of epiphytic orchids in general are somewhat thickened, they appear genuinely fleshy in many species (e.g. Kaushik 1983; Figueroa & al. 2008). Water-storage idioblasts are common in the cortex of the roots esp. of epiphytic taxa (Pridgeon in Stern (2014: 35)).

In stem-succulent species, the stem is either uniformly thickened over part or all its length (e.g. many species of Dendrobium), or succulence is confined to the basal internode or few internodes, and such thickened stem parts are termed a pseudobulb. The stem tissue of such orchids is usually parenchymatous and mucilaginous, with scattered large water-storage cells (Kaushik 1983). In many epiphytic Epidendroideae, a 1- to 5-layered hypodermis is present, with scleric, parenchymatous or collenchymatous cells and a “probable function in water storage” (Pridegon in Stern (2014: 27)). The ground tissue is continuous or commonly interrupted by a sclerotic sheath (referred to as sclerenchyma sheath, pericyclic ring, sclerenchymatous ring, or sclerenchyma band in the literature), and when present, this sheath delimits the outer cortex from the pith. It is often continous with the outermost vascular bundles, while the cortex is only rarely also provided with vascular bundles. The more peripheral cells of the cortex are usually smaller and contain chloroplasts, while the inner cells have fewer or no chloroplasts. Within the ground tissue, conspicuous larger water-storage idioblasts with or without spiral thickenings are common (Pridgeon in Stern (2014: 27)). According to Stern & Carlsward (2006), these storage cells are usually devoid of a protoplast.

Leaf-succulence is without doubt the prevailing type of succulence in the family, but the anatomy of water storage is variable. According to Metzler (1924), four different types of water storage anatomies can be distinguished, ranging from epidermal water storage to hypodermal or central water storage tissues, and to isolated water-storage idioblasts scattered in the photosynthetic parenchyma. A true hypodermis is present in many epiphytic Epidendroideae, commonly consisting of 1–4 (rarely up to 10) cell layers on the adaxial side, and 1–2 layers on the abaxial side. Its cells are often elongate and the hypodermis then resembles a palisade mesophyll that accounts for up to 80% of the leaf volume and “almost certainly functions in water storage” (Pridgeon in Stern (2014: 21)). The water-storage idioblasts show various wall sculpturings (annular or helical), and are usually devoid of protoplasts, except for Maxillariinae (Stern & Carlsward 2006: 265). The wall thickenings are cellulosic according to Pridgeon (1982). while Olatunji & Nengim (1980) report varying degrees of lignification. These mesophyll idioblasts likely serve for mechanical support to prevent tissue collapse during drought conditions. Olatunji & Nengim (1980) use the term “tracheoidal elements” for these cells, but they are not of tracheoidal origin and are better referred to as spirally thickened water-storage idioblasts. The mesophyll chlorenchyma can be differentiated into palisade and spongy mesophyll, or it is uniform spongy mesophyll, but conditions are variable even within genera (Pridgeon in Stern (2014: 22)). Pridgeon (1981) reports the presence of specialized trichomes on the leaves of species of subtribe Pleurothallidinae. The trichomes are likely comparable in function to the much better known water-absorbing multicellular trichomes in Tillandsia. They consist of an apical thin-walled cell and a stalk formed by 1 or 2 cells with thick lateral walls, and their bases are adjacent to the water-storing hypodermal layer.

Numerous studies have investigated the occurrence of CAM photosynthesis in the orchid family. While CAM is neither a prerequisite for succulence nor dependent on the existence of succulence in orchids, there is some correlation between the degree of succulence (measured as leaf thickness) and the degree of CAM expression. CAM is most prevalent in epiphytes from low elevations and markedly seasonal climates with a dry season (Earnshaw & al. 1987; Silvera & al. 2010). Winter & Smith (1996) estimate the number of CAM orchid species as c. 7100, which is close to the 7800 species estimate of Silvera & al. (2010). Conventional C3 photosynthesis has been identified as ancestral condition for the family, and CAM photosynthesis has evolved several times in parallel (Reyes-García and Andrade 2009).

The number of succulent orchid species is unknown, and is difficult to establish in view of the complete grade from slightly thickened and coriaceous leaves to distinctly succulent leaves. A first guess of c. 2200 species (Eggli 2007) is certainly far too low, and a recent superficial analysis (pers. obs.) results in a figure of at least 4500 succulent taxa.

Despite the undisputed claim to succulence, the orchids are not covered in detail in the present handbook series. Orchids rival with cacti and bromeliads as the horticulturally most important group of plants, and there is a vast selection of specialist literature, and numerous hobby groups are devoted to cultivating and admiring orchid diversity.

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Authors and Affiliations

  1. 1.Sukkulenten-Sammlung ZürichGrün Stadt ZürichZürichSwitzerland

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