Chap4 Morphology

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    MORPHOLOGY

    4Food-Hair Form and Diversificationin Orchids

    KEVIN. L. DAVIES

    Contents

    Introduction .............................................................................................................................. 160Food-Hairs and Pseudopollen .................................................................................................. 161Terminology ............................................................................................................................. 163Occurrence of Pseudopollen .................................................................................................... 165Trichome Morphology ............................................................................................................. 169Development of Pseudopollen ................................................................................................. 170Food Content and Ecology ....................................................................................................... 170

    Trichomal Secretion of Resin-Like and Waxy Materials ......................................................... 172Trichomal Elaiophores ............................................................................................................. 173Evolution of Food-Hairs and Food-Hairs as Taxonomic Characters ....................................... 176Conclusions .............................................................................................................................. 177Glossary ................................................................................................................................... 178Literature Cited ........................................................................................................................ 180

    Kull, Arditti and Wong (eds.), Orchid Biology: Reviews and Perspectives,X, Springer Science + Business Media B.V. 2009

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    160 K.L. Davies

    Introduction

    Orchids are renowned for their diverse and often elaborate pollination strategies

    (van der Pijl and Dodson, 1969; van der Cingel, 2001). Some reward pollinatorswith food (e.g., nectar, food-hairs and oils), floral fragrances and other compoundssuch as resin-like substances and wax (van der Pijl and Dodson, 1969; Dressler,1990, 1993; van der Cingel, 2001) and these rewards, in turn, reinforce pollinatorforaging behaviour (van der Pijl and Dodson, 1969; Proctor and Yeo, 1975;Dressler, 1990; Proctor, Yeo, and Lack, 1996; van der Cingel, 2001). Many, how-ever, produce no rewards whatsoever, and here attraction by mimicry and deceittend to predominate (Porsch, 1908; van der Pijl and Dodson, 1969; Ackerman,1984; Neiland and Wilcock, 1998, 2000; van der Cingel; 2001). In fact, some one-third of orchid species attract potential pollinators solely by deceit (Ackerman,1984) and it is thought that deceptive pollination evolved from reward-mediatedpollination systems (Ackerman, 1986). The former may involve complex mimicrystrategies such as food-fraud, pseudocopulation and pseudoantagonism (van derPijl and Dodson, 1969; Dressler, 1990; van der Cingel, 2001) and once attracted tothe flower by olfactory and visual cues, the precise configuration of the floral parts,the presence of honey guides and tactile stimuli provided by floral hairs and papil-lae ensure that orientation of the insect upon the flower is optimal for pollination.

    Although the rewardless condition is common amongst orchids, a significantnumber of species, nonetheless, produce food rewards (van der Pijl and Dodson,1969; Dressler, 1990, 1993; van der Cingel, 2001). Many angiosperm familiesreward pollinators with pollen (Proctor and Yeo, 1975; Proctor et al., 1996).However, that of epidendroid orchids is bound within pollinia and is thus inacces-sible to foraging insects (van der Pijl and Dodson, 1969; Dressler, 1990, 1993; vander Cingel, 2001). Even so, floral, food rewards such as nectar, food-hairs and floraloils play an important role in the successful pollination of many orchids (van derPijl and Dodson, 1969; Proctor and Yeo, 1975; Dressler, 1990; Proctor et al., 1996;van der Cingel, 2001) and their effectiveness in the attraction of pollinators hasbeen convincingly demonstrated for a number of species (Dafni and Ivri, 1979;

    Inoue, 1986; Johnson and Bond, 1997; Johnson and Nilsson, 1999; Neiland andWilcock, 1994, 1998, 2000; Smithson, 2002). Moreover, they have been shown tobe potent even in small quantities (Ackerman, Rodriguez-Robles, and Melndez,1994) and Neiland and Wilcock (1998) have reported that species that offer rewardsoften double their chances of developing fruit and seed. However, reward productionand the subsequent processes of fruit- and seed-maturation are costly both in termsof materials and energy expenditure and this may outweigh the benefits (Southwick,1984; Pyke, 1991; Ackerman et al., 1994; Melndez-Ackerman, Ackerman, andRodriguez-Robles, 2000 and references therein). Despite the cost, floral rewards, nev-

    ertheless, generally confer evolutionary advantage.Although primitive orchid species reward pollinators with pollen (Kocyan and

    Endress, 2001; Sugiura, Miyazaki, and Nagaishi, 2006), nectar is the most commonfood reward in Orchidaceae and, in some species this is secreted by trichomes.

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    4 Food-Hair Form and Diversification in Orchids 161

    Thus, floral food-hairs can conveniently be divided into four main categories: pro-tein- or starch-containing trichomes including pseudopollen that are nibbled orgathered by insect pollinators (Janse, 1886; Porsch, 1905, 1906; Beck, 1914; vander Pijl and Dodson, 1969; Davies and Winters, 1998; Davies, Winters, and Turner,

    2000; Davies, Roberts, and Turner, 2002; Davies, Turner, and Gregg, 2003a; Daviesand Turner, 2004a, b, c; Matusiewicz, Stpiczyska, and Davies, 2004; Davies andStpiczyska, 2006), resin-secreting trichomes (Porsch, 1905; von Kirchner, 1925;Macpherson and Rupp, 1935; van der Pijl and Dodson, 1969; Roberto Vsquez andDodson, 1982; Davies et al., 2002, 2003a; Davies, Turner, and Gregg, 2003b;Davies and Turner, 2004a; Flach et al., 2004; Matusiewicz et al., 2004; Davies andStpiczyska, 2006; Davies, Stpiczyska and Turner, 2006), oil-secreting, trichomalelaiophores (Vogel, 1974; Buchmann, 1987; Toscano de Brito, 2001; Mickeliunas,Pansarin, and Sazima, 2006) and nectar-secreting trichomes such as those lining the

    nectar spurs of Aeranthes arachnites (Thouars) Lindl., A. grandiflora Lindl.(Roberts, 2001), Platanthera bifoliaL. (Stpiczyska, 1997), P. chlorantha(Custer)Rchb. (Stpiczyska, 2003; Stpiczyska et al., 2005) and Gymnadenia conopsea(L.)R. Br. (Stpiczyska and Matusiewicz, 2001). This fourth category, however, liesbeyond the scope of this paper.

    Food-Hairs and Pseudopollen

    The development of food-hairs (including pseudopollen) in orchids represents amajor stride in their evolution. Pseudopollen is a mealy material superficially resem-bling pollen and is produced as food-hairs become detached or fragment to formindividual or small groups of cells. It is significant that flowers that produce pseudo-pollen tend to lack nectar (van der Pijl and Dodson, 1969) and, with the exceptionof some notable examples such as members of theMaxillaria grandiflora(Humb.,Bonpl. & Kunth) Lindl. andMaxillaria lepidotaLindl. alliances, fragrance percepti-ble to humans (Flach et al., 2004; Singer et al., 2006). Moreover, a mentum, possiblya vestigial nectary spur, is usually present in pseudopollen-producing and rewardless

    species ofMaxillariaRuiz & Pav., indicating that pseudopollen may have replacednectar as the main reward and that the production of nectar preceded the rewardlesscondition in that genus (Davies et al., 2003a). Indeed, Davies et al. (2005) estimatethat some 56% of Maxillariaspp. are rewardless, 1623% produce pseudopollen,13% produce wax or a viscid, resin-like material rich in lipids and aromatic aminoacids and only 8% produce nectar.

    Food-hairs, including pseudopollen-forming hairs, usually occur upon the label-lum (Figs. 4-14-13). They contain rich reserves of food, mainly protein, and aregathered or nibbled by insects. Of those orchid species that offer food rewards,

    most offer one type only. In some cases, however, several types of food reward mayoccur in a single species. For example, Maxillaria violaceopunctataRchb.f. and

    M. lepidota produce both food-hairs and a resin-like substance (Davies et al.,2003a; Matusiewicz et al., 2004) and food-laden papillae and viscid material have

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    162 K.L. Davies

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    4 Food-Hair Form and Diversification in Orchids 163

    Figs. 4.14.13. Examples of food-hairs and pseudopollen in orchids. Fig. 4-1: Labellum ofPolystachya foliosa(Hook.f.) Rchb.f. (sect. PolystachyaRchb.f.) showing uniseriate, moniliform,pseudopollen-forming hairs. Scale bar = 100m. Fig. 4-2: Labellar surface ofMaxillaria johni-anaKraenzl. (M. grandiflora(Humb., Bonpl. & Kunth) Lindl. alliance) with similar pseudopol-len-forming hairs. Scale bar = 100m.Fig. 4-3: Detail of above showing uniseriate, moniliformtrichomes consisting of ellipsoid cells. Scale bar = 25m. Fig. 4-4: Transmission electronmicro-graph of section through trichome cell ofMaxillaria sanderianaRchb.f. (M. grandiflora(Humb.,Bonpl. & Kunth) Lindl. alliance) showing cluster of amyloplasts containing starch grains (A) andamorphous protein body (P). Scale bar = 2.5 m (by kind permission of the editor of The Annalsof Botany). Fig. 4-5: Labellar surface of Maxillaria ochroleucaLodd. ex Lindl. (M. splendensPoepp. & Endl. alliance) showing few-celled trichomes that ar