Floral elaiophores in Lockhartia Hook. (Orchidaceae: Oncidiinae): their distribution,diversity and anatomy

Background and Aims

A significant proportion of orchid species assigned to subtribe Oncidiinae produce floral oil as a food reward that attracts specialized bee pollinators. This oil is produced either by glabrous glands (epithelial elaiophores) or by tufts of secretory hairs (trichomal elaiophores). Although the structure of epithelial elaiophores in the Oncidiinae has been well documented, trichomal elaiophores are less common and have not received as much attention. Only trichomal elaiophores occur in the genus Lockhartia, and their distribution and structure are surveyed here for the first time.

Methods

Flowers of 16 species of Lockhartia were studied. The location of floral elaiophores was determined histochemically and their anatomical organization and mode of oil secretion was investigated by means of light microscopy, scanning electron microscopy and transmission electron microscopy.

Key Results and Conclusions –

All species of Lockhartia investigated have trichomal elaiophores on the adaxial surface of the labellum. Histochemical tests revealed the presence of lipoidal substances within the labellar trichomes. However, the degree of oil production and the distribution of trichomes differed between the three major groups of species found within the genus. All trichomes were unicellular and, in some species, of two distinct sizes, the larger being either capitate or apically branched. The trichomal cuticle was lamellate, and often appeared distended due to the subcuticular accumulation of oil. The labellar trichomes of the three species examined using transmission electron microscopy contained dense, intensely staining cytoplasm with apically located vacuoles. Oil-laden secretory vesicles fused with the plasmalemma and discharged their contents. Oil eventually accumulated between the cell wall and cuticle of the trichome and contained electron-transparent profiles or droplets. This condition is considered unique to Lockhartia among those species of elaiophore-bearing Oncidiinae studied to date.     

Elaiophore structure and oil secretion in flowers of Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst (Oncidiinae: Orchidaceae)

BACKGROUND AND AIMS: Many orchid flowers have glands called elaiophores and these reward pollinating insects with oil. In contrast to other reward-producing structures such as nectaries, the anatomy of the elaiophore and the process of oil secretion have not been extensively studied. In this paper, elaiophore structure is described for two members of Oncidiinae, Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst. METHODS: Elaiophores of both species were examined using light microscopy, scanning electron microscopy and transmission electron microscopy. KEY RESULTS AND CONCLUSIONS: In flowers of Oncidium trulliferum and Ornithophora radicans, oil is secreted by morphologically distinct elaiophores associated with the labellar callus. However, in O. trulliferum, elaiophores also occur on the lateral lobes of the labellum. In both these species, the epithelial elaiophores are composed of a single layer of palisade-like epidermal cells and a distinct subepithelial layer. Secretory elaiophore cells may contain numerous, starchless plastids, mitochondria and smooth endoplasmic reticulum profiles. In O. trulliferum, the cytoplasm contains myelin-like figures but these are absent from O. radicans. In the former species, cavities occur in the cell wall and these presumably facilitate the passage of oil onto the elaiophore surface. In O. radicans, the accumulation of oil between the outer tangential wall and the cuticle causes the latter to become distended. Since it is probable that the full discharge of oil from the elaiophores of O. radicans occurs only when the cuticle is ruptured by a visiting insect, this may contribute towards pollinator specificity. The structure of the elaiophore in these species resembles both that found in previously investigated species of Oncidiinae and that of certain members of the Malpighiaceae.     

Comparative histology of floral elaiophores in the orchids Rudolfiella picta (Schltr.) Hoehne (Maxillariinae sensu lato) and Oncidium ornithorhynchum H.B.K. (Oncidiinae sensu lato)

Background and Aims

Floral elaiophores, although widespread amongst orchids, have not previously been described for Maxillariinae sensu lato. Here, two claims that epithelial, floral elaiophores occur in the genus Rudolfiella Hoehne (Bifrenaria clade) are investigated. Presumed elaiophores were compared with those of Oncidiinae Benth. and the floral, resin-secreting tissues of Rhetinantha M.A. Blanco and Heterotaxis Lindl., both genera formerly assigned to Maxillaria Ruiz & Pav. (Maxillariinae sensu stricto).

Methods

Putative, floral elaiophore tissue of Rudolfiella picta (Schltr.) Hoehne and floral elaiophores of Oncidium ornithorhynchum H.B.K. were examined by means of light microscopy, histochemistry, scanning electron microscopy and transmission electron microscopy.

Key Results and Conclusions

Floral, epithelial elaiophores are present in Rudolfiella picta, indicating, for the first time, that oil secretion occurs amongst members of the Bifrenaria clade (Maxillariinae sensu lato). However, whereas the elaiophore of R. picta is borne upon the labellar callus, the elaiophores of O. ornithorhynchum occur on the lateral lobes of the labellum. In both species, the elaiophore comprises a single layer of palisade secretory cells and parenchymatous, subsecretory tissue. Cell wall cavities are absent from both and there is no evidence of cuticular distension in response to oil accumulation between the outer tangential wall and the overlying cuticle in R. picta. Distension of the cuticle, however, occurs in O. ornithorhynchum. Secretory cells of R. picta contain characteristic, spherical or oval plastids with abundant plastoglobuli and these more closely resemble plastids found in labellar, secretory cells of representatives of Rhetinantha (formerly Maxillaria acuminata Lindl. alliance) than elaiophore plastids of Oncidiinae. In Rhetinantha, such plastids are involved in the synthesis of resin-like material or wax. Despite these differences, the elaiophore anatomy of both R. picta (Bifrenaria clade) and O. ornithorhynchum (Oncidiinae) fundamentally resembles that of several representatives of Oncidiinae. These, in their possession of palisade secretory cells, in turn, resemble the floral elaiophores of certain members of Malpighiaceae, indicating that convergence has occurred here in response to similar pollination pressures.Key words: Bifrenaria clade, elaiophore, floral oil, Heterotaxis, Maxillariinae, Oncidiinae, Oncidium ornithorhynchum, Rhetinantha, Rudolfiella picta, secretion     

Elaiophore diversity in three contrasting members of Oncidiinae (Orchidaceae)

The elaiophores of Trichocentrum cavendishianum (Bateman) M.W. Chase & N.H. Williams, Oncidium loefgrenii Cogn., and Gomesa recurva R. Br. display considerable morphological and anatomical diversity. Oil secretion by flowers of T. cavendishianum and O. loefgrenii can be related to the presence of saddle-like, labellar elaiophores and the labellar callus, respectively, whereas, in G. recurva , although oil is present, no obvious structure appears to be involved in its secretion. In the first two species, the secretory tissue consists of palisade-like cells, whereas, in G. recurva , these cells are oval. Many Oncidiinae are thought to mimic members of the Malpighiaceae, and the elaiophores of that family also contain palisade-like cells that may indicate evolutionary convergence. As oils accumulate below the elaiophore cuticle, that of T. cavendishianum becomes distended, whereas that of the other two species does not. Full discharge of oil from the elaiophores of T. cavendishianum probably occurs only after the cuticle is ruptured by a visiting insect, and this may contribute towards pollinator selection.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 155 , 135–148.     

Elaiophores in Gomesa bifolia (Sims) M.W. Chase & N.H. Williams (Oncidiinae: Cymbidieae: Orchidaceae): structure and oil secretion

Background and Aims

Oils are an unusual floral reward in Orchidaceae, being produced by specialized glands called elaiophores. Such glands have been described in subtribe Oncidiinae for a few species. The aims of the present study were to identify the presence of elaiophores in Gomesa bifolia, to study their structure and to understand how the oil is secreted. Additionally, elaiophores of G. bifolia were compared with those of related taxa within the Oncidiinae.

Methods

Elaiophores were identified using Sudan III. Their structure was examined by using light, scanning electron and transmission electron microscopy.

Key Results

Secretion of oils was from the tips of callus protrusions. The secretory cells each had a large, centrally located nucleus, highly dense cytoplasm, abundant plastids containing lipid globules associated with starch grains, numerous mitochondria, an extensive system of rough and smooth endoplasmatic reticulum, and electron-dense dictyosomes. The outer tangential walls were thick, with a loose cellulose matrix and a few, sparsely distributed inconspicuous cavities. Electron-dense structures were observed in the cell wall and formed a lipid layer that covered the cuticle of the epidermal cells. The cuticle as viewed under the scanning electron microscope was irregularly rugose.

Conclusions

The elaiophores of G. bifolia are of the epithelial type. The general structure of the secretory cells resembles that described for other species of Oncidiinae, but some unique features were encountered for this species. The oil appears to pass through the outer tangential wall and the cuticle, covering the latter without forming cuticular blisters.Key words: Elaiophore, Gomesa bifolia, Orchidaceae, Oncidiinae, oil secretion, anatomy, micromorphology, ultrastructure     

The evolution of beetle pollination in a South African orchid

The pollination biology of the orchid Ceratandra grandiflora was investigated in order to determine whether the partial loss of a specialized floral reward (i.e., oil) was the result of an incomplete shift from one specialized pollinator to another. In the three-species clade of section Ceratandra, there has been a progressive loss of the oil-secreting callus. lt is always present in C atrata, sometimes present in C. grandiflora, and never present in C. globosa. Thirty-nine to 67% of individuals in populations of C. grandiflora bear the callus gland, but gland presence has no signifikant effect on the proportion of flowers pollinated. Pollinator observations show that the shift in pollinators is complete and that the oil-secreting callus is a vestige of the ancestral oil-bee pollination system that no longer plays a role in pollination. C grandiflora is pollinated almost exclusively by a single species of hopliine beetle (Scarabaeidae). Experiments with artificial flower traps indicate that color alone can explain the attraction of beetles to C. grandiflora, despite the absence of a floral reward. The proportion of C. grandiflora flowers pollinated (50.2 and 61.1%; N = 524 and 324 flowers, respectively) is unusually high for a plant that relies on generalized food deception and is probably due to the use of inflorescences as mating sites (i.e., rendezvous pollination).     

Elaiophores in three Neotropical Malpighiaceae species: a comparative study

Malpighiaceae species are recognized for their sepal elaiophores of which secretions reward oil-collecting bees. Information on elaiophore location, structure and functioning is likely to provide valuable insights into pollination ecology and evolution of the family. We characterized the elaiophores in three Malpighiaceae species and compared the patterns of distribution and dimensions of these glands, their structural organization, their histochemistry and their life spans. Intact elaiophores from buds and 1-day flowers (bagged and un-bagged) of Banisteriopsis variabilis, Byrsonima coccolobifolia and Peixotoa reticulata were collected for structural, histochemical and ultrastructural studies. We also reported the behavior of elaiophore-visiting insects. Elaiophores exhibit uniseriate secretory epithelium covered by a thick cuticle and vascularized parenchyma. The secretory surfaces can be flat (B. coccolobifolia and P. reticulata) or convoluted (B. variabilis). In B. variabilis and P. reticulata the epithelium has longer cells than in B. coccolobifolia and these become papillose, taking an appearance similar to trichomal elaiophores. The mixed secretions accumulate within subcuticular spaces and may be released either by a natural rupture of the cuticle (B. coccolobifolia and P. reticulata) or by a sudden rupture caused by the bee activity (B. variabilis). Different bees were observed exploiting the elaiophores, acting as potential pollinators or oil robbers. A greater diversity of oil-collecting bees was registered in B. variabilis. The differences identified, mainly in relation to the location of the glands on the sepals, in the fine structure of secretory epithelia and cuticle architecture, and in their secretion release mechanisms, in some way, can affect the behavior of visitors.     

Osmophore and elaiophores of Grobya amherstiae (Catasetinae,Orchidaceae) and their relation to pollination

Grobya amherstiae flowers release a honey‐like scent produced by an osmophore, comprising a papillate epidermis. The scent attracts bee pollinators (Paratetrapedia fervida), which collect floral oils produced by elaiophores on the lip apex and column base. The secretory tissue of the elaiophore on the lip apex consists of both palisade‐like epidermal cells and conspicuously elongated unicellular trichomes. From an anatomical point of view, this elaiophore differs in structure from those known in angiosperms to date. The elaiophore on the column base is exclusively composed of short unicellular trichomes. In addition, there is an elaiophore comprising a papillate epidermis on the internal surface of the lip. The elaiophores produce a heterogeneous secretion, composed of fatty acids and mucilage. The elaiophore on the internal surface of the lip produces oil in non‐collectible amounts, but it is enough to maintain the interest of the bees, guiding them to the elaiophore on the column base, a necessary step in pollination. The former elaiophore is here identified as an oil guide and it plays an essential role in ensuring pollination. The presence of three types of elaiophores on the flowers of this species of Orchidaceae is peculiar and noteworthy. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 408–415.     

New evidence of floral elaiophores and characterization of the oil flowers in the subtribe Oncidiinae (Orchidaceae)

Elaiophores seems to be uncommon in Orchidaceae; however, the number of known species with floral oil glands has increased in recent years, principally in Oncidiinae. Oil rewards are used by bees of the tribes Centridini, Tapinotaspidini and Tetrapediini. Our aims were to identify the presence of elaiophores and to describe their structure in species of Gomesa, Grandiphyllum and Trichocentrum, and to compare our results with other studies of elaiophores in Oncidiinae. We selected a set of characters presumably associated with oil production in flowers of Oncidiinae, which were evaluated using a cluster analysis to identify different floral morphologies of the oil flowers. The correlation between morphological types of oil flowers and species of pollinators was examined. The cluster analysis distinguished two groups of species, one of them principally linked with pollination by bees of genus Centris and the other type associated to species of Paratetrapedia and Tetrapedia. The evaluation of these results into a phylogenetic framework of the Oncidiinae, adding more evidence that species of this subtribe with similar floral morphology associated with floral oil secretion arise in many independent clades, in parallel evolution with the oil-bee pollination.     

Oil-producing flowers within the Iridoideae (Iridaceae): evolutionary trends in the flowers of the New World genera

Background and Aims

Oil-producing flowers related to oil-bee pollination are a major innovation in Neotropical and Mexican Iridaceae. In this study, phylogenetic relationships were investigated among a wide array of New World genera of the tribes Sisyrinchieae, Trimezieae and Tigridieae (Iridaceae: Iridoideae) and the evolution of floral glandular structures, which are predominantly trichomal elaiophores, was examined in relation to the diversification of New World Iridaceae.

Methods

Phylogenetic analyses based on seven molecular markers obtained from 97 species were conducted to produce the first extensive phylogeny of the New World tribes of subfamily Iridoideae. The resulting phylogenetic hypothesis was used to trace the evolutionary history of glandular structures present in the flowers of numerous species in each tribe. Hypotheses of differential diversification rates among lineages were also investigated using both topological and Binary-State Speciation and Extinction methods.

Key Results and Conclusions

Floral glandular structures and especially trichomal elaiophores evolved multiple times independently in the American tribes of Iridoideae. The distribution pattern of species displaying glandular trichomes across the phylogeny reveals lability in the pollination system and suggests that these structures may have played a significant role in the diversification of the Iridoideae on the American continent.     

Comparative reproductive biology and pollinator specificity among sympatric Gomesa (Orchidaceae: Oncidiinae)

• The incredible pollination mechanisms displayed by orchid flowers has inspired biologists over the centuries. Based on the intriguing flower structures, the relationship among orchid species and their pollinators has been frequently regarded as very specialised.
• Given that visits on flowers pollinated by oil‐collecting bees are regularly rare, and in Oncidiinae the flowers frequently attractexclusively species that act as effective pollinators, the comparative reproductive biology and pollinator specificity of two sympatric Gomesa (G. varicosa and G. montana; Oncidiinae) were analysedbased on records of floral morphology, production of floral rewards, pollinators and pollination mechanisms. Furthermore, experimental pollinations were carried out in order to examine the breeding systems.
• The results have show that in the studied population, both Gomesa are visited by several bee species, but these orchids present a specific pollination system.Pollinaria are deposited on the head of Centridini (G. varicosa and G. montana) and Epicharitini (G. varicosa) bees when landed on the central callus of the labellumto collect lipoidal substances produced by glandular elaiophores on lateral lobes of the labellum. Both species are dependent on a biotic pollen vector to set fruits. Gomesamontana is completely self‐incompatible, while G. varicosa is partially self‐compatible.
• Our results indicate that although the occurrence of self‐sterile species seems to be common in Oncidiinae, in partially self‐incompatible species, as is the case of G. varicosa, self‐compatibility has been considered as an important factor favouring reproductive assurance in populations with low visitation frequencies, despite occurrence of inbreeding depression.

     

Labellar anatomy and secretion in Bulbophyllum Thouars (Orchidaceae: Bulbophyllinae) sect. Racemosae Benth. & Hook. f.

Background and Aims

Floral secretions are common in Bulbophyllum Thouars, and the labella of a number of Asian species are said to produce secretions rich in lipids that act as food rewards for insect pollinators. Although some of these reports are based on simple histochemical tests, a much greater number are anecdotal and, hitherto, neither the ultrastructure of the labellum nor the secretory process has been investigated in detail. Furthermore, sophisticated histochemical approaches have generally not been applied. Here, both the labellar structure and the secretory process are investigated for four species of Asian Bulbophyllum sect. Racemosae Benth. & Hook. f., namely Bulbophyllum careyanum (Hook.) Spreng., B. morphologorum Kraenzl., B. orientale Seidenf. and B. wangkaense Seidenf., and compared with those of unequivocal lipid-secreting orchids.

Methods

Labellar, secretory tissue was investigated using light microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry.

Key Results

The adaxial median longitudinal groove of the labellum contained secretory tissue comprising palisade-like epidermal cells, similar to those of certain lipid-secreting Oncidiinae Benth. However, these cells and their secretions gave positive results mainly for protein and mucilage, and their organelle complement was consistent with that of cells involved in protein and mucilage synthesis. Sub-cuticular accumulation of secretion resulted in cuticular distension and blistering. The sub-epidermal layer of isodiametric parenchyma contained starch and, like the epidermal cells, ultrastructure consistent with mucilage synthesis. Lipids were mainly confined to the cuticle, and hardly any intracellular lipid droplets were observed.

Conclusions

It is proposed that mucilage is produced by dictyosomes present in the palisade-like epidermal cells. Mucilage precursors may also be produced by these same organelles in sub-epidermal cells and are thought to pass along the symplast via plasmodesmata into the adjoining palisade-like secretory cells, which contain abundant arrays of rough endoplasmic reticulum. Here, they become chemically modified and form a protein-rich, mucilaginous secretion that, following vesicle-mediated transport across the cytoplasm, traverses the cell wall and accumulates in blisters formed from the distended cuticle. Rupture of these blisters releases the secretion onto the labellar surface. However, in certain species, there is some evidence that the secretion may traverse the cuticle via cuticular pores, and micro-channels may permit the passage of fragrance. Hydrolysis of sub-epidermal starch probably generates the carbohydrate and, together with mitochondria, much of the energy required for the secretory process. This anatomical organization resembles that found in certain lipid-secreting, Neotropical species of Bulbophyllum and Oncidiinae, but since the chemical composition of their secretions is different, and these taxa occur on a separate continent and have different insect pollinators, parallelism of floral anatomy is likely.     

Labellar micromorphology of two euglossine-pollinated orchid genera; Scuticaria Lindl. and Dichaea Lindl

Background and Aims

Until recently, there was no consensus regarding the phylogenetic relationships of the Neotropical orchid genera Scuticaria Lindl. and Dichaea Lindl. However, recent evidence derived from both gross morphological and molecular studies supports the inclusion of Scuticaria and Dichaea in sub-tribes Maxillariinae and Zygopetalinae, respectively. The present paper describes the labellar micromorphology of both genera and seeks to establish whether labellar characters support the assignment of Scuticaria and Dichaea to these sub-tribes.

Methods

The labella of four species of Scuticaria and 14 species of Dichaea were examined using light microscopy and scanning electron microscopy, and their micromorphology was compared with that of representative species of Maxillariinae sensu lato and Zygopetalinae (Huntleya clade).

Key Results and Conclusions

In most specimens of Scuticaria examined, the papillose labella bear uniseriate, multicellular, unbranched trichomes. However, in S. steelii (Lindl.) Lindl., branched hairs may also be present and some trichomes may fragment and form pseudopollen. Multicellular, leaf-like scales were also present in one species of Scuticaria. Similar, unbranched hairs are present in certain species of Maxillaria Ruiz & Pav. (Maxillariinae sensu stricto) and Chaubardia Rchb.f. (Huntleya clade). As yet, moniliform, pseudopollen-forming hairs have not been observed for Zygopetalinae, but their presence in Scuticaria steelii, Maxillaria and Heterotaxis Lindl. supports the placing of Scuticaria in Maxillariinae. As other genera are sampled, the presence of branched hairs, hitherto unknown for Maxillariinae sensu lato, may prove to be a useful character in taxonomy and phylogenetic studies. Euglossophily occurs in Dichaea, as well as Chondrorhyncha Lindl. and Pescatorea Rchb.f. (Huntleya clade), and all three genera tend to lack distinctive labellar features. Instead, lip micromorphology is relatively simple and glabrous or papillose. However, two of the Dichaea species examined bear unicellular, labellar trichomes very similar to those found in Bifrenaria Lindl. (pollinated by both euglossine bees and Bombus spp.), and this feature may have arisen by convergence in response to similar pollination pressures.Key words: Bifrenaria, Bifrenaria clade, Chaubardia, Chondrorhyncha, Dichaea, Dichaeinae, Heterotaxis, Huntleya clade, Huntleyinae, labellum, Maxillaria, Maxillariinae, papillae, Pescatorea, scales, Scuticaria, trichomes, Zygopetalinae     

Lipoidal labellar secretions in Maxillaria ruiz & pav. (Orchidaceae)

The labella of Maxillaria acuminata Lindl., M. cerifera Barb. Rodr. and M. notylioglossa Rchb.f., all members of the M. acuminata alliance, produce a viscid wax-like secretion. Histochemical analysis revealed that the chemical composition of the secretion is similar in all three species, consisting largely of lipid and protein. Light microscopy and low-vacuum scanning electron microscopy were used to investigate the secretory process. In a fourth taxon, M. cf. notylioglossa, transmission electron microscopy showed that lipid bodies are associated with smooth endoplasmic reticulum or occur as plastoglobuli within plastids. Lipid bodies vary in appearance and this may reflect differences in chemical composition. They become associated with the plasmalemma and eventually accumulate between the latter and the cell wall. The wall contains no pits or ectodesmata, and it is speculated that lipid passes through the wall as small lipid moieties before eventually reassembling to form lipid globules on the external surface of the cuticle. These globules are able to coalesce forming extensive viscid areas on the labellum. The possible significance of this process to pollination is discussed.     

Taxonomic separation of the genera Prosthechea and Encyclia (Laeliinae: Orchidaceae) using leaf and root anatomical features

Despite formal separation based on molecular and morphological evidence, the genera Prosthechea Knowles & Westc. and Encyclia Hook. have not been studied in terms of their vegetative anatomy. In this study we examine 16 Brazilian species of these genera. Additionally, one species of subtribe Laeliinae and another from subtribe Oncidiinae were studied in order to evaluate the taxonomic consistency of the anatomic characters observed. Except for Epidendrum crassifolium Lindl., all species possess a velamen differentiated into epivelamen and endovelamen. Endodermal cells with uniformly thickened walls, calcium oxalate raphides and cells with phi thickenings in the cortical region were observed in all species. Silica bodies, raphides and fibre bundles in the mesophyll were common in leaves of all species studied. By contrast, flavonoid crystals were observed in both roots and leaves of Prosthechea only. Encyclia is characterized by the presence of fibre bundles on the subepidermal layer, and a cuticle, which is clearly thicker than that of Prosthechea . These data strongly support the separation of Prosthechea and Encyclia .  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 293–303.     

Is the variation of floral elaiophore size in two species of Stigmaphyllon (Malpighiaceae) dependent on interaction with pollinators?

Background: Intraspecific variations in floral traits of species over its geographic range can be associated with differences in pollinator assemblages and/or with environmental conditions.

Aims: We evaluated the area of elaiophores in different populations of Stigmaphyllon bonariense (n = 9) and S. jatrophifolium (n = 6), and we hypothesised a marked reduction in their size towards their southern limits of distribution, associated with different oil-collecting bee assemblages.

Methods: Area of elaiophores was calculated and we carried out linear correlations with floral size, pollinators, visitation rate and pollinator size along the latitudinal gradient of the plants’ distributions. Moreover, we examined the relative size relationships using allometric analyses, to verify this reduction.

Results: Floral elaiophore area decreased with latitude. However, for S. bonariense we observed an allometric reduction in elaiophore area with respect to floral size, while for S. jatrophifolium an isometric reduction was found. In both species, pollinator richness and visitation rate did not diminish with latitude, but pollinator size for S. bonariense varied.

Conclusions: Our results show a reduction in the size of elaiophores in both species along their distribution range, with dissimilar tendencies, suggesting that these species may have different selection pressures that cause variation of their phenotypic traits.     

Fine structural observations on the epidermis

Summary In species of Apium, Eryngium and Humulus, the cuticular membrane of the petiole could be resolved into two parts, of which the inner one appeared amorphous and after staining appeared to be penetrated by an electron-dense reticulum, whereas the outer layer showed a lamellate structure consisting of electron-dense and electron-transparent plates, 50–80 Å in thickness. These layers are considered to correspond with the cuticular layer and the cuticle proper, respectively. In species of Abutilon and Rumex the cuticle proper did not exhibit the lamellate structure. In the leaves of Eryngium the outer lamellated structure was present in the cuticle of both young and mature leaves. Both the lamellate and non-lamellate types of the cuticle proper increased in thickness with age of the specimen. The results are discussed in relation to earlier investigations.