Pseudopollen in Dendrobium unicum Seidenf. (Orchidaceae): Reward or Deception?

BACKGROUND AND AIMS: In 1987, Kjellsson and Rasmussen described the labellar trichomes of Dendrobium unicum Seidenf. and proposed that these hairs function as pseudopollen. Pseudopollen is a mealy material that superficially resembles pollen, is usually laden with food substances and is formed when labellar hairs either fragment into individual cells or become detached from the labellum. However, the trichomes of D. unicum are very different from pseudopollen-forming hairs found in other orchid genera such as Maxillaria and Polystachya. Moreover, Kjellsson and Rasmussen were unable to demonstrate the presence of food substances within these trichomes and argued that even in the absence of food substances, the hairs, in that they superficially resemble pollen, can still attract insects by deceit. The aim of this paper is to investigate whether the labellar trichomes of D. unicum contain food reserves and thus reward potential pollinators or whether they are devoid of foods and attract insects solely by mimicry. METHODS: Light microscopy, histochemistry and transmission electron microscopy. KEY RESULTS: Dendrobium unicum produces pseudopollen. Pseudopollen here, however, differs from that previously described for other orchid genera in that the pseudopollen-forming trichomes consist of a stalk cell and a 'head' of component cells that separate at maturity, in contrast to Maxillaria and some Polystachya spp. where pseudopollen is formed by the fragmentation of moniliform hairs. Moreover, the pseudopollen of Maxillaria and Polystachya largely contains protein, whereas in D. unicum the main food substance is starch. CONCLUSIONS: Flowers of D. unicum, rather than attracting insects solely by deceit may also reward potential pollinators.     

Atypical pseudopollen-forming hairs in Maxillaria (Orchidaceae)

Pseudopollen is found on the labella of several species of Maxillaria and is formed by the fragmentation of uniseriate, multicellular, moniliform trichomes. The resultant component cells are rich in protein. Since flowers of pseudopollen-forming species generally lack nectar, it is probable that pseudopollen gradually replaced nectar as the pollinator reward. However, direct evidence for this hypothesis is lacking. The present paper examines the labellar micromorphology of five Brazilian species of Maxillaria assigned to the M. discolor alliance. The flowers of two of these species, namely M. violaceopunctata and M. villosa , produce both food hairs and a lipoidal, labellar secretion which is rich in aromatic amino acids. Moreover, in the case of M. violaceopunctata , the secretion was found to contain reducing sugars. This may represent an intermediate stage in the transition from nectar-producing to pseudopollen-forming flowers. SEM studies indicate how pseudopollen-forming, moniliform trichomes, such as those found in the Maxillaria grandiflora complex, may have evolved from simple, uniseriate, multicellular hairs.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 143 , 151−158.     

Trichome anatomy and taxonomy of Saxifraga (Saxifragaceae)

Trichomes on flower pedicels and leaves were examined in 213 species of Saxifraga L. and in the monotypic, segregate genera Zahlbrucknera Reichenb., Saxifragella Engl., Saxifragodes D. M. Moore and Saxifragopsis Small. Pedicel trichomes were found to be better taxonomic characters than foliar ones. Six main trichome types were identified: i) multiseriate, glandular; ii) uniseriate, glandular; iii) sessile, multicellular, glandular; iv) multiseriate, eglandular; v) uniseriate, eglandular; vi) unicellular, eglandular. The different types were distributed (except in certain species) on pedicels as follows. Multiseriate, glandular types occur in four series of Saxifraga sect. Micranthes (Haw.) D. Don, part of sect. Gymnopera D. Don, and in sects Ciliatae Haw., Trachyphyllum Gaudin, Xanthizoon Griseb., Ligulatae Haw., Porophyllum Gaudin, Porphyrion Tausch and Irregulares Haw. They also occur in the genus Saxifragopsis. Uniseriate, glandular types occur in five series of Saxifraga sect. Micranthes , part of sect. Gymnopera and Irregulares , and in sects. Miscopetalum (Haw.) Sternb., Cymbalaria , Griseb. Tridactylites (Haw.) Engl., Saxifraga and Stoloniferae Haw. They also occur in the genus Zahlbrucknera. Eglandular, multicellular trichomes occur occasionally in Saxifraga sects Micranthes and Gymnopera , more frequently in sects Ciliatae and Ligulatae , and are ubiquitous in sects Xanthizoon, Porophyllum and Porphyrion. Eglandular, unicellular trichomes occur only in Saxifragopsis and Saxifragodes. Sessile, glandular trichomes occur in a group of species from Saxifraga sect. Stoloniferae. Saxifragella is glabrous. This taxonomic distribution of trichome types generally supports Engler's classification of the genus, and is compatible with other evidence from palynology, embryology and seed morphology. The taxonomic disposition of the few exceptional species is discussed.     

Pseudopollen in Eria Lindl. section Mycaranthes Rchb.f. (Orchidaceae)

BACKGROUND AND AIMS: Pseudopollen is a whitish, mealy material produced upon the labella of a number of orchid species as labellar hairs either become detached or fragment. Since individual hair cells are rich in protein and starch, it has long been speculated that pseudopollen functions as a reward for visiting insects. Although some 90 years have passed since Beck first described pseudopollen for a small number of Eria spp. currently assigned to section Mycaranthes Rchb.f., we still know little about the character of pseudopollen in this taxon. The use of SEM and histochemistry would re-address this deficit in our knowledge whereas comparison of pseudopollen in Eria (S.E. Asia), Maxillaria (tropical and sub-tropical America), Polystachya (largely tropical Africa and Madagascar) and Dendrobium unicum (Thailand and Laos) would perhaps help us to understand better how this feature may have arisen and evolved on a number of different continents. METHODS: Pseudopollen morphology is described using light microscopy and scanning electron microscopy. Hairs were tested for starch, lipid and protein using IKI, Sudan III and the xanthoproteic test, respectively. KEY RESULTS AND CONCLUSIONS: The labellar hairs of all eight representatives of section Mycaranthes examined are identical. They are unicellular, clavate with a narrow 'stalk' and contain both protein and starch but no detectable lipid droplets. The protein is distributed throughout the cytoplasm and the starch is confined to amyloplasts. The hairs become detached from the labellar surface and bear raised cuticular ridges and flaky deposits that are presumed to be wax. In that they are unicellular and appear to bear wax distally, the labellar hairs are significantly different from those observed for other orchid species. Comparative morphology indicates that they evolved independently in response to pollinator pressures similar to those experienced by other unrelated pseudopollen-forming orchids on other continents.     

Scanning electron microscopy of silica bodies and other epidermal features in Gibasis (Tradescantia) leaf

The scanning electron microscope is used to examine epidermal preparations belonging to three species of Gibasis , a genus allied to Tradescantia. The surface topography of the leaf is exposed for investigation at high magnifications, the observations add new information about structural projections from the surface and contribute to an understanding of silica deposition and its organization. Hairs of three main types occur: (1) short two-celled trichomes, hook or prickle hairs, (2) long uniseriate hairs consisting of four to six cells, (3) three-celled glandular micro-hairs. The arrangement and shape of epidermal cells and stomata are reported. Epidermal papillae were observed for the first time in two of the three species; their morphology is described in detail and their spacing expressed mathematically. Variations in the structure and distribution of silica cells are depicted, and new aspects of the silica bodies are displayed in relief. The taxonomic significance of these anatomical characters is discussed in relation to species and chromosome numbers.     

LEAF HAIRS IN ENCELIA (ASTERACEAE)

Four different kinds of leaf hairs occur in Encelia species. These are unicellular-based and multicellular-based uniseriate hairs, moniliform hairs, and biseriate glandular hairs. The unicellular-based uniseriate hairs appear responsible for increased leaf spectral reflectance by species within the genus. In particular, it appears that elongation of the distal cell of the uniseriate hair is necessary for increased leaf reflectance.     

Ultrastructure of the labellar epidermis in selected Maxillaria species (Orchidaceae)

The ultrastructure of the labellar epidermis of 13 species of Maxillaria and one hybrid was examined using low-vacuum scanning electron microscopy (SEM). The labellum may be homogeneous and glabrous or papillose, comprising one type of cell only, or heterogeneous with papillae, uniseriate trichomes and/or glands in various combinations. The trichomes are unbranched and multicellular with pointed or truncated tips. Moreover, in some taxa, moniliform trichomes occur, and these are thought to fragment with the formation of pseudopollen. Homogeneous and heterogeneous labellar organization may represent separate evolutionary lines. Preliminary results suggest that labellar features may provide additional taxonomic characters allowing determination of intrageneric affinities.     

Variable responses of tuber moth to the leaf trichomes of wild potatoes

This study examines the response of tuber moth, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae), during the initial stages of attack, to variability in trichome density and composition on foliage of Solanum berthaultii (Hawkes) and Solanum tarijense (Hawkes) (Solanaceae). Solanum berthaultii bears two types of glandular trichome (type A and type B) that together reduced oviposition by the moth. Females were often completely deterred from ovipositing on foliage with >300 trichomes per cm². In contrast, neonate establishment on S. berthaultii was generally positively related to trichome densities, indicating that trichomes may be a poor defense against P. operculella when the moth oviposits in soil and neonate larvae select the host plant. Solanum tarijense has only one type of glandular trichome (type A) and eglandular hairs. Most eggs were deposited on the adaxial leaf surfaces that had lower trichome densities. Although the density of type A trichomes was negatively related to oviposition, high densities of hairs on the abaxial and adaxial leaf surfaces appeared to stimulate oviposition, leading to stronger positive relations between hair densities and oviposition. Larvae generally established on the abaxial surface where hair densities were greatest. Relationships between the abaxial densities of leaf hairs and neonate establishment on S. tarijense were positive. The results indicate that the responses by P. operculella to the types and density of trichomes are complex. Whereas type A and type B trichomes may act synergistically to reduce oviposition by the moth, leaf hairs do not defend against oviposition and neither leaf hairs nor type A and B trichomes reduce neonate establishment by this herbivore species.     

Systematic distribution of foliar trichome types in Crotovt (Euphorbiaceae)

Variation in foliar trichomes in Croton (Euphorbiaceae) is reviewed, and the terminology clarified by redefinitions and illustrations of terms. Evolution of trichome types appears to proceed from branched (stellate or fasciculate) hairs to simple and dendritic ones. A systematic enumeration characterizes the trichome morphology in 36 of the 40 sections, with citation of the 120 species examined.     

Characteristic Trichomes and Indumentum Specialization in African and European Parasitic Scrophulariaceae

Trichomes and their distribution patterns have been observed in the aerial parts of plants of the genera Striga, Buchnera, Rhamphicarpa, Euphrasia, Melampyrum and Rhinanthus, using both light (LM) and scanning electron microscopy (SEM). All trichomes belong to the multicellular, uniseriate category and can be classified as glandular and nonglandular types. On nearly all organs, the two types can be observed side by side. Apart from a large group of ubiquitous trichomes, some characteristic trichome types are reported; they are restricted either to chlorophyllous organs (foliage leaves, bracts, calyx and stem) or to the corolla. In some nonglandular trichomes mineral deposits have been found. Other morphological and anatomical trichome features are discussed with regard to their possible function.     

Evolution of trichome types and its systematic significance in the genus Phlomoides (Lamioideae–Lamiaceae)

Despite a number of recent molecular phylogenetic studies on Phlomoides, in terms of trichome morphology the genus is still among the most poorly studied taxa in the family Lamiaceae. In order to test the utility of trichome characters for delimitation of sections, subsection and species of Phlomoides, we examined trichomes of 64 species representing all recognized sections and subsections using stereomicroscopy and scaning electron microscopy. Two basic types of trichomes could be identified: non‐glandular and glandular. Both trichome types can be simple or branched. The glandular trichomes were sessile, short stalked or long stalked. Different kinds of branched trichomes were observed in most species of P. sect. Phlomoides, i.e. symmetrically stellate, stellate with a central long branch, bi‐ or trifurcate. The species of P. sect. Filipendula were mostly covered by simple trichomes. Moreover, variation in trichome characters appears to have particular value, not only for the classification at sectional or subsectional rank, but also for delimitation of species within each section. For example, all studied species of P. subsect. Fulgentes are characterized by various kinds of stellate trichomes, while the trichome variability in P. subsect. Tetragonae was sufficiently high for species discrimination. An ancestral character state reconstruction was performed in order to investigate the evolution of trichome types and it revealed the following evolutionary trends in trichome characters of Phlomoides: 1) branched trichomes are primitive in Phlomoides as compared to simple ones, 2) long simple non‐glandular trichomes are derived as compared to short simple ones and 3) the presence of stalked glandular trichomes is advanced as compared to subsessile or sessile ones.     

Leaf epidermis of Quercus subgen. Cyclobalanopsis (Oerst.) Schneid. (Fagaceae)

The characters of leaf epidermis (mainly epidermal cells, stomata, trichomes) have been examined in 48 species of Quercus subgen. Cyclobalanopsis of the Fagaceae. The adaxial leaf epidermis was investigated under light microscope. The epidermal cells of this surface are of two types in terms of the outline of their anticlinal wall. One type is characterized by having straight wall, and this type has been observed in most of the species studied, while the other is characterized by having sinuate or sinuolate wall, and this type has been found only in eight species, such as Q. blakei and Q. hui. There exist two types of trichome-bases on this surface: in one type the bases are singlecelled, and this type has been found in most of the species studied, such as Q. oxyodon and Q. augustinii; while in the other the bases consist of a group of cells, and this type has been found only in nine species, such as Q. thorelii and Q. chungii. The abaxial leaf epidermis was investigated under light microscope and scanning electron microscope. The epidermal cells and the trichome-bases on this surface under LM are in morphology essentially similar to those on the adaxial surface. The stomata under LM include three types: cyclocytic, subcyclocytic and anomocytic. The stomatal density is of some diagnostic value to distinguish between closely related species, e.g. the stomatal density in Q. glauca is higher than that in Q. schottkyana. Under SEM the stomata are elliptical or roundish. The most noteworthy feature of the adaxial leaf epidermis is the trichomes. Eight types of trichomes have been observed under SEM: simple-uniseriate, solitary, papillae, stellate, fused-stellate, stipitate-fasciculate, multiradiate and jellyfish-like. The first type, i.e. the simple-uniseriate trichome, is glandular, other six types, i.e. the solitary, papillae, stellate, fused-stellate, stipitate-fasciculate and multiradiate trichomes, are non-glandular, and the last one, i.e. the jellyfish-like trichome, may represent an intermediate type in structure between the glandular and the non-glandular trichomes. Papillae are very common in subgen. Cyclobalanopsis although in the Fagaceae they were previously reported only in a few species of Lithocarpus. The jellyfish-like trichome, observed only in Q. sichouensis, is reported in the Fagaceae for the first time. The evolutionary trends of trichomes are discussed. The trichomes seem to evolve toward the direction of increasing complexity in structure. Papillae are considered as the pedomorphism of trichomes. From papillae the solitary trichomes and then the stellate ones have evolved. The complicated trichome types with more arms may be produced from relatively simple ones with fewer arms by means of varions fusion: several stellate trichomes are fused to form the fused-stellate ones by basal fusion, and by the same way the fused-stellate ones to form the stipitate-fasciculate ones; the multiradiate trichomes are formed by the stellate ones, in which the arms emerge in a variety of seemingly random directions from a typically rounded common base. Based on the above results, it is considered that the trichome features are of important significance for a better understanding of the infrageneric division of Quercus and the phylogenetic relationships between this genus and the other genera in the Fagaceae. The very common presence of papillae in Quercus subgen. Cyclobalanopsis and in a few species of Lithocarpus may indicate that subgen. Cyclobalanopsis is a quite natural group on the one hand, and may have close relationship with Lithocarpus on the other hand. The thin-walled, jellyfish-like trichomes found in Q. sichouensis are in structure similar to the “glandular” peltate trichomes in Trigonobalanus, a genus generally regarded as the most primitive in subfam. Quercoideae, indicating that the affinity between subgen. Cyclobalanopsis and Trigonobalanus may becloser than that between subgen. Quercus and Trigonobalanus.     

栎属青冈亚属（壳斗科）的叶表皮研究

利用光镜和扫描电镜观察了栎属青冈亚属Quercus subgen. Cyclobalanopsis 48种植物的叶表皮,尤以对叶下表皮的毛被特征观察较为仔细。共观察到8种不同类型的叶表皮毛：单列毛、单毛、乳突、星状毛、溶和星状毛、具柄束毛、多出毛和水母状毛。其中乳突在青冈亚属中较常见,而在壳斗科其他属中仅在石栎属 Lithocarpus 少数种类中有报道;水母状毛首次在壳斗科中发现。毛被可能遵循以下的演化规律：乳突→单毛→星状毛;星状毛依照从简单→复杂的演化途径,分化出各种形态各异和结构复杂的毛系。初步讨论了毛被以及叶表皮其他特征(如毛基细胞和表皮细胞的形态、气孔的类型和密度等)的分类和系统学意义。     

中国猕猴桃属植物叶表皮毛策形态特征及数量分类分析

选取国产猕猴桃３５个分类群的代表植株,应用光学显微对其新鲜叶表皮毛的微形态特征,形体大小、细胞结构、分布和密度等多态性状和数量性状进行了观察和测量,该属植物的叶表皮毛微形态特征可归纳为６个类型：１）单细胞毛;２）单列多细胞毛,每形单列毛、泡状单列毛、扭曲毛、直壁单列毛、曲壁单列毛;３）多列渐尖毛和急尖毛,包括多列曲壁渐尖毛和急尖毛,多列直壁渐尖毛和急尖毛;４）多列粗毛,包括柱状毛,多列渐尖粗毛,多     

TOMATO TRICHOMES AND MUTATIONS AFFECTING THEIR DEVELOPMENT

A new trichome type for the genus Lycopersicon is described in L. esculentum Mill. It is a short (0.03–0.08 mm), pendant, glandular hair with a club-shaped head consisting of 8–12 cells. Two previously described “hairless” mutations were examined microscopically. One, hl, does not affect the frequency of hairs nor the number of cells per hair, but causes abnormal enlargement of the stalk cells of all hair types, and thus produces shortened, extremely bent and twisted hairs. Observations on the time of action of this gene indicate that in trichome development two to four cell divisions occur prior to any appreciable cell enlargement. The second mutation, h, affects only the large type of trichome. This mutation effects a developmental shift from trichome to stomatal apparatus at the apex of the multicellular base normally supporting the large trichomes.     

Glandular Trichomes ofCalceolaria adscendensLidl. (Scrophulariaceae): Histochemistry,Development and Ultrastructure

This paper reports the results of a study of the morphology and development of glandular trichomes in leaves ofCalceolaria adscendensLidl. using light and electron microscopy. Secretory trichomes started as outgrowths of epidermal cells; subsequent divisions gave rise to trichomes made up of a basal epidermal cell, a stalk cell and a two-celled secretory head. Ultrastructural characteristics of trichome cells were typical of terpene-producing structures. Previous phytochemical studies had revealed thatC. adscendensproduces diterpenes. Comparison withC. volckmanni,which produces triterpenes, and has trichomes with eight-celled secretory heads, suggests that there could be a relationship between the type of glandular trichome and the class of terpene produced. Further work is needed to test the hypothesis and to develop trichome characters as taxonomic tools.     

Quantitative taxonomic analyses of Actinidia (Actinidiaceae) in China based on micromorphological characters of foliar trichomes

There are various arguments on classification of the genus Actinidia Lindl., a genus with approximately 63 species, 59 of which have been found in China. The paper investigated the characteristics of foliar trichomes of 35 taxa from China under optical microscope, including size, celluar structure, distribution and density. According to their micromorphological characteristics, foliar trichomes can be classified into the following six categories: 1) single-cell hairs; 2) uniseriate hairs, including linear, bulbous, twisted, straight-walled, and bent-walled hairs; 3) multiseriate hairs, including twisted, straight-walled and gradually sharpening, straight-walled and suddenly sharpening, bent-walled and gradually sharpening, and suddenly sharpening hairs; 4) multiseriate thick hairs, including pillar hairs, gradually sharpening thick hairs, and suddenly sharpening thick hairs; 5) stellate hairs, including parenchyma-stellate and sclerenchyma-stellate (normal state and special states such as rosulate, peltate-stellate, and overlopping-stellate) hairs; and 6) dichotomous hairs. On the basis of the micromorphological characteristics of foliar trichomes in Actinidia, with Clematoclethra lasioclada as an outgroup, both the quantitative cladistic analysis and phenetic analysis were performed using Wagner method and UPGMA clustering method respectively to reconstruct the phylogeny of Actinidia in China. The phylogenetic tree generated by cladistic analysis suggests that the sect. Leiocarpae be a monophyletic group, but other three sections, i.e., sect. Maculatae, sect. Strigosae and sect. Stellatae, be non-monophyletic groups. The results obtained from the phenetic analysis reflect relationships among the taxa of Actinidia in China, especially a close relationship between A. chinensis and A. deliciosa, and a relatively remote relationship between A. callosa var. henryi and A. callosa var. discolor. In conclusion, the micromorphological characters of foliar trichomes and the methods of quantitative taxonomic analysis are of key importance tostudies on phylogenetic and phenetic relationships of Actinidia.     

SIAMESE, a gene controlling the endoreduplication cell cycle in Arabidopsis thaliana trichomes

Cell differentiation is generally tightly coordinated with the cell cycle, typically resulting in a nondividing cell with a unique differentiated morphology. The unicellular trichomes of Arabidopsis are a well-established model for the study of plant cell differentiation. Here, we describe a new genetic locus, SIAMESE (SIM), required for coordinating cell division and cell differentiation during the development of Arabidopsis trichomes (epidermal hairs). A recessive mutation in the sim locus on chromosome 5 results in clusters of adjacent trichomes that appeared to be morphologically identical 'twins'. Upon closer inspection, the sim mutant was found to produce multicellular trichomes in contrast to the unicellular trichomes produced by wild-type (WT) plants. Mutant trichomes consisting of up to 15 cells have been observed. Scanning electron microscopy of developing sim trichomes suggests that the cell divisions occur very early in the development of mutant trichomes. WT trichome nuclei continue to replicate their DNA after mitosis and cytokinesis have ceased, and as a consequence have a DNA content much greater than 2C. This phenomenon is known as endoreduplication. Individual nuclei of sim trichomes have a reduced level of endoreduplication relative to WT trichome nuclei. Endoreduplication is also reduced in dark-grown sim hypocotyls relative to WT, but not in light-grown hypocotyls. Double mutants of sim with either of two other mutants affecting endoreduplication, triptychon (try) and glabra3 (gl3) are consistent with a function for SIM in endoreduplication. SIM may function as a repressor of mitosis in the endoreduplication cell cycle. Additionally, the relatively normal morphology of multicellular sim trichomes indicates that trichome morphogenesis can occur relatively normally even when the trichome precursor cell continues to divide. The sim mutant phenotype also has implications for the evolution of multicellular trichomes.