Stigmatic Chamber Research Articles

While Prosopanche (Hydnoraceae) flowers gently heat: mutualistic pollination relationships among the perianth-bearing Piperales

Abstract Flowers of most Piperales do not reward pollinators. However, a few mutualistic pollination relationships have been proposed among the perianth-bearing species. To test the hypothesis of a mutualistic relationship between Prosopanche and beetle pollinators, we studied the pollination biology of three species (P. americana, P. bonacinae, P. panguanensis). For all three species, we recorded flower visitors and flower volatile organic compounds (VOCSs). In addition, for P. americana we investigated flower phases, thermogenesis, visitors’ behaviour, and viability of transported pollen. Using a behavioural experiment, we identified the role of flower heat and fragrance in pollinator attraction. We recorded Neopocadius nitiduloides and Lasiodactylus sp. sap beetles (Nitidulidae) as main pollinators and Hydnorobius hydnorae and H. helleri weevils (Belidae) as occasional pollinators. Thermogenic female flowers heat up to 8°C above the ambient temperature. Flowers only trap the small-sized sap beetles. Methyl-3-methyl-2-butenoate dominated the VOC profile. This was a powerful attractant for sap beetles in controlled bioassays. We conclude that pollination in Prosopanche is mutualistic. This was observed through a pollinator-size-based access limit to the stigmatic chamber and a strong olfactory attractant. Mutualism in Prosopanche therefore contrasts with that of most perianth-bearing Piperales, suggesting it is a novel pollination relationship in early-diverging angiosperms.

Pollen tube growth in Calotropis procera is controlled by environmental changes: does it have an impact on delayed fertilization?

Calotropis procera (Apocynaceae) is a Sudanian plant that grows throughout the eastern Saharo-Arabian region. In Israel, it grows along the Rift Valley under extremely hot and dry climatic conditions. In C. procera, as in many other Apocynaceae, the nectar is secreted in the flowers from the nectaries located inside the stigmatic chamber, with the excess flowing via the capillary system into special reservoirs (cucculi). The nectar has two functions: it is used as a reward to attract pollinating insects and it serves as the germination medium for pollen grains. Under natural conditions the nectar concentration is subjected to a large variability, ranging from 22-68% sucrose. The aim of this study was to elucidate the effects of the natural fluctuations of nectar concentration on pollen germination and pollen tube growth, and their possible role in delaying fertilization in Calotropis procera. We followed the process of pollen germination under various experimental sucrose concentrations simulating the nectar. We found that the optimal concentration of a sucrose medium for pollen germination is 20%. However, if the already-germinated pollen grains are subjected to high sucrose concentration for different periods of time (between one and three hours), elongation of the pollen tubes is inhibited. In all the experimental groups, the pollen tubes renewed their elongation following a reduction of the sucrose. In conclusion, we found that C. procera pollen grains’ germination is able to adjust to the large fluctuations in sucrose concentration, caused by the changes in temperature and relative humidity conditions of the plant’s habitat during the day. This phenomenon probably enables postponing the fertilization towards a time of better conditions and enables the plant to retain the pollen tubes alive, albeit inactive, and thus allow the plant to overcome temporary harsh conditions and develop seeds.

Beyond taxonomy: anther skirt is a diagnostic character that provides specialized noctuid pollination in Marsdenia megalantha (Asclepiadoideae–Apocynaceae)

Taxonomic diagnostic floral structures can be very informative about details of the floral mechanism, promoting baselines for inferences about pollination system. In a number of species in the milkweed tribe Marsdenieae, the already complex basic asclepiad flower structure is further elaborated by the evolution of one morphological novelty: the anther skirt. Since the functional significance of this trait is poorly understood, we investigate its role in the pollination mechanism on Marsdenia megalantha. We tested the phenotypic integration of anther skirt; we described its functional anatomy; we identified and recorded the function of its nectaries; we described and tested in laboratory its contribution to the pollination mechanism; and we studied the pollination effectiveness of this system. The characteristics of an anther skirt are essential in the noctuid pollination of M. megalantha. This is due to its particular morphological and anatomical features related to its ability both to secrete at night and to contain nectar independently from the stigmatic chamber. The skirt also functions coordinately with the corona as a funnel that directs the noctuid legs to gynostegium. Its effectiveness in producing pollination was confirmed by field observations and manipulative experiments. We suggest that nectar offers at night and mechanical adjustment may be applicable to other representatives of the tribe as a feature that determines noctuid pollination, assigning a functional value for the anther skirt beyond the taxonomy.

Flowering phenology and trap pollination of the rare endemic plantCeropegia thaithongiaein montane forest of northern Thailand

Although Ceropegia species are well known for their complex pitfall flowers that temporarily imprison their pollinators, various aspects of their pollination ecology are still unknown. This study investigated flowering phenology, functional floral traits, and insect visitation in a natural population of a rare endemic lithophyte, Ceropegia thaithongiae Kidyoo. Flowering of C. thaithongiae was not synchronous but staggered. Anthesis lasted mostly 1–2 days, but its duration was shorter in flowers with a pollinium inserted into the stigmatic chamber. Several different insects visited flowers, but only chloropid and milichiid flies were effective pollinators. At anthesis, the epithelial osmophores on the corolla lobes emitted a floral scent that was simple in composition. Nectar of high viscosity was exuded from the nectaries hidden behind the guide rails. When transported by an insect, the pollinarium was attached to bristles on the mouthparts. Size and shape of the thin pellucid margin of the pollinium enable it to fit into the stigmatic chamber in a lock-and-key arrangement. The pollen transfer efficiency was 6.8%. The plant’s staggered flowering and pollination-induced ending of anthesis are advantageous in decreasing competition for pollinators when flower-visiting insects are scarce.

Corona development and floral nectaries of Asclepiadeae (Asclepiadoideae, Apocynaceae)

Flowers of Asclepiadoideae are notable for possessing numerous nectaries and elaborate coronas, where nectar can accumulate but is not necessarily produced. Given the complexity and importance of these structures for reproduction, this study aimed to analyze the ontogeny of the corona, the structure and position of nectaries and the histochemistry of the nectar of species of Asclepiadeae. Two types of coronas were observed: androecial [C(is)] and corolline (Ca). The development of the C(is)-type of corona initiates opposite the stamens in all species examined with the exception of Matelea in which it begins to develop as a ring around the filament tube. Despite their morphological variation, coronas typically originate from the androecium. A notable difference among the studied species was the location of the nectaries. Primarily, they are located in the stigmatic chamber, where nectar composed of carbohydrates and lipids is produced. A secondary location of nectaries found in species of Peplonia and Matelea is within the corona, where nectar is produced and stored, composed of carbohydrates and lipids in Peplonia and only carbohydrates in Matelea. The functional role of nectar is related to the location of its production since it is a resource for pollinators and inducers of pollen germination.

Morpho-Anatomical Analysis of Cosmostigma racemosum (Asclepiadoideae) Flowers

<em>Cosmostigma racemosum</em> is a plant species belonging to the family Apocynaceae, the subfamily Asclepiadoideae. <em>Cosmostigma racemosum </em>is found in Nglanggeran Mountain Gunungkidul, Yogyakarta. The local name and its original distribution are not known. Information or study of <em>Cosmostigma racemosum</em> in Indonesia is not available. Comprehensive characterization of this species is important for authentication and addition of data base. Characterization was conducted by analyzing the morphology and anatomy of flower. The objectives of this study were to describe and analyze the morphology and anatomy of <em>C. racemosum</em> flowers. The method of research was based on observation method and exploration of plant systematics evidence or taxonomy evidence, including analyses and description of morphology and anatomy of flower structure and its development. The results showed that the characteristics of flower morphology are in accordance with the existing description in literatures. Characteristics of pollinia are specific characters of morphological aspect of flower. Data of anatomy of flower and its parts development are the new ones which confirm the position of <em>C.racemosum</em> as a member of the tribe Marsdenieae. The data of anatomy also show new information of the ontogeny of the important parts of flower: pollinia formation, pollinia corpusculum, anther wall, anther sac, stigma, stamen, staminal tube, stigmatic chamber, and structure of ovary in Asclepiadoideae

Preliminary Observations of Insect Visitation to Flowers of Vincetoxicum pycnostelma (Apocynaceae: Asclepiadoideae), an Endangered Species in Japan

Vincetoxicum pycnostelma Kitag. (Apocynaceae) is a perennial herb that grows in grassland habitats in East Asia and the western part of Amur. Urbanization, land development, abandonment of traditional management of grasslands, or any combination of these factors are rapidly reducing grassland habitats, and V. pycnostelma is now ranked as a “Near Threatened” species on the Red List of Japan. Identification and characterization of pollinators of this endangered species are important in its conservation. We observed and collected insect visitors to the flowers at night and in the morning in the populations of V. pycnostelma on the banks of the Kidzu River, Kyoto, Japan from June to August in 2011. Pollinators were characterized as those insects on which flower pollinaria were attached. Twenty-seven species (9 families; 3 orders) were observed visiting the flowers; individuals of 14 lepidopteran species (6 families) had pollinaria attached to their bodies and most visited the flowers at night. Adult lepidopterans with a proboscis length of 0.9 - 4.5 mm were the primary carriers of pollinaria. Examination of stigmatic chambers of randomly selected flowers showed that the percentage of flowers in which pollinaria was inserted was much higher than the percentage of fruit set observed. Reasons for the observed low fruit set may be self-incompatibility or resource limitation. These preliminary observations were conducted at only 1 site and were restricted to a short period of time; yet, these data could be useful for elucidation of effective pollinators of this species and, thus, its conservation.

ViolasectionMelanium(Violaceae) in Italy. New data on morphology ofViola tricolor-Group

Summary Italian species of the Viola tricolor group, Sect. Melanium are examined from the morphological view-point. Additional diagnostic characters have been highlighted on the basis of in situ and ex situ experimental data, direct analysis of exsiccata and high resolution digital images of several Herbaria, check of previous international literature. Measures have been taken on well developed flowers after a given vegetative period. Diagnostic features of stigma, labellum, position of opening to the stigmatic chamber and detailed photos are provided. Scanned images of stipules and corresponding leaves taken from the lower, median and upper part of the stem are also provided. These images provide clearer discrimination and correct identification of this difficult group. A synoptic table with the essential features of V. kitaibeliana, V. hymettia, V. parvula, V. arvensis subsp. arvensis and V. tricolor subsp. tricolor has been framed. Exsiccata are checked whilst some of them are revised. Therefore the It...

Palp-Faction: An African Milkweed Dismembers Its Wasp Pollinators

Interactions between pollinators and nectar-producing flowers are usually assumed to be mutualistic, but the exploitative basis of these relationships can lead to antagonistic interactions. Flowers of the African milkweed, Pachycarpus appendiculatus E. Mey, produce concentrated nectar that is consumed primarily by the large spider-hunting wasp Hemipepsis dedjas Guerin (Hymenoptera: Pompilidae). Pollinaria of this milkweed become attached to the palps of these wasps during nectar feeding. Broken wasp palps were found between guide rails, attached to corpuscula that were trapped behind the guide rails, and attached to pollinia that were inserted into the stigmatic chambers of the flowers. Approximately 85% of wasps captured on flowers of P. appendiculatus were missing one or more palps, whereas only 9% of wasps captured on flowers of another asclepiad species were missing any palps. It thus seems that wasps face a high risk of losing their palps when foraging on these flowers. The interaction may thus be antagonistic for the wasps if the cost of losing their sensory palps (not yet established) is greater than the benefits of the nectar reward. The plants, however, gain clear benefit from the interaction, as verified by the removal and insertion of pollinia in flowers exposed solely to visits by pompilid wasps.

Morphological aspects and phylogenetic analyses of pollination systems in the Tylophora-Vincetoxicum complex (Apocynaceae-Asclepiadoideae) in Japan

Pollinators of two Cynanchum, five Tylophora, and 16 Vincetoxicum species were observed in 26 populations in Japan. The following pollination systems were observed in 18 species: moth pollination, generalized insect pollination, wasp pollination, dipteran pollination, both dipteran and moth pollination, and autogamy. Principal component analysis based on 13 floral characters indicated that the size of the pollinator tended to increase with sizes of all characters measured. Furthermore, species that have developed interstaminal parts of the corona and concealed stigmatic chambers tend to be pollinated by long-tongued insects. The phylogenetic distribution of pollinator types showed that species belonging to Clade I are pollinated exclusively by Diptera, whereas those of Clade II are pollinated by four insect orders. The most prominent pollinator transition in the Tylophora–Vincetoxicum complex is dipteran to moth pollination. The most common morphological change of the plants from dipteran pollination to moth pollination, or vice versa, is modification of the corolla. In the Tylophora–Vincetoxicum complex, pollination mode shifts are generally accompanied by modifications of the corona and the structure of gynostegium. One hypothesis for the rapid radiation observed in Clade II is that a widely distributed species may have partitioned its distribution in the relatively near past and adapted to various environments, in which the dominant pollinators were different, and that the local races may then have diverged from each other after they were isolated. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 93, 325–341.

Oxypetalum banksii subsp. banksii: a taxon of Asclepiadaceae with an extragynoecial compitum

The floral morphology of seven Oxypetalum species and, in particular, the spatial relationship between the five stigmatic chambers and two separate ovaries of their flowers with respect to transmission of the pollen tube are studied. In all species, except O. banksii subsp. banksii, floral morphology is similar to that in other Asclepiadeae, and the flowers pollinated with one pollinium develop only one follicle, which means compitum absence. In O. banksii subsp. banksii flowers, the secretory interstaminal tissue lines the inner walls of the stigmatic chambers as in the other species studied, but it also reaches the upper part of the inner surface of the filament tube, where it surrounds the styles, an unprecedented feature for Asclepiadaceae. This tissue secretes nectar and mucilage; the latter acts as transmitting medium for the growth of pollen tubes from pollinia inserted and hydrated in stigmatic chambers (“hyperstigmas”). Mucilage also functions as an extragynoecial compitum: in flowers pollinated with one pollinium both carpels develop into a follicle.

Towards an Understanding of the Mixed Breeding System of Swamp Milkweed (Asclepias incarnata)

which set fruit following self-pollinations. For the self-fertile plants, mean fruit-set from self-pollinations (10.0%) was significantly lower than that from cross-pollinations (35.1%), but the relative success of self- versus crosspollination varied between plants. Fruits produced from self-pollinations contained significantly lower total seed mass and a higher proportion of unfilled seeds than did those from cross-pollinations. Pollen competition experiments were also performed, in which individual flowers received both self- and cross-pollinations in stigmatic chambers leading to different ovaries. The results showed no evidence of increased abortion of self-pollinated fruits under conditions of competition with cross-pollinated fruits within the same flower or between flowers within umbels. Instead, the success of self-pollinations appeared to increase in flowers that had also received cross-pollinations. These results are surprising, given the high outcrossing rates previously reported in natural populations of A. incarnata ssp. incarnata.

THE CONSEQUENCES OF SELF‐POLLINATION IN ASCLEPIAS EXALTATA, A SELF‐INCOMPATIBLE MILKWEED

We investigated the effect of self‐pollination on fruit‐ and seed‐set in Asclepias exaltata, a self‐incompatible milkweed. A total of 1,380 hand‐pollinations were performed on 138 flowers in each of five treatments. In each treatment, two pollinia were placed in adjacent stigmatic chambers, which transmitted pollen tubes to the same ovary. Treatments 1 and 2 involved simultaneous placement of two self‐pollinia (treatment 1) or two cross‐pollinia (treatment 2) into adjacent stigmatic chambers. No fruits were produced after self‐pollinations, but 23% of the cross‐pollinated flowers matured fruits. The three remaining experimental pollination treatments were self‐pollination preceding cross‐pollination by 24 hr (treatment 3), simultaneous self‐ and cross‐pollination (treatment 4), and cross‐pollination preceding self‐pollination by 24 hr (treatment 5). Compared with that in flowers receiving only cross‐pollen, fruit‐set in treatments 3–5 was reduced 81%, 49%, and 29%, respectively. Seed‐set was also significantly reduced in flowers receiving self‐pollen 24 hr in advance of the cross‐pollen. Using genetic markers, we observed that only 0.6% of seeds resulted from self‐pollination. Our data strongly suggest that self‐pollination in milkweeds not only wastes pollen but also greatly reduces the number of ovules and ovaries that might otherwise mature fruits and seeds after cross‐pollination.

Evidence for Short-Distance Dispersal of Pollinia in Asclepias syriaca L.

In milkweeds (Asclepiadaceae) pollen is produced in paired packages (pollinia) and pollination occurs when a pollinium is inserted by a pollinator into a stigmatic chamber. Previous studies have suggested that this mode of pollen delivery results in predominantly long-distance pollen dispersal. This assertion was tested for Asclepias syriaca L. A plant whose pollinia had been labelled with 14C was placed within a large population of milkweeds and a single subject umbel was exposed to honey-bee pollinators for 5 days. Hand pollinations involving another umbel on this plant verified that labelled pollinia dispersed to flowers within the umbel (geitonogamy), or to flowers on other plants, could be detected by scintillation counts. The labelled plant was at the centre of a 6 X 6m study plot. Of the 38 insertions on the labelled umbel, 14 (37%) were from the labelled umbel itself (geitonogamy). Thirty labelled pollinia were detected in umbels on neighbouring plants within the study plot. It is estimated that few labelled pollinia were inserted into flowers outside of the study plot, although the possibility of some long-distance dispersal cannot be excluded. The results indicate that much of the pollen dispersal was short distance. This makes pollen dispersal characteristics in this species similar to those of non-milkweed species. Shortdistance pollen dispersal can have important implications for A. syriaca which reproduces vegetatively and for which geitonogamous pollination does not result in fruit set. Key-words: Asclepias, 14C, milkweeds, pollen dispersal, pollinia

THE NECTAR-INHABITING YEASTS AND THEIR EFFECT ON POLLEN GERMINATION IN COMMON MILKWEED, ASCLEPIAS SYRIACA L.

ABSTRACT In Asclepias, nectar is secreted by the stigmatic chamber and has two functions: (i) it is a carbohydrate reward for pollinating insects required by the plants for sexual reproduction and (ii) it is the germination medium for pollen at the onset of pollen tube growth. The nectar easily becomes infected by yeasts, mostly Metschnikowia reukaufii Griiss, which are vectored by pollinators. Nectar from flowers which opened in the laboratory was yeast-free and did not inhibit pollen germination, but samples of nectar from older, open flowers from the field were almost always (94–98%) inhibitory and supported populations of yeast When nectar samples from buds which opened in the laboratory or artificial nectars (solutions of D-glucose and sucrose which normally support vigorous germination of pollen) were contaminated with yeast-infected nectar, they did not support pollen germination. Yeast-infected nectar or sugar solutions were inhibitory even when filtered to remove the yeast cells. Laboratory yeast...

THE RELATIONSHIP BETWEEN THE FIVE STIGMATIC CHAMBERS AND TWO OVARIES OF MILKWEED (ASCLEPIAS AMPLEXICAUUS SM.) FLOWERS: A THREE-DIMENSIONAL ASSESSMENT

To assess the spatial relationships between the five stigmatic chambers and two separate ovaries of milkweed flowers with respect to pollen tube transmission, we performed experimental cross-pollinations of Asclepias amplexicaulis Sm. and examined cleared or sectioned gynostegia of these flowers 12–24 h after pollination. Three adjacent stigmatic chambers transmit pollen tubes to one of the two separate ovaries, and the remaining two chambers transmit pollen tubes to the second ovary. Despite the potential for pollen tubes to cross over at the point of fusion of the two styles, we never observed this phenomenon in our sections. Thus, it should be possible to perform controlled pollinations of selected ovaries in milkweed flowers. We have also discovered a furrow on the upper surface of the gynostegium that enables one to identify which stigmatic chambers transmit pollen tubes to particular styles and ovaries: three stigmatic chambers transmitting to one ovary are on one side of this furrow, and the two chambers transmitting to the second ovary are on the opposite side.

The Role of Nectar in the Germination of Pollen in Asclepias syriaca L.

The nectary of Asclepias syriaca is in the stigmatic chamber. Nectar flows from there by a capillary system to the nectar reservoirs (cuculli). The fluid connection from the stigmatic chamber to the cuculli allows quick mixing of liquids in the system. Nectar in the stigmatic chamber is the germination medium for the pollen. Pollinia do not germinate in dry stigmatic chambers. The nectar must contain 5%-30% sugars for pollen germination. The nectar in the cuculli is the reward taken by insect pollinators and may become altered by physical (e.g., evaporation or dilution) or biotic (e.g., microbes, enzymes) factors.

Nectar Production and Pollination of Asclepias exaltata

Nectar production and pollination of herbaceous perennial milkweed Asclepias exaltata were studied in three populations in southwestern Virginia. Individual flowers were available to pollinators, chiefly large bees and wasps, an average of 6.2 days. Volumes, concentrations, and amounts of sucrose in nectar decreased as flowers aged. Most nectar was secreted overnight, and standing stocks increased in concentration from 20-30% in morning to 40-60% in afternoon. There were significant differences among plants in all components of nectar production, with sucrose amounts varying up to four-fold. The average number of pollinaria removed per flower was significantly greater in 1981 (1.38 ? 1.25; x ? s) than in 1982 (0.78 ? 0.09), as was average number of insertions per flower (0.66 ? 0.93 versus 0.39 ? 0.66). Pollination efficiency, ratio of insertions to removals, was nearly same in 1981 (0.43 ? 0.66) as in 1982 (0.36 ? 0.59). Rates of pollination were positively correlated with nectar concentration. In addition, plants producing concentrated nectar matured more fruits and had higher percentage fruit-set than did plants with dilute nectar. These data from natural populations agree with general results of laboratory trials suggesting that bees prefer concentrated nectar and that bees can respond to differences in nectar quality among plants. The extremely specialized morphology of milkweed flowers is rivaled only by that of orchids. Unlike orchids, however, milkweeds have been studied intensively by plant reproductive biologists only within past ten years. This is surprising in view of elegantly contrived ... pollinating apparatus of and general agreement that the entomophilous pollination of asclepiads provides one of most fascinating chapters in (Woodson 1954). The flower of Asclepias exaltata L., like those of other 107 North American species (Woodson 1954), consists of five showy, reflexed petals covering five smaller, greenish sepals (fig. 1). Two free, superior ovaries are joined by their styles to form a gynostegium with five lateral stigmatic surfaces. These surfaces are enclosed by tightly abutting wings of adjacent anthers to produce five stigmatic chambers. Five coronal extensions from base of stamen (hoods), each of which usually contains an arching structure (horn), serve as reservoirs for nectar secreted by nectaries located within stigmatic chambers (Galil and Zeroni 1965). There are five pollinaria, each of which consists of paired pollen sacs (pollinia) from adjacent anthers joined by translator arms to a corpusculum situated just above slit or opening to stigmatic chamber (Lynch 1977). Pollination is a two-stage process: 1) removal of a pollinarium occurs when a groove in corpusculum catches on a bristle or other appendage of an insect and is forcibly pulled from flower, and 2) insertion is effected when a pollinium lodges in a stigmatic chamber. The unique morphology of milkweed flowers makes it possible to quantify aspects of pollination and fruit-set that are difficult to study in other plants, whose small pollen grains are transported individually. Pollination rates can be scored directly and easily in field by examining flowers and recording numbers of pollinaria removed. Removal rates give some indication of relative success of plants as male parents. Dissection of preserved flowers allows scoring of numbers of insertions, an index of relative female success. Several workers have taken advantage of these features in studies of reproductive biology of milkweeds (e.g., Willson and Rathcke 1974; Wyatt 1976, 1978, 1980a, 1980b, 1981; Willson and Price 1977; Lynch 1977; Bertin and Willson 1980; Kephart and Heiser 1980; Kephart 1981; Fritz and Morse 1981; Morse 1982; Chaplin and Walker 1982; Pleasants and Chaplin 1983). Milkweeds also are favorable subjects for studies of nectar production because nectar collects in hoods and can be removed with relative ease. The durable, relatively long-lived

Morpho-ecological aspects on the pollination ofCalotropis procera (Asclepiadaceae) in Israel

Calotropis procera (Asclepiadaceae) grows in Israel under extremely hot climatic conditions. Flower morphology is adapted towards the concealment of nectar, in order to protect it against robbery by ants and from evaporation. Pollinia while being inserted within the stigmatic chamber, are soaked in the liquid nectar which, by its concentration controls pollen germination. Two Carpenter bees (Xylocopa) are the only pollinators and their behavior on flowers is discussed.

THE NECTARIES OF CYNANCHUM VINCETOXICUM (ASCLEPIADACEAE)

ABSTRACT The nectary epithelia of Cynanchum vincetoxicum cover cavities formed between the base of the corolla and the gynostegium, and they are in open contact with the stigmatic chambers. In addition to sugars, insoluble substances are secreted, presumably mainly lipidie in nature. The wall of epithelial cells at the apical dome is traversed by channellike structures. The mitochondria are partly aggregated into large complexes. Golgi apparatus and ER are well developed and may participate in the secretory process, although unequivocal evidence is lacking.