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Development and Morphology |
Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland
Received for publication May 1, 2001. Accepted for publication August 9, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Arecaceae • development • flowers • Geonoma interrupta • Geonomeae • morphology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Nevertheless, based on the available publications, reproductive structures of only 4% of the nearly 2000 palm species have been studied in detail. Fragmentary information on floral development was sometimes included in these studies, but it proved to be conspicuously unevenly distributed among the six subfamilies. Coryphoideae were preliminarily studied in all their tribes, but only Morrow (1965)
, who studied 24 genera, really improved the knowledge on floral morphology and anatomy of this subfamily. Within Calamoideae, two species of Salacca (S. zalacca and S. wallichiana) (van Heel, 1977, 1988
) and four species of Eugeissona (E. insignis, E. minor, E. tristis, and E. utilis) (Uhl and Dransfield, 1984
) were treated developmentally. Among Ceroxyloideae, only the organogenesis of the male flowers of Ceroxylon alpinum was studied (Uhl and Moore, 1980
). In the subfamily Arecoideae, the largest number of investigations have been carried out in Areceae (Arecinae) and Cocoeae (Butiinae, Bactridinae). Furthermore, with the exception of Podococceae, at least one taxon in each of the remaining six tribes of Arecoideae has been treated. For the rather small neotropical subfamily Phytelephantoideae, the paper of Barfod and Uhl (2001)
on the organogenesis of the male flowers of Aphandra natalia completes the developmental investigations started by Uhl and Moore (1977b)
and Uhl and Dransfield (1984)
in this group. It should be pointed out that even whole tribes of recognized phylogenetic or ecological importance, such as Lepidocaryeae (Calamoideae) and Cyclospatheae (Ceroxyloideae), are still awaiting developmental and morphological approaches.
Geonoma, which belongs to the widespread subfamily Arecoideae, is one of the most diverse genera in the Neotropics (
51 species sensu Henderson, Galeano, and Bernal, 1995
), and the only one with a pseudomonomerous gynoecium within Geonomeae (Wessels Boer, 1968
; Uhl and Moore, 1971
). Because of its unilocular, uniovulate gynoecium, Geonoma has been interpreted as the most derived genus in the tribe (Moore, 1966
). Geonomeae have traditionally been considered as a natural group (Punt and Wessels Boer, 1967
), mainly on the basis of three easily identified synapomorphies: triads of flowers sunken in pits of the inflorescence axis, floral parts united in all whorls, and styles slender and elongated. Monophyly of the tribe was indicated by Asmussen (1999)
, Asmussen, Baker, and Dransfield (2000)
, and Asmussen and Chase (2001)
.
Some aspects of floral morphology of geonomoid palms have been studied, such as flower structure (Gassner, 1941
; Uhl, 1966
; Wessels Boer, 1968
; Schmid, 1983
; Uhl and Dransfield, 1987
) and palynology (Punt and Wessels Boer, 1967
). However, these studies are incomplete, and except for Uhl and Moore (1971, 1980)
, Uhl and Dransfield (1984)
, and Uhl (1988)
, data on floral development are scant, and there are none on Geonoma.
Geonoma interrupta (Ruiz & Pav.) Mart. is a small palm to 2 m tall, with an inflorescence up to 40 cm long. It is widely distributed in central and northwestern South America and most of the Antilles and relatively frequent in the understory of the lowland and premontane forest (Henderson, Galeano, and Bernal, 1995
). Since its original description in 1798, at least 20 different names have been published for this species, and a high level of polymorphism has been found within this taxon (Wessels Boer, 1965, 1968, 1988
; Henderson, Galeano, and Bernal, 1995
). This is the first of a series of studies that aim to improve the knowledge on the developmental morphology of flowers in Geonomeae.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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and Henderson, Galeano, and Bernal (1995)
, as well as by comparison with exsiccatae deposited at the Venezuelan National Herbarium (VEN), Herbarium of the Agronomy Faculty of Maracay, Venezuela (MY), Herbarium of the New York Botanical Garden (NY), and the United States National Herbarium (US).
Sectioning and scanning electron microscopy
For anatomical investigations, flowers in bud or at anthesis were evacuated, dehydrated, and embedded in Kulzer's Technovit 7100 (2-hydroxyethyl methacrylate [HEMA]). Perianth and sometimes staminodial tube were removed to facilitate infiltration. Further details on this method are explained in Igersheim and Cichocki (1996)
. The material was later sectioned at 6–7 µm using a rotary microtome (Microm HM-355, Microm Laborgeräte GmbH, Walldorf, Germany) and then stained with ruthenium red (40 sec) and toluidine blue (3 min), and enclosed in Histomount. For scanning electron microscopy (SEM), the dissected specimens were dehydrated, critical-point dried, and sputter-coated with gold. Micrographs were obtained with a Cambridge S4 scanning electron microscope (Cambridge, UK).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Triads of flowers immersed in pits are arranged spirally according to the Fibonacci pattern, showing most obviously sets of two and three parastichies (Fig. 1). Each pit is closed by two raised lips. The lower lip (proximal in relation to the base of the inflorescence) has an entire, but glabrous, or at least less hairy, margin. The upper lip (distal in relation to the base of the inflorescence) is very short, with an entire and rather pilose margin. The inner surface of the pit is either glabrous or covered with short hairs.
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), each triad is formed by two male flowers and one female flower and represents a short cincinnus. In the case of G. interrupta, left-handed and, more rarely, right-handed cincinni, are present in the spiral succession. The female flower bud is located below and adaxial to the two male flowers of each triad (Fig. 2). Pedicels of male flowers shortly before anthesis are almost 1 mm long and closely touch the lateral sides of the female flower.
The first male flower of the cincinnus, and the cincinnus as a whole, is subtended by the pit-closing bract, which represents the lower lip of the pit. According to the interpretation of Wessels Boer (1968)
the largest bract inside the pit subtends the second male flower, a medium-sized bract subtends the female flower, and a small bract represents the prophyll of the female flower. The medium-sized and the small bracts surround, at least basally, all but the adaxial region of the female flower, acting as a protective structure in addition to the perianth (Fig. 2).
Female flowers become visible outside the pit only when the first male flower of the triad has fallen. While the second male bud opens and its anthers start to spread, the female bud emerges to one-third of its length (stage III). During male anthesis, the developed female bud reaches its maximal emergence from the pit, nearly two-thirds of its length. When the female flower of a triad is at anthesis, both male flowers have fallen (stage IV). The rachillae studied did not show overlap between the male and female phases. Therefore, functional dioecy, at least within an entire inflorescence, seems to occur in the samples collected.
Stages of floral development in the rachillae
Based on external morphological characters of the rachillae and the flowers, four different stages of development were assigned to the studied material. Thus, the presence and shape of the inflorescence prophyll and the peduncular bract, the degree of expansion of the rachillae, the number of male flowers exposed in each pit, the degree of emergence of all the flowers from the pits, and the development of conspicuous organs (e.g., style, stigmatic branches) were selected as characters that could be correlated with the relative age of the rachillae and the growth of the female flowers themselves. The definitions provided below are based on the evaluation of these characters and compared with some phenological studies in several species of Geonoma (e.g., Olesen and Balslev, 1990
; Listabarth, 1993, 1999
; Borchsenius, 1997
; Silberbauer-Gottsberger, 1999
).
Stage I
The inflorescence prophyll and the peduncular bract enclose the entire inflorescence, and the rachillae are completely folded and twisted. Pits are completely closed by the lips and therefore rather inconspicuous. A secretion, which is white in the pickled material, covers the union between the lip margins. The flower triads are completely hidden in the pits.
Stage II
The inflorescence prophyll and the peduncular bract have longitudinally split into 2–4 parts, or they have already fallen. The rachillae start to spread. The lips begin to open, with the lower lip becoming more prominent than the upper one. Both male flowers of the triad emerge from the pit to between one-quarter and one-third of their length; the female flower is still sunken in the pit.
Stage III
Inflorescence prophyll and peduncular bract fallen. Rachillae have expanded; the lips have opened, with the lower lip becoming prominent. Different patterns were observed depending on the position along the rachillae. Towards the base: the first male flower fallen, the second in bud, emerging to almost one-half of its length, and the female flower also in bud, emerging to one-quarter of its length. Towards the medial part of the rachilla: the first male flower fallen, the second in anthesis, emerging almost totally from the pit, and the female flower in bud, emerging from one-third to one-half of its length. Towards the apex: the first male flower fallen, the second in bud, emerging to three-quarters of its length, and the female flower also in bud, emerging to one-quarter of its length. Triads of two male flowers in bud, slightly emerged, and one female flower, still hidden, are also frequent in the apical region of the rachilla. Anatomical sections and SEM photos of female flowers from the basal, medial, and apical regions of the rachilla did not allow the detection of obvious differences in their degree of development; nevertheless, only results on flowers dissected from the medial region are included in this study.
Stage IV
Rachillae are expanded. Of the triads, only solitary female flowers or young fruits are left; male flowers have fallen. One predominant pattern was observed in the rachilla: solitary female flowers in anthesis were present, extending from the pit to one-half of their length, with their stigmatic branches becoming reflexed. Some pits contain young fruits of 2–3 mm in length and 2 mm diameter. Triads with one male flower at anthesis, totally emerged, and one female flower in bud exposed to one-half its length were also observed. Additional sections and SEM photos at this stage were made from material collected by Dr. Christian Listabarth in Peruvian Amazonia (C.L. G43-3 19/10/1989-WU).
Perianth
As in other species of Geonomeae, the sepals are keeled and briefly imbricate to two-thirds of their length; the petals are congenitally united and the free tips are valvate (Fig. 3). Raphide idioblasts are widespread in the mesophyll of the calyx and the corolla.
Androecium
Observations in the region of attachment of the floral organs suggest that the staminodial tube is congenitally united with the petals and the carpels at the base; it becomes thicker during development in its upper part but not in its lower part. The rim is six-toothed crenate, and six short slits, apically between each of the six staminodes, are visible only in late development (stages III and IV) (Figs. 4 and 5). The tube mainly consists of parenchymatous tissue with some tanniferous cells. Raphide idioblasts, similar to those found in the calyx and the corolla, occur in stages older than II and are more concentrated towards the apical region of the tube; however, isolated raphide idioblasts were also observed at lower levels. Vascular bundles corresponding to the six staminodes are present as procambial strands already in stage I (Fig. 3) and are more differentiated in stages III and IV.
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Sterile carpels
Although the gynoecium always has three carpels, two of them are reduced and are never fertile. The sterile carpels may develop to different degrees; one of them commonly becomes larger than the other. For example, at stage III if the sterile carpels cease their development very early, they will remain as short basal knobs in the gynoecium (Fig. 6); however, if they grow further, one (Figs. 7, 20, and 26), or, less commonly, both (Fig. 8), may reach up to one-half of the length of the fertile carpel. At stage IV, the presence of the sterile carpels is obscured by the predominant development of the fertile one (Figs. 14 and 15).
Styles
Styles are formed in each carpel from stage II and become "basifixed" from stage III by the dorsal bulging out of the ovary (Figs. 14–16). At stage III, and more so during anthesis (stage IV), the ventral slits of the three styles are confluent above the level of attachment of the ovule (Fig. 16). The ventral slits remain joined up to mid-length of the styles and then separate. Postgenital fusion between the styles is more conspicuous in the upper part than in the lower part, but it is not present at the level of the stigmas (Figs. 11 and 16). Tanniferous cells are irregularly spread in the base of the styles (Fig. 19) and almost completely make up the mesophyll in the middle and apical regions of the styles.
Stigmatic branches
The three stigmatic branches are prominent from stage I and are, at least apically, slightly papillate from this stage (Fig. 9). The branches remain free and plicate during the entire sequence of development, and it is only from stage III that postgenital union in the basal region of the branches begins. From stage III, the uppermost parts of the stigmas overtop the staminodial tube, and therefore, are protected only by the perianth. At anthesis (stage IV) the stigmas are reflexed, and unicellular papillae are concentrated on ventral ridges (Figs. 10, 11, and 23). The branch of the fertile carpel is always the thickest one but not necessarily the highest (Fig. 16). The stigmas seem to be of the dry type (terminology from Heslop-Harrison [1981
]), or at least the pickled material did not show remnants of secretion. Tanniferous cells are widespread in the mesophyll of the stigmatic branches.
Pollen tube transmitting tract (PTTT)
During anthesis (stage IV) a PTTT develops on the epidermal cells of the ventral slits of the stigmas and styles. The tracts coming from the stigmatic branches join in a compitum (united pollen tube transmitting tract of all carpels in the flower) at the levels of postgenital fusion of the styles and remain this way down to the level of attachment of the ovule (Fig. 16). The PTTT communicates with the ovule through the ventral slit of the fertile carpel (Fig. 22) and reaches the micropyle by surrounding both flanks of the funiculus (Fig. 16). According to the terminology of Endress (1994)
, the PTTT studied may have a weakly secretory epidermis, and the pathway of pollen tubes is superficial and formed by a one-cell-layer-thick tissue. At least where the compitum is present, the style has a large inner surface and a small lumen.
Septal nectary
In stage III, a septal nectary begins to differentiate. The nectary is a triradiate cavity in the base of the gynoecium, and it is formed by the incomplete fusion of the flanks of the carpels (Fig. 16). Nectar is secreted from the epidermis, which is differentiated as an epithelium with columnar, uninucleate cells (Figs. 27 and 28). The septal nectaries extend diagonally upwards and outwards between the carpels. At stage IV, the septal nectary reaches its maximal differentiation, and the secretory cells are heavily stained (Fig. 24). The central protrusion between the carpels also has a secretory epidermis (Fig. 25). At the base of the ovary, the septal nectary is just a single slit between the fertile and most commonly the largest sterile carpel (Fig. 16). Just below the level of insertion of the ovule, two openings of the nectary to the outer surface of the gynoecium are present: one between the two sterile carpels and the other between the fertile and one of the sterile carpels (Fig. 24). A third opening between the fertile carpel and the other sterile carpel is present above the level of ovule insertion. The openings are not secretory, but are just nectar ducts formed by isodiametric epidermal cells (following the terminology of Vogel [1998]
) (Fig. 28).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Dransfield and Uhl, 1998
); however, further investigations are necessary to define whether pseudomonomery has the same developmental origin within all Arecoideae. Carpels with ventral slits that remain open above the level of attachment of the ovule have already been reported in Asterogyne spicata (Uhl, 1966
) and in Juania (Uhl, 1969
), and our observation of it in this study suggests that this character is probably more widely distributed within the family. Presence and partial differentiation of sterile carpels in Geonoma interrupta could be correlated with the extra production of nectar at their flanks and the formation of a compitum between all three styles.
The anatropous ovule of Geonoma interrupta corresponds with the most common pattern for the tribe, shared with Pholidostachys, Calyptronoma, and Calyptrogyne (Uhl and Dransfield, 1987
). Our observations at stage I suggest that the single ovule is initiated directly at the base of the carpel, as already reported in Eugeissona (Uhl and Dransfield, 1984
), Elaeis (van Heel, Breure, and Menendez, 1987
), and Palandra (Uhl, 1988
). With the material available to us, it was not possible to see whether one or two integuments were present in the mature ovule.
Geonomeae and Phytelephantoideae exhibit the longest styles in the family (Uhl and Dransfield, 1987
). Thus, our investigation supports that the long style in Geonoma is correlated with the mostly internal growth of the flowers in pits and the lack of an elongate pedicel, as was reported by Uhl and Dransfield (1987)
. Observations on the short stigmatic tips with surface papillate already in flowers at stage I agree with the early stigmatic development in Phoenix dactylifera (De Mason, Stolte, and Tisserat, 1982
) and probably many other monocots, e.g., Ornithogalum caudatum (Tilton and Horner, 1980
). Since the first report of a transmitting tissue in the styles of some Arecaceae (Venkata Rao, 1959b
) and the description of a compitum in palms (Cocucci, 1964
), this study on Geonoma interrupta represents a contribution towards a better understanding of the pollen tube transmitting tract in the family; nevertheless, additional studies on this matter are still greatly needed.
In Geonoma, lack of histological protective devices in the long styles and the ovules is compensated by enclosure of young flowers in pits of the rachillae (Uhl and Moore, 1971, 1973
). According to our study, in addition to the pit, the perianth and the staminodial tube constitute the most important structural protective devices for the gynoecium. Raphide idioblasts and tanniferous cells are widespread in the mesophyll of all organs. The stigmatic branches of Geonoma interrupta become exserted from the pit only during anthesis and this behavior was interpreted by Uhl and Moore (1977a)
as a protective device for the stigmas in Asterogyne martiana. Tanniferous cells have been identified in several parts of the palm flower. They occur around the openings of the septal nectary, in the style, and in the stigmas of Asterogyne martiana (Uhl and Moore, 1977a
) and occur in the receptacle of Aphandra (Barfod and Uhl, 2001)
. Tanniferous cells in the gynoecium were reported in Licuala sp. (Venkata Rao, 1959a
), several Arecoideae (Venkata Rao, 1959b
), Caryota urens (Shirke and Mahabale, 1972
), Wallichia densiflora (Uhl and Moore, 1973
), Asterogyne martiana (Schmid, 1983
), and in Phoenix dactylifera (De Mason and Tisserat, 1980
). Mature ovules of Geonoma interrupta showed a hypodermal tanniferous layer in the periphery. Tanniferous cells were also found in the chalaza and the outer integument of several Corypheae (Venkata Rao, 1959a
) and in Asterogyne martiana (Uhl and Moore, 1977a
). Contrary to the report of Uhl and Moore (1971)
for Geonoma interrupta, we could not detect the presence of raphide idioblasts in the base of the styles at anthesis. Furthermore, according to our study, raphide idioblasts are restricted to the perianth and the staminodial tube. Raphide idioblasts in the staminodial tube have been found in Asterogyne martiana (Uhl and Moore, 1977a
) and are widespread in the ovaries of several Ceroxyloideae (Uhl, 1969
; Uhl and Moore, 1973
), Caryota urens (Shirke and Mahabale, 1972
), and Zombia (Uhl and Moore, 1973
).
Septal nectaries, which among angiosperms are only known from monocots, are also the most common type in the monocots (Daumann, 1970
; van Heel, 1988
; Vogel, 1998
). They are reported as predominant within the Arecaceae (Schmid, 1983
). Wessels Boer (1968)
could not identify nectaries in the Geonomeae, but the present study and the investigations of Uhl and Moore (1971)
on Geonoma interrupta, of Schmid (1970a, b, 1983)
on Asterogyne martiana, and personal observations on Geonoma simplicifrons do support that septal nectaries are also widespread within the tribe. The septal nectary of Geonoma interrupta matches the common model of a triradiate cavity formed by the incomplete union of the carpel flanks, lined by a secretory epithelium. The nectary stains intensively from late bud through anthesis of the flowers. Nectar secretion in other Arecaceae has been reported shortly before anthesis (Schmid, 1983
), during anthesis (Moncur, 1988
), and also after anthesis (Daumann, 1970
; Fahn, 1979
).
Wind pollination, suggested by Wessels Boer (1968)
for the Geonomeae, has been subsequently rejected by later exhaustive field work. Further approaches on the study of the insect-mediated reproductive systems in Geonoma, most of them at the population level, were carried out by Olesen and Balslev (1990)
, Listabarth (1993, 1999)
, Borchsenius (1997)
, and Silberbauer-Gottsberger (1999)
. Nectaries have developed as adaptations to insect pollination (Faegri and van der Pijl, 1979
), and their presence in the gynoecium of Geonoma interrupta (Uhl and Moore, 1971
; current study) also suggest entomophily in the genus. Floral scents in Geonoma have been reported by Olesen and Balslev (1990)
, Listabarth (1993, 1999)
, Knudsen (1999)
, and Knudsen, Andersson, and Bergman (1999)
. Nectar secretion has been detected by Silberbauer-Gottsberger (1999)
in flowers of Geonoma schottiana, and Listabarth (1993)
mentioned "exudates" from styles and flower bases in Geonoma interrupta. Rewardless female flowers and mimicry of male flowers by female flowers have been proposed for Geonoma by Olesen and Balslev (1990)
, Listabarth (1999)
, and recently supported by Knudsen, Andersson, and Bergman (1999)
, based on the resemblance in the chemical composition of floral scent between male and female phase inflorescences. Together with the color and the general morphology of the flower, the digitately lobate apical rim of the staminodial tube in some species of Geonoma has been regarded as an important component of the mimicry mentioned above (Olesen and Balslev, 1990
). Wessels Boer (1968)
identified five different shapes of apical rim (crenulate, shortly crenulate, digitately lobed, truncate, shortly dentate) in the staminodial tubes of the 69 species of Geonoma studied in his monograph. Geonoma interrupta, with its shortly crenulate apical rim, is one of the 23 species reported by Wessels Boer (1968)
with this feature. Based on the study of Wessels Boer (1968)
, only 14% of the species in the genus present a staminodial tube with digitately lobed apices, whereas the remaining 86% shows poorly elaborated apical rims. Thus, we suggest here that in Geonoma as a whole, attraction of female flowers combines two components: (1) nectar reward and (2) partial mimicry of male flowers, the latter probably more due to shape, color, and scent of the flowers than by strong modification of the staminodial tube resembling fertile stamens.
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FOOTNOTES |
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2 Author for reprint requests (stauffer.pappa{at}bluewin.ch
).
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