Successive Microsporogenesis Research Articles

Androecium development of Oxypetalum appendiculatum Mart. (Apocynaceae): A Taxonomic Approach

Circa 80% of the neotropical species of Asclepiadoideae (Apocynaceae) belong to the “MOOG Clade”, whose interrelations are poorly established. This study describes the floral morphology, anther development, microsporogenesis, microgametogenesis, and pollinarium and pollen grain morphology of Oxypetalum appendiculatum to foster future taxonomic work. In addition to typical morphological features of Asclepiadoideae, as a bifid vinaceous appendix and a translator with a laterally expanded retinaculum, flowers present a clear division of stamens into three groups, with respect to size and arrangement in the flower, which has not yet been reported in the literature. Stamens present connate, subsessile filaments, bisporangiate anthers with six parietal layers, and dicotyledonous development. Endothecium is unthickened; secretory tapetum is biseriate on the adaxial side and multiseriate on the abaxial surface. Successive microsporogenesis produces linear tetrads. Single pollen grains, united in pollinia, are tricellular and inaperturate. Features as bisporangiate anthers and successive microsporogenesis delimit the subfamily. Unthickened endothecium may indicate a derived position of O. appendiculatum in the “MOOG Clade”.

Floral anatomy of Peristethium leptostachyum (Loranthaceae)

Peristethium leptostachyum is a hemiparasite species of the family Loranthaceae, distributed in Colombia, Costa Rica, Ecuador, Peru, Venezuela and Panama. Previously treated as Struthanthus leptostachyus, the species was recently transferred to Peristethium together with other species of Cladocolea and Struthanthus. The present research describes the inflorescence and floral morphoanatomy of Peristethium leptostachyum, detailing the structure of the androecium and gynoecium and the processes of microgametogenesis and megagametogenesis, thus allowing comparison with Struthanthus and Cladocolea. Flowering material was collected in February and August 2012, in Santa María, Boyacá, Colombia. Histological sections were prepared and stained with astrablue-fuchsin and floral dissections were performed under a stereomicroscope. Peristethium leptostachyum shares inflorescence characters with Cladocolea (determinate inflorescence, ebracteate terminal flower), but also with Struthanthus (pairs of triads along the axis, deciduous bracts and actinomorphic flowers). The flowers of P. leptostachyum from Santa María are clearly hermaphrodites with androecium and gynoecium fully developed. This observation contradicts the description by Kuijt who reported this species to be dioecious. The androecium was observed to be similar to that of Struthanthus vulgaris, with a glandular tapetum and simultaneous microsporogenesis; in contrast, Cladocolea loniceroides has a periplasmodial tapetum and successive microsporogenesis. The gynoecium of P. leptostachyum, like that of Cladocolea, Struthanthus and Phthirusa, has a unilocular ovary with a mamelon and arquesporial tissue isoriented towards the style, which in turn is solid and amyliferous. Peristethium leptostachyum is similar to Cladocolea loniceroides and differs from Strutanthus vulgaris in presenting multiple embryo sacs and an unlignified pelvis (hipostase). The presence of a solid stylar canal is proposed as a synapomorphy of the tribe Psittacanthinae. Given that P. leptostachyum shares characters with both Cladocolea and Struthanthus generic placement cannot be clearly determined on the basis of anatomical evidence. Phylogenetic studies that include representative species of all three genera are desirable to test hypotheses of monophyly. The sexual system observed here in P. leptostachyum is different from that reported by Kuijt and more studies are needed to identify the factors (geographic, ecological, etc.) that influence this variation.

Ultrastructure and development of sporoderm in Aristolochia clematitis (Aristolochiaceae)

The purpose of this study was to investigate the sporoderm development of inaperturate pollen grains in Aristolochia clematitis (Aristolochiaceae). During successive microsporogenesis several types of tetrads are formed; no tetrahedral tetrads were observed. In the tetrad period, the primexine is observed around the perimeter of microspores. In the free microspore period, a thick lamellar endexine develops around the perimeter of the microspore; the thick tubular intine appears later in this period. At the maturation of pollen grains, the intine is stratified into two layers: an outer tubular ectintine and an inner homogeneous endintine. Finally, the intine forms an aperture thickening. The thickened intine deforms and displaces the endexine and tears the ectexine apart. This shows that pollen grains of A. clematitis are aperturate. The apertures are colpi.

Anther Wall Development, Microsporogenesis, and Microgametogenesis inAbolbodaandOrectanthe: Contributions to the Embryology of Xyridaceae (Poales)

Premise of research. Xyridaceae includes two subfamilies with a total of five genera. Xyridoideae has only the single genus Xyris, and Abolbodoideae includes the remaining genera. The inter- and intrafamilial relationships among Xyridaceae have not yet been clarified, and this study therefore aimed to investigate the anther wall and pollen grain development in Abolboda and Orectanthe species, searching for characters that can be used in taxonomic and phylogenetic analyses.Methodology. Anthers from flowers at different stages of development were examined using both LM and SEM.Pivotal results. All studied species present the following: monocotyledonous type of anther wall development, single-layered endothecium with band-like thickenings in the mature anther, single ephemeral middle layer, multilayered and plasmodial tapetum, successive microsporogenesis, and spherical inaperturate pollen grains. Characteristics such as reduced type of anther wall development, single-layered and secretory tapetum with uninucleate or binucleate cells, and elliptical sulcate pollen grains with a reticulate exine are present in Xyris and differentiate this genus from Abolboda and Orectanthe. The characteristics reported for Xyris are also present in Mayacaceae, which used to form the xyrid clade of Poales with Eriocaulaceae and Xyridaceae. Abolboda and Orectanthe share with Eriocaulaceae the monocotyledonous type of anther wall development with an ephemeral middle layer, an endothecium with band-like thickenings, and pollen grain exine with projections. The presence of a multilayered tapetum is exclusive to Abolboda and Orectanthe among the Xyridaceae and also among the Poales.Conclusions. The embryological characteristics support the division of Xyridaceae into two subfamilies. In contrast with the megasporogenesis and megagametogenesis, the microsporogenesis and microgametogenesis characteristics were quite variable within the xyrid clade, with successive microsporogenesis being the only characteristic shared by Xyridaceae, Eriocaulaceae, and Mayacaceae.

Microtubule organization during successive microsporogenesis in Allium cepa and simultaneous cytokinesis in Nicotiana tabacum

Microtubule cytoskeleton organization during microspore mother cell (MMC) meiosis in Allium cepa L. and microsporogenesis in Nicotiana tabacum L. was examined. The MMC microtubules (MTs) were short and well dispersed in the cytoplasm of both taxa. As the MMCs of both species entered metaphase of meiosis I, the MTs constructed a spindle that facilitated the chromosomes to orient in the meridian plane. At anaphase of meiosis I, the spindle MTs differentiated into two types: one MT type became short, pulled the chromosomes toward the two poles, and was designated as centromere MTs; the second type of MT connected the two poles, and was designated as pole MTs. In A. cepa, where successive cytokinesis was observed, pole MTs assumed a tubbish shape. Some new short MTs aggregated in the meridian plane and constricted to form a phragmoplast, which developed into a cell plate, divided the cytoplasm into two parts and produced a dyad. However, in tobacco, a phragmoplast was not generated in anaphase of meiosis I and II and cytokinesis did not occur. The spindle MTs depolymerized and reorganized the radial arrangement of MTs from the nucleate surface to the periplasm during anaphase. Following telophase of meiosis II, the cytoplasm produced centripetal furrows, which met in the center of the cell and divided it into four parts, serving as a form of cytokinesis. In this process, MTs appeared to bear no relationship to cytokinesis.

Morphology and development of anthers and ovules in Croton and Astraea (Euphorbiaceae)

We examined the embryological development of anthers and ovules from Astraea (A. lobata and A. praetervisa) and Croton (C. floribundus, C. fuscescens, C. glandulosus, C. lundianus, C. piptocalyx, C. urucurana and C. triqueter) focusing on features with systematic significance for the group. Some of these features are common in Euphorbiaceae including: a dicotyledonous type of anther wall formation, a secretory tapetum, a mixed origin of the outer ovule integument, an epidermal origin of the inner ovule integument, the occurrence of many archesporial cells inside the ovules and a megagametophyte of the Polygonum‐type. Other features, such as the presence of styloid crystals in the tapetum, an idioblast with a druse in the endothecium, simultaneous and successive microsporogenesis, and a functional micropylar megaspore, have not previously been reported in the family. These characters appear to distinguish Croton and Astraea from other Euphorbiaceae and possibly represents autapomorphies for the tribe Crotoneae.

Anther and pollen development in some species of Poaceae (Poales)

Anther and pollen development were studied in Olyra humilis Nees, Sucrea monophylla Soderstr, (Bambusoideae), Axonopus aureus P. Beauv., Paspalum polyphyllum Nees ex Trin. (Panicoideae), Eragrostis solida Nees, and Chloris elata Desv. (Chloridoideae). The objective of this study was to characterise, embryologically, these species of subfamilies which are considered basal, intermediate and derivate, respectively. The species are similar to each other and to other Poaceae. They present the following characters: tetrasporangiate anthers; monocotyledonous-type anther wall development, endothecium showing annular thickenings, secretory tapetum; successive microsporogenesis; isobilateral tetrads; spheroidal, tricellular, monoporate pollen grains with annulus and operculum. Nevertheless, the exine patterns of the species studied are distinct. Olyra humilis and Sucrea monophylla (Bambusoideae) show a granulose pattern, whereas in the other species, it is insular. In addition, Axonopus aureus and Paspalum polyphyllum (Panicoideae) have a compactly insular spinule pattern, while Chloris elata and Eragrostis solida (Chloridoideae) show a sparsely insular spinule pattern. The exine ornamentation may be considered an important feature at the infrafamiliar level.

Development of male gametophyte of Larix leptolepis Gord. with emphasis on diffuse stage of meiosis

A basic developmental framework of the Larix leptolepis Gord male gametophyte is presented in detail by squashing technique. The duration of the meiosis stage was more than 6 months, and included a long diffuse stage during winter. This long duration of the diffuse appearance of the diplotene stage makes L. leptolepis a unique suitable experimental material for studying the structure and function of the diffuse stage of meiosis. In particular, the processes of desynapsing and unpairing, which so far have received little attention, can be examined in detail. In L. leptolepis, the chromosomes undergo a dramatic structural reorganization during the diffuse diplotene stage. Based on the clearly visible differences in chromosome morphology, the diffuse diplotene stage was divided into four periods with suggested nomenclature as follows: schizonema, pre-diffuse diplotene, diffuse diplotene and post-diffuse diplotene. Both simultaneous and successive microsporogenesis were observed within L. leptolepis, and there was no strict relationship between the microsporogenesis types and the tetrad configurations, which are strongly influenced by spindle orientation, especially during meiosis II. The mature pollen grain at pollination consists of five cells aligned in an axial row. The prothallial cells cannot be regarded as senescent cells because they remain capable of division.

Successive microsporogenesis in eudicots, with particular reference to Berberidaceae (Ranunculales)

The eudicot clade of angiosperms is characterised by simultaneous microsporogenesis and tricolpate pollen apertures. Successive microsporogenesis, where a distinct dyad stage occurs after the first meiotic division, is relatively rare in eudicots although it occurs in many early branching angiosperms including monocots. An extensive literature survey shows that successive microsporogenesis has arisen independently at least six times in eudicots, in five different orders, including Berberidaceae (Ranunculales). Microsporogenesis and pollen apertures were examined here using light and transmission electron microscopy in eleven species representing six genera of Berberidaceae. Successive microsporogenesis is a synapomorphy for the sister taxa Berberis and Mahonia (and possibly also Ranzania), the remaining genera are simultaneous. Callose wall formation in Berberis and Mahonia is achieved by centripetal furrowing, though centrifugal cell plates are more usual for this microsporogenesis type. This discrepancy could reflect the fact that the successive type in Berberidaceae is derived from the simultaneous type, and centripetal furrowing has been retained. Eudicots with successive microsporogenesis usually produce tetragonal or decussate tetrads, though occasional tetrahedral or irregular tetrads in Berberis and Mahonia indicate that the switch from simultaneous to successive division is incomplete or “leaky”. In contrast, linear tetrads produced by successive microsporogenesis in Asclepiadoideae (Apocynaceae s.l.) are the result of a highly specialised developmental pathway leading to the production of pollinia. Pollen in successive eudicots is dispersed as monads, dyads, tetrads, and as single grains in pollinia. Apertures are diverse, and patterns include spiraperturate, clypeate, irregular, monocolpate, diporate and inaperturate. It is possible that successive microsporogenesis, although rare, potentially occurs in other eudicots, for example, in species where pollen is inaperturate.

Reproductive morphology of Nuphar (Nymphaeaceae), a member of basal angiosperms

Recent phylogenetic analyses position Nuphar as the basalmost genus in the Nymphaeaceae, a member of the ANITA grade that is constituted of the three most basal angiosperm lineages [Amborella, Nymphaeales, and Austrobaileyales (Austrobaileyaceae, Illiciaceae, Schisandraceae, and Trimeniaceae)]. In Nuphar the anther is tetrasporangiate. The endothecial cells elongate radially and develop fibrous thickenings. The glandular tapetum persists up to the two-celled pollen stage. Successive microsporogenesis results in the formation of tetrahedral and isobilateral microspore tetrads. Anasulcate pollen is two-celled when shed, with long stout echinae on the surface. A longitudinal ridge occurs in the center of the pollen furrow. The ovules are anatropous, bitegmic and crassinucellate. The inner integument arises as an annular ridge, whereas the outer one initiates as a semi-annular ridge interrupted on the concave side of the funiculus. Later the outer integument becomes a hood-shaped envelope. The micropyle structure is variable in various stages of ovule development. At anthesis the micropyle is formed by both integuments, while before and after anthesis it is formed by the inner integument. This contrasts with all previous reports, which state that the micropyle of Nuphar is formed by the inner integument alone throughout ovule development. Megasporogenesis forms a linear triad of megaspores. At the binucleate stage of female gametophyte ontogeny, the two nuclei are not separated by a large vacuole from the chalazal and micropylar ends. The wide distal region of stigma is covered with high density in unicellullar papillae whereas the narrow adaxial region has sparse unicellular papillae. The present study compares the reproductive morphological characters of Nuphar with those of other nymphaeaceous genera.

Pollen and anther ontogeny inCabomba caroliniana(Cabombaceae, Nymphaeales)

Cabomba is a small water lily genus that is native to the New World. Studies of pollen development and associated changes in the anther yield valuable characters for considering the evolution of reproductive biology in seed plants. Here we characterized the complete ontogenetic sequence for pollen in Cabomba caroliniana. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using scanning electron, transmission electron, and light microscopy. Tetragonal and decussate tetrads both occur in C. caroliniana, indicating successive microsporogenesis. The exine is tectate-columellate, and the infratectal columellae are the first exine elements to form, followed by a continuous tectum and a thin foot layer. A lamellate endexine initiates in the early free microspore stage, but becomes compressed in mature grains. Tectal microchannels and sculptural rods also initiate during the early free microspore stage, and significant pollenkitt deposition follows, supporting the hypothesis that these elements function in entomophily. The tapetum is morphologically amoeboid, with migratory tapetal cells directly contacting developing free microspores within the anther locule. Results from this study illustrate the importance of including ontogenetic data in analyzing pollen characters and in developing evolutionary and ecological hypotheses. The new palynological data also emphasize the character plasticity that occurs in basal angiosperms.

Embryology and seed development of Syngonanthus caulescens (Poir.) Ruhland (Eriocaulaceae-Poales)

The embryology and the seed development of Syngonanthus caulescens are presented. This species possesses: a bithecous and tetrasporangiate anther, with a four-layered wall, a conspicuous endothecium of the baseplate type, a secretory tapetum formed by uninucleate cells, successive microsporogenesis resulting in isobilateral microspore tetrads, spiraperturate and binucleate pollen grains, an orthotropous, pendulous, bitegmic and tenuinucellate ovule, with a micropyle formed only by the inner integument, a megagametophyte of the Polygonum type, with formation of an antipodal cyst, free-nuclear and starchy endosperm, a broad and bell-shaped embryo, operculate and endotestal seeds, a seed coat derived from the inner layers of both integuments, and tanniniferous endotegmen. These embryological aspects are characteristic not only for Syngonanthus, but for the whole family, with few differences between genera. Furthermore, the pollen grain of the spiraperturate type and the cystic arrangement of the antipodals in the megagametophyte are peculiar and very distinctive features of Eriocaulaceae within the other Poales (commelinids).

Pollen ontogeny in Brasenia (Cabombaceae, Nymphaeales)

Brasenia is a monotypic genus sporadically distributed throughout the Americas, Asia, Australia, and Africa. It is one of eight genera that comprise the two families of Nymphaeales, or water lilies: Cabombaceae (Brasenia, Cabomba) and Nymphaeaceae (Victoria, Euryale, Nymphaea, Ondinea, Barclaya, Nuphar). Evidence from a range of studies indicates that Nymphaeales are among the most primitive angiosperms. Despite their phylogenetic utility, pollen developmental characters are not well known in Brasenia. This paper is the first to describe the complete pollen developmental sequence in Brasenia schreberi. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using combined scanning electron, transmission electron, and light microscopy. Both tetragonal and decussate tetrads have been identified in Brasenia, indicating successive microsporogenesis. The exine is tectate-columellate. The tetrad stage proceeds rapidly, and the infratectal columellae are the first exine elements to form. Development of the tectum and the foot layer is initiated later during the tetrad stage, with the tectum forming discontinuously. The endexine lamellae form during the free microspore stage, and their development varies in the apertural and non-apertural regions of the pollen wall. Degradation of the secretory tapetum also occurs during the free microspore stage. Unlike other water lilies, Brasenia is wind-pollinated, and several pollen characters appear to be correlated with this pollination syndrome. The adaptive significance of these characters, in contrast to those of the fly-pollinated genus Cabomba, has been considered. Brasenia does not produce pollenkitt nor develop tectal microchannels as does Cabomba. Instead, the discontinuity of the tectum reduces the amount of sporopollenin in the wall, which may allow for more effective wind dispersal. The importance of reassessing palynological characters in light of new ontogenetic data and the phylogenetic implications of this reevaluation are also discussed.

Comparative Structure and Development of Pollen and Tapetum in Pandanales

The newly recircumscribed monocot order Pandanales now includes five families: Cyclanthaceae, Pandanaceae, Stemonaceae, Triuridaceae, and Velloziaceae. We present new data and a review of pollen morphology, microsporogenesis, and tapetal characters in Pandanales in a systematic context. Probable plesiomorphic character states include pollen dispersed as monads, pollen small to medium in size with an ovoid shape, monosulcate apertures, perforate to reticulate sculpturing, a columellate exine with a foot layer, successive microsporogenesis, and a secretory tapetum. These correspond to generalized monocot or lilioid pollen and anther character states, and all occur within Dioscoreales, the sister group to Pandanales. Within Pandanales, there are apparently homoplastic trends in each family toward either reduction in aperture size (e.g., from sulci to pores) or covering the aperture with exine (e.g., from monosulcate to monosulcate‐operculate), resulting in a reduction in the delicate apertural area exposed t...

Embryology and seed development of Blastocaulon scirpeum and Paepalanthus scleranthus (Eriocaulaceae)

The embryology and seed structure of Blastocaulon scirpeum (Mart.) Giul. and Paepalanthus scleranthus Ruhland were studied in order to contribute to the embryology of Eriocaulaceae and supply data for future taxonomic studies. Both species present: anther with 4-layered wall; conspicuous endothecium with fibrous thickenings; secretory tapetum with uninucleate cells; successive microsporogenesis forming isobilateral microspore tetrads; bicellular pollen grains; orthotropous, bitegmic and tenuinucellate ovule; micropyle formed by the inner integument alone; megagametophyte of the Polygonum type, with a conspicuous antipodal cyst; nuclear and starchy endosperm; reduced, undifferentiated, and bell-shaped embryo; operculate and endotestal seed; seed coat derived from the two ovule integuments; and tanniniferous endotegmen. In addition, Blastocaulon scirpeum shows a bisporangiate anther and a 3-layered ovary wall, while P. scleranthus presents a tetrasporangiate anther that becomes bisporangiate at maturity, and a 2-layered ovary wall. This investigation shows that the bisporangiate condition does not suffice to separate Blastocaulon from Paepalanthus, since it is common to both. It also indicates, based on several embryological aspects, the proximity of Eriocaulaceae and Xyridaceae, which comply mainly with the features presented by the other commelinid families. These results may be used in future cladistic analysis of the family, and contribute to a better under-standing of its phylogeny.

Evolution of Microsporogenesis in Angiosperms

Microsporogenesis is highly labile in early‐branching angiosperms, i.e., those with mostly sulcate pollen, compared with the tricolpate and tricolpate‐derived eudicots. New records of microsporogenesis in basal angiosperms (19 taxa were examined), together with a review of the literature, demonstrate that the existing typology has been too strictly applied; several basal angiosperms have apparently intermediate forms and therefore do not fit easily into simultaneous or successive categories. Intermediate forms include the “modified simultaneous” type, where ephemeral cell plates are formed after the first meiotic division but then disperse, and simultaneous cleavage follows the second meiotic division. This relative diversity reflects a range of variation in number and position of pollen apertures in basal angiosperms, although both monosulcate and inaperturate pollen may occur in conjunction with either simultaneous or successive microsporogenesis. However, many taxa with inaperturate pollen have successive microsporogenesis, whereas many monosulcate taxa have the simultaneous type (although successive and monosulcate is common in monocotyledons). The predominance of simultaneous microsporogenesis in extant basal angiosperms and in land plants in general (including gymnosperms) indicates that simultaneous microsporogenesis is plesiomorphic in angiosperms, despite the occurrence of the successive type in the putative first‐branching extant angiosperm, Amborella. This conclusion contradicts earlier views on the evolutionary polarity of this character.

Microsporogenesis and systematics of Aristolochiaceae

Within Aristolochiaceae, a secretory tapetum and orbicules are ubiquitous, but both simultaneous and successive types of microsporogenesis occur. Simultaneous cytokinesis is apparently plesiomorphic within the order Piperales, in which Aristolochiaceae are now placed. Successive microsporogenesis was found only in species of Aristolochia confined to a crown clade in the proposed phylogeny of this genus. In contrast to many other taxa, within Aristolochiaceae there is no strict relationship between microsporogenesis type and tetrad configuration, which is strongly influenced by spindle orientation, especially during meiosis II. There is also no direct correlation between microsporogenesis type and the aperture of mature pollen grains.

Microsporogenesis and systematics of Aristolochiaceae

Within Aristolochiaceae, a secretory tapetum and orbicules are ubiquitous, but both simultaneous and successive types of microsporogenesis occur. Simultaneous cytokinesis is apparently plesiomorphic within the order Piperales, in which Aristolochiaceae are now placed. Successive microsporogenesis was found only in species of Aristolochia confined to a crown clade in the proposed phylogeny of this genus. In contrast to many other taxa, within Aristolochiaceae there is no strict relationship between microsporogenesis type and tetrad configuration, which is strongly influenced by spindle orientation, especially during meiosis II. There is also no direct correlation between microsporogenesis type and the aperture of mature pollen grains.

Pollen and anther characters in monocot systematics

Pollen and anther characters are potentially informative in higher-level systematics of monocotyledons. Several characters of monocot pollen and anthers (tapetum type, microsporogenesis type and inaperturate pollen) are reviewed here in relation to recent phylogenetic concepts of the group, and new data are presented for some critical taxa. The first-branching monocotyledon, Acorus , has a secretory tapetum but most other early branching taxa (i.e., most Alismatales, except Tofieldia ) are plasmodial. The lilioid orders, Pandanales, Dioscoreales, Liliales and Asparagales are almost uniformly secretory. The tapetum is more diverse within the commelinoid clade. Successive microsporogenesis predominates in the monocotyledons although the simultaneous type is of systematic significance within some orders, such as Dioscoreales,Asparagales and Poales. Inaperturate pollen (either "functionally monoaperturate" or "omniaperturate") occurs in every major monocot group. It predominates in Alismatales and Zingiberales, and is a synapomorphy for some Liliales and Asparagales.

Microsporogenesis in Monocotyledons

This paper critically reviews the distribution of microsporogenesis types in relation to recent concepts in monocot systematics. Two basic types of microsporogenesis are generally recognized: successive and simultaneous, although intermediates occur. These are characterized by differences in tetrad morphology, generally tetragonal or tetrahedral, although other forms occur, particularly associated with successive division. Successive microsporogenesis is predominant in monocotyledons, although the simultaneous type characterizes the ‘lower’ Asparagales. Simultaneous microsporogenesis also occurs inJaponolirion and Petrosavia (unplaced taxa), some Araceae, Aponogeton, Thalassia andTofieldia (Alismatales), Dioscorea, Stenomeris and Tacca (Dioscoreales), and some Commelinanae: Arecaceae (Arecales), and Cyperaceae, Juncaceae and Thurniaceae (Poales). Simultaneous microsporogenesis is of phylogenetic significance within some of these groups, for example, Asparagales, Dioscoreales and Poales. An intermediate type is recorded in Stemonaceae (Pandanales), Commelinaceae (Commelinales) and in Eriocaulaceae and Flagellariaceae (Poales). There is little direct relationship between microsporogenesis type and pollen aperture type in monocots (except for trichotomosulcate and pantoporate apertures), although trichotomosulcate apertures in monocot pollen, and equatorial tricolpate and tricolporate apertures in eudicot pollen, are all related to simultaneous microsporogenesis.