Fossil Records in the Lythraceae

The Lythraceae (Myrtales) are a family of 28 genera and ca. 600 species constituting with the Combretaceae and sister family Onagraceae a major lineage of the Myrtales and including the former Sonneratiaceae, Duabangaceae, Punicaceae, and Trapaceae. The fossil record of the family is extensive and significant new discoveries have been added to the record in recent years. This review provides a vetted summary of fossils attributed to the Lythraceae, their geographic distributions, and their stratigraphic ranges. It anticipates the use of the information to generate robustly dated molecular phylogenies to accurately reconstruct the evolutionary and biogeographic history of the family. Fossils of 44 genera or form genera have been attributed to the Lythraceae; 24 are accepted here as lythracean. Fourteen of the 28 modern genera have fossil representatives: Adenaria, Crenea, Cuphea, Decodon, Duabanga, Lafoensia, Lagerstroemia, Lawsonia, Lythrum, Pemphis, Punica, Sonneratia, Trapa, and Woodfordia. Ten extinct genera are recognized. The most common kinds of fossil remains are seeds and pollen. The only fossil flower confidently accepted in the family is the extinct genus Sahnianthus from the Early Paleocene of India. The oldest confirmed evidence of the Lythraceae is pollen of Lythrum/Peplis from the Late Cretaceous (early Campanian, 82?81 Ma) of Wyoming. Seeds of Decodon from the late Campanian (73.5 Ma) of northern Mexico are next oldest. Sonneratia, Lagerstroemia, and extinct Sahnianthus first appear in the Paleocene of the Indian subcontinent; extinct Hemitrapa fruits first occur in the Paleocene of northwestern North America. Diversification of the Lythraceae occurred primarily during two major periods of global temperature change, during the Paleocene-Eocene Thermal Maximum and from the middle Miocene forward when temperatures decreased markedly and seasonality and dry-adapted vegetation types became more prominent. Fossils of the Lythraceae from South America and Africa are limited in number. The few dates available for South American genera are comparatively young and diversification of the largest genus, Cuphea (ca. 240 species), was mainly a Quaternary event. A phylogeny of the family is briefly explored and examples of specialized characters occurring in the oldest known genera are noted. The fossil record of the Lythraceae is presently too fragmentary to confidently reconstruct the early history of the family. The record indicates, however, that the family was well-diversified and widely dispersed globally over a wide latitudinal range by the end of the Paleocene.     

Phylogenetic Analysis of the Sonneratiaceae and its Relationship to Lythraceae Based on ITS Sequences of nrDNA

Lagerstroemia nested within the Sonneratiaceae. The Sonneratiaceae occurred within the Lythraceae with high bootstrap value support (96%). The two traditional genera constituting Sonneratiaceae were in different well-supported clades. Duabanga (Sonneratiaceae) is sister to the clade of Lagerstroemia (Lythraceae) (82%). The mangrove genus Sonneratia (100%) formed the other monophyletic group. It was located terminally within the Lythraceae clade and comprised two clades: one consisting of S. apetala, S. alba, S. ovata, and S. hainanensis; the other including S. caseolaris and S. paracaseolaris. The results indicated that species previously placed in two different sections (Sect. Sonneratia and Sect. Pseudosonneratia) of Sonneratia occurred within the same clade, and the taxonomic classification was not supported by the molecular analysis of the ITS region sequences. Based on the phylogenetic analyses of the ITS regions, the Sonneratiaceae were shown to be nested within the family Lythraceae. Therefore, the sequence data presented here do not support the recognition of the Sonneratiaceae as a distinct family, but instead support the inclusion of Sonneratiaceae in the Lythraceae proposed by other authors. Received 27 December 1999/ Accepted in revised form 25 June 2000     

PALYNOLOGY AND SYSTEMATICS OF THE LYTHRACEAE. I. INTRODUCTION AND GENERA ADENARIA THROUGH GINORIA

A palynological survey, including LM, SEM, and TEM, is presented for 10 genera of the Lythraceae s.s. (plus Alzatea to be assigned separate family status): Adenaria, Ammannia, Capuronia, Crenea, Cuphea, Decodon, Didiplis, Diplusodon, Galpinia, and Ginoria. Discussion of the results is deferred until the survey is complete, but already information of taxonomic interest is available for the 11 genera studied. The pollen data, consistent with results from cytology, anatomy, and embryology, suggest the current subgeneric classification into the Tribes Lythreae and Nesaeeae based on complete or incomplete septation of the ovary is not valid. The pollen of Alzatea is a generalized type and lacks the morphological features most characteristic of the Lythraceae (pseudocolpi, sculptured exine). The genus is being described as a separate family, and the pollen morphology is consistent with this treatment. The pollen of Cuphea is more diverse and shows little similarity to other genera in the family. It also has a unique combination of floral characters and lipid chemistry that suggests a separate category within the developing subfamilial classification of the Lythraceae.     

The Complete Plastid Genome of Lagerstroemia fauriei and Loss of rpl2 Intron from Lagerstroemia (Lythraceae)

Lagerstroemia (crape myrtle) is an important plant genus used in ornamental horticulture in temperate regions worldwide. As such, numerous hybrids have been developed. However, DNA sequence resources and genome information for Lagerstroemia are limited, hindering evolutionary inferences regarding interspecific relationships. We report the complete plastid genome of Lagerstroemia fauriei. To our knowledge, this is the first reported whole plastid genome within Lythraceae. This genome is 152,440 bp in length with 38% GC content and consists of two single-copy regions separated by a pair of 25,793 bp inverted repeats. The large single copy and the small single copy regions span 83,921 bp and 16,933 bp, respectively. The genome contains 129 genes, including 17 located in each inverted repeat. Phylogenetic analysis of genera sampled from Geraniaceae, Myrtaceae, and Onagraceae corroborated the sister relationship between Lythraceae and Onagraceae. The plastid genomes of L. fauriei and several other Lythraceae species lack the rpl2 intron, which indicating an early loss of this intron within the Lythraceae lineage. The plastid genome of L. fauriei provides a much needed genetic resource for further phylogenetic research in Lagerstroemia and Lythraceae. Highly variable markers were identified for application in phylogenetic, barcoding and conservation genetic applications.     

PALYNOLOGY AND SYSTEMATICS OF THE LYTHRACEAE. III. GENERA PHYSOCALYMMA THROUGH WOODFORDIA,ADDENDA, AND CONCLUSIONS

Pollen of the 27 genera presently recognized as comprising the family Lythraceae have been surveyed with light microscopy and scanning and transmission electron microscopy. Results for five genera (Physocalymma, Pleurophora, Rotala, Tetrataxis, Woodfordia), in addition to Duabanga, Sonneratia, and Punica (assigned to the Lythraceae in some classifications), are presented here; the remaining genera were treated previously in the series. The family is revealed as the most diverse palynologically of the order Myrtales. The most simple pollen type and the one common to the largest number of genera is prolate-spheroidal to prolate; tricolporate, without pseudocolpi; psilate, scabrate or finely verrucate; and 16–28 μm or less in length. Specializations include oblate grains, development of pseudocolpi (three or six in number), diversification of exine sculpturing, broadening of the colpal and pseudocolpal areas, and reduction in the conspicuousness of the colpi. Pollen evidence provides qualified support for inclusion of Punica in the Lythraceae, the generalized nature of the pollen tempering the conclusion, and little support for inclusion of Sonneratia and Duabanga in the family. Completion of the survey provides a data base of pollen characters that will be integrated in future studies with other evidence into an overall phenetic and cladistic assessment of the family leading to production of a more natural classification.     

POLLEN MORPHOLOGY AND DETAILED STRUCTURE OF FAMILY COMPOSITAE,TRIBE CICHORIEAE. I. SUBTRIBE STEPHANOMERIINAE

A survey of pollen morphology in 20 species representing the 11 genera of the North American subtribe Stephanomeriinae by light, scanning electron, and transmission electron microscopy revealed 10 of the 11 genera to have echinate, tricolporate pollen grains, Lygodesmia being the only genus with echinolophate pollen. Sectioned exines of most of the species examined are similar, being composed of ektexine and endexine. The ektexine surface is composed of spines which typically have globose perforate bases. A cavus occurs as a separation between the basis (foot layer) and the columellae in all of the genera examined except Chaetadelpha. Pollen of the two species of Glyptopleura were found to be strikingly different in exomorphology. Pollen of the putatively self-fertile G. marginata has much shorter spines than the closely related G. setulosa. Atrichoseris, Anisocoma, Calycoseris, Glyptopleura, Pinaropappus, Prenanthella, and most species of Malacothrix have pollen which lack paraporal ridges. The remaining genera, Chaetadelpha, Lygodesmia, Rafinesquia, and Stephanomeria have well-developed ridges of fused spine bases around the apertures. Pollen characters, particularly those of the aperture region, have been found to be systematically useful in the subtribe, therefore acetolyzed material gives more useful information than untreated pollen.     

CONTRIBUTION OF POLLEN MORPHOLOGY TO SYSTEMATICS OF COLLOMIA (POLEMONIACEAE)

Pollen morphology of 14 species of Collomia (Polemoniaceae) was examined by light microscopy, and by both scanning and transmission electron microscopy. Four distinct pollen types were observed which are based principally upon 1) shape, number and distribution of apertures, and 2) surface sculpturing: Type 1—zonocolporate with striate ridges; Type 2—zonocolporate with striato-reticulate ridges; Type 3—pantoporate with radiate ridges; Type 4—pantoporate with irregularly reticulate ridges. Evaluation of pollen morphology reveals considerable discrepancy with respect to presently accepted sectional classification. Collomia grandiflora of sect. Collomia has a pollen type similar to that of members of sect. Collomiastrum and is now interpreted as representing an independent evolutionary line derived from the latter section. Collomia diversifolia of sect. Courtoisia has a pollen morphology similar to that of sect. Collomia. whereas C. heterophylla of the same section possesses pollen unique within the genus. This last pollen type shows close similarity to the pollen of members of Polemonium, Gilia, Leptodactylon, and Ipomopsis. Pollen of C. tinctoria and C. tracyi of sect. Collomia are anomalous within Polemoniaceae. No significant difference in exine stratification was discernible among the four pollen types.     

Exine architecture and function in some lythraceae and sonneratiaceae

The pollen morphology of selected species of Crenea, Lagerstroemia, Lafoensia and Diplusodon (Lythraceae) and of Sonneratia (Sonneratiaceae) is compared from a functional point of view. From a basically tricolporate type, multiplication of colpi, reduction of colpi, development of meridional ridges, development of polar caps, differentiation of pores and reduction of pores have led to partly diverging, partly converging structural types which are interpreted as adaptation to controlled harmomegathy. In some species of Lagerstroemia with dimorphic stamens, the exine of the pollen produced by the feeding anthers shows a specialized structure, which enables the pollen grain to retain moisture and become sticky. For Sonneratia the fossil record allows a check on the postulated evolutionary trends.     

Pollen morphology of the Flueggeinae (Euphorbiaceae,Phyllanthoideae)

A total of 129 species from the subtribe Flueggeinae of the tribe Phyllantheae (Euphorbiaceae, Phyllanthoideae) were investigated using light and scanning electron microscopy, and 10 species using transmission electron microscopy, in order to evaluate the relationships between the eight constituent genera: Breynia, Flueggea, Glochidion, Margaritaria, Phyllanthus, Reverchonia, Richeriella, and Sauropus. Of these genera, Flueggea, Margaritaria and Richeriella share pollen with a prolate spheroidal meridional outline and a 3-colporate aperture system. Pollen of Reverchonia is also 3-colporate, but differs from that of the Flueggea alliance by its clearly prolate shape, tilioid ornamentation and absence of costae endopori. Breynia and Sauropus have 4–12 and 3–16-colporate pollen, respectively, with diploporate colpi. Two pollen types are recognised in Breynia, and four in Sauropus, one of which supports the recognition of Sect. Hemisauropus. Glochidion pollen is 3–6-colporate, and similar to that of Breynia in having reticulate sculpture with Y-shaped sexine structures, but it has monoporate colpi. Of the genus Phyllanthus, only species with pollen with diploporate colpi have been studied. Seven types are described. Diploporate Phyllanthus pollen can be distinguished from that of Breynia and Sauropus by its distinct colpus margins consisting of parallel muri. Colpal irregularities and endoaperture configurations in the subtribe are discussed, and pollen morphological trends are hypothesised. Placed in the successiform aperture series, the Flueggea alliance and Reverchonia form a basal group. Glochidion is considered intermediate, giving rise to the Breynia-Sauropus group. The relationship with Phyllanthus remains unclear.     

COMPARATIVE POLLEN MORPHOLOGY AND TAXONOMIC AFFINITIES IN CYCADALES

Scanning and transmission electron microscopy of pollen grains of 29 species, representing the ten extant genera of Cycadales, has provided valuable insight into their relationships. Pollen grains of these taxa are boat-shaped, monosulcate, and bilaterally symmetrical. They range from narrowly to widely elliptical or subcircular when viewed distally, and have an exine surface of psilate, foveolate, or fossulate. Pollen wall ultrastructure of Cycadales is typically tectate with alveolate∗∗∗spongy exine. The nexine is laminated in all genera. Nexine 1 (footlayer) is present in most species as a thin and often discontinuous layer. There is consistent variation in thickness of the sporoderm layers among the genera but relative uniformity within them. Pollen characteristics are well correlated with macro- and micromorphological features, chromosome numbers, geographical distribution, and postulated pollination mode. A close affinity between Encephalartos, Lepidozamia, and Macrozamia is recognized. Pollen characteristics of the genus Bowenia show some similarity with those of the latter group. Except for two species of Macrozamia which are narrowly elliptic, all of the genera have widely elliptic pollen and share a psilate exine surface and the thinnest sexine with nearly identical arrangement of alveoli. Pollen grains of the species in the genus Dioon exhibit a unique morphology but are more similar to Stangeria than they are to those of taxa in Zamiaceae. The circular outline of the grains and the foveolate exine surface are characters shared by these two genera, but several morphological features distinguish Dioon from Stangeria. Ceratozamia and Zamia share a widely elliptic shape, foveolate exine surface and nearly identical sexine, as well as morphological features and chromosome numbers. They differ from Microcycas in sexine thickness, gross morphology and chromosome numbers. The pollen grains of Cycas circinalis and C. revoluta differ in size and structure of the sexine from all other genera and from each other, substantiating their distinct subgeneric delimitations.     

Pollen morphology of Trigonostemon and its relatives (Euphorbiaceae)

The pollen of Trigonostemon and the related genera Dimorphocalyx, Ostodes, Tritaxis and Jatropha (outgroup) has been studied with light microscopy, and scanning and transmission electron microscopy. The two major pollen types within Trigonostemon correlate well with macromorphological characters. Species belonging to the Trigonostemon reidioides type have pollen with ‘croton pattern’ ornamentation, a pistil with deeply divided stigmas (to at least half the length of the stigma arm) and stamens with a protruding appendage on the connective, while species of the Trigonostemon verrucosus type have verrucate (to almost gemmate) pollen, stigmas that are shortly cleft and stamens without an appendage on the connective. Dimorphocalyx, Ostodes, Tritaxis and Jatropha (outgroup) have similar pollen morphology, while Trigonostemon deviates from these genera in the absence of the ‘vertically’ striate ornamentation on the subunits. Therefore, when compared with an existing phylogeny of the Euphorbiaceae, the pollen characters of Trigonostemon appear to be derived. Moreover, because the ‘croton pattern’ ornamentation itself is widely shared by the ‘inaperturate crotonoids’, the loss of that structure in the Trigonostemon verrucosus type pollen is considered a further apomorphy.     

The geologic history of the Lythraceae

The known fossil record of the Lythraceae has been amplified by recent studies in northern Latin America. A total of 18 genera is recognized in geologic strata ranging in age from lower Eocene to Recent, and among the 22 or 23 modern genera, seven have a documented geologic history. The oldest remains are from an Indo-Malayan Old World warm-temperate to subtropical vegetation preserved in the lower Eocene London Clay flora. The most ancient of extant genera isLagerstroemia and the most recent (among those with an adequate fossil record) isCuphea (middle Miocene to Recent). These represent, respectively, primitive and advanced members of the family, and the paleobotanical record supports current concepts concerning phylogenetic relationships among genera of the Lythraceae. The family apparently had an Old World origin and became differentiated into a distinct modern taxon during Paleocene and early Eocene time.     

Pollen morphology and systematics of Atripliceae (Chenopodiaceae)

Pollen morphology of 58 species from 17 putative genera of the tribe Atripliceae (Chenopodiaceae) was investigated using light (LM) and scanning electron microscopy (SEM). Morphological variation was analyzed based on a dense sampling of the subtribes Atriplicinae and Eurotiinae, including many of the species in the two largest genera: Atriplex and Obione. The pantoporate pollen grains of Atripliceae are characterized by their spheroidal or subspheroidal shape, flat or moderately vaulted mesoporia with 21–120 pores, tectum with 1–8 spinules and 5–28(?38) puncta per?µm², and 1–13 ectexinous bodies bearing 1–7 spinules each. Taxonomic relevance of the most important pollen morphological characters is discussed (pollen diameter, pore number, pore diameter, interporal distance, spinule and puncta density and ratio, number of ectexinous bodies, and their spinules). Pollen morphological data support the exclusion of Suckleya from the tribe and the recognition of subtribe Eurotiinae, but suggest that it needs to be reviewed. Pollen does not support generic recognition of Atriplex, Neopreissia and Obione and infrageneric subdivisions as currently recognized, and suggests the need to review them. Smaller or monotypic genera, such as Axyris, Ceratocarpus, Endolepis, Krascheninnikovia, Microgynoecium, Proatriplex and Spinacia have distinctive pollen morphological characters that support their generic status. Grayia needs to be reevaluated; although its two species are distinct from all the other species in the study, there are notable differences between each of them, and this suggests they may not form a natural group. Multivariate techniques were employed to investigate if there are discrete patterns of variation within Atripliceae. Principal Component Analyses (PCA) weakly differentiates four groups based on variation in pore number, puncta density per?µm², and ratio between spinule and puncta density per?µm²; species of Ceratocarpus, Haloxanthium, Krascheninnikovia, Manochlamys, Microgynoecium, Spinacia, and some species of Atriplex and Obione are isolated. Preliminary results indicate that pollen data are potentially useful in the classification of the tribe, and further studies will be of taxonomic value.     

Comparative pollen morphology and ultrastructure of Platanus: Implications for phylogeny and evaluation of the fossil record

Pollen of Platanus was studied using light (LM) and electron microscopy (SEM and TEM). Overall, pollen is uniform in modern Platanus (small, tricolpate, prolate to spheroidal, reticulate, semitectate). A number of characters, however, display remarkable variability within a taxon and even a single anther (size; foveo‐reticulate, fine to coarse reticulate ornamentation). Platanus kerrii (subgenus Castaneophyllum) differs from the remaining species by its high and “folded” reticulum and possibly the smooth colpus membrane. Moreover, to our knowledge, pollen of the P. kerrii – type is not known from the fossil record. The exine in modern and fossil Platanaceae shows great structural similarity, but the thickness of the foot layer within the ectexine is less variable and normally smaller in modern taxa. Furthermore, in Early Cretaceous to Early Cainozoic Platanaceae a number of distinct pollen types occurred that are not known within the modern Platanus. Considering pollen of Platanaceae from the Early Cretaceous to today, a dynamic picture of the evolution of the family emerges. In the first phase (Early Cretaceous) pollen of extinct genera such as Aquia differed considerably from modern Platanus and shows strong similarity to basal eudicot taxa such as Ranunculales (e.g. Lardizabalaceae). The Late Cretaceous Platananthus hueberi displays a distinct coarse reticulum that is unknown from modern Platanus but similar to some taxa of Hamamelidaceae (e.g. Exbucklandia). After the first phase of eudicot radiation that appears to have been characterized by strongly reticulate evolution, platanaceous diversity decreased in the course of the Cainozoic. Despite this, the pollen type of the modern subgenus Castaneophyllum (P. kerrii type) seems to be an innovation that originated after the initial radiation of the family.     

Floral morphology and taxonomic relations among the genera of Amaranthaceae in the New World and the Hawaiian Islands

Twenty-three genera of Amaranthaceae occur in the New World. Two endemic genera occur in the Hawaiian Islands. Among the genera of the subfamily Amaranthoideae, Celosia, Cyathula and Achyranthes have their main distributions in the Old World; the two last-named genera are represented in the Americas only by widespread weeds. All the New World genera of the subfamily Gomphrenoideae are mainly or entirely restricted to this region. Characters of androecium and gynoecium are fundamental in the recognition of genera within the family. Androecia of different genera may be structurally and phylogenetically more similar than would appear from a cursory examination. It is suggested that the type of staminal tubes found in Pseudogomphrena and Froelichia can be derived from that in Alternanthera and Froelichiella by reduction of filament length and a fusion of pseudostaminodia with the filaments. The staminal tube in Gomphrena could result from a further decrease in distance between pseudostaminodia of the Pseudogomphrena type, and a deeper forking of the pseudostaminodia; each so-called apical filament lobe in Gomphrena would then be homologous with half a pseudostaminodium in Pseudogomphrena. Much of the variation in the androecia of these and other genera, as well as within genera such as Pfaffia, can be explained as the combined results of coalescence and splitting-up tendencies. Splitting up of staminal tubes may not necessarily take place along the borders of phylogenetically original filaments and pseudostaminodia. The Amaranthus-type of pollen is found in the majority of genera of the subfamily Amaranthoideae, but also in the Chenopodiaceae. A group of genera within the subfamily Gomphrenoideae also has pollen very similar to, or identical with, this type. Most genera of the subfamily Gomphrenoideae have pollen of the Gomphrena-type. Pseudoplantago has unilocular (at anthesis) anthers, a characteristic of the subfamily Gomphrenoideae, but floral structure as well as pollen morphology connect the genus to a group of genera within the Amaranthoideae, subtribe Achyranthinae. The combination of subcuboidal shape and opercula with radially arranged hooked protuberances, makes the pollen of Pseudoplantago unique among the angiosperms studied so far. Floral morphology and palynological characteristics indicate a close relationship between Pfaffia and Alternanthera. Both genera, as currently accepted, are relatively homogeneous from pollen morphological points of view. There are no correlations between pollen morphology and the variation in the androecium in Pfaffia, nor would pollen structure support recognition of Hebanthe as a distinct genus. Woehleria and Irenella may be derived from, or be of the same origin as, Dicraurus and Iresine. All four genera are placed in the subfamily Gomphrenoideae because of the bisporangiate anthers, but their pollen structure is very close to, or identical with, that of the Amaranthus-type. Pseudogomphrena combines characteristics of Gomphrena and Pfaffia.     

Morphology and Molecular Phylogeny of Staurojoenina mulleri sp. nov. (Trichonymphida,Parabasalia) from the Hindgut of the Kalotermitid Neotermes jouteli

Staurojoenina is a large and structurally complex genus of hypermastigont parabasalians found in the hindgut of lower termites. Although several species of Staurojoenina have been described worldwide, all Staurojoenina observed to date in different species of North American termites have been treated as the same species, S. assimilis. Here, we characterize Staurojoenina from the North American termite Neotermes jouteli using light microscopy, scanning electron microscopy, and phylogenetic analysis of small subunit ribosomal RNA, and compare it with S. assimilis from its type host, Incisitermes minor. The basic morphological characteristics of the N. jouteli symbiont, including its abundant bacterial epibionts, are similar as far as they may be compared with existing data from S. assimilis, although not consistently identical. In contrast, we find that they are extremely distantly related at the molecular level, sharing a pairwise similarity of SSU rRNA genes comparable to that seen between different genera or even families of other parabasalians. Based on their evolutionary distance and habitat in different termite genera, we consider the N. jouteli Staurojoenina to be distinct from S. assimilis, and describe a new species, Staurojoenina mulleri, in honor of the pioneering parabasalian researcher, Miklos Muller.     

SYSTEMATIC SIGNIFICANCE OF POLLEN NUCLEAR NUMBER IN EUPHORBIACEAE,TRIBE EUPHORBIEAE

Pollen nuclear number is determined in 139 species of 5 genera in the Euphorbieae, subtribe Euphorbiinae. The 111 new determinations are tabulated along with previous reports, and the results indicate that the distribution of binucleate (II) and trinucleate (III) pollen is strongly associated with the taxonomic groupings within the Euphorbieae. Although binucleate pollen is probably primitive within the tribe Euphorbieae, as suggested by the nuclear condition in Neoguillauminia, the situation in Euphorbia still requires further elucidation. Within Euphorbia, the morphologically most primitive species studied have III pollen despite the fact that II pollen is presumably the original condition for the subtribe Euphorbiinae. In Euphorbia, II pollen only is reported from nine sections and III pollen only from ten sections, while in four sections (Esula, Goniostema, Aphyllis, and Deuterocalli) both II and III pollen have been found. The New World species of Euphorbia nearly all have III pollen, whereas the vast majority of the African succulents have II pollen. The genera of New World origin, Chamaesyce and Pedilanthus, have III pollen, while the African genera Monadenium and Synadenium have II pollen. Independent derivations of III pollen from II pollen appear to have occurred in sections Goniostema, Aphyllis, and Deuterocalli (all of subg. Euphorbia). There is no evidence that reversals from III to II pollen have occurred.     

POLLEN MORPHOLOGY AND THE RELATIONSHIPS OF CIRCAEASTER,OF KINGDONIA,AND OF SARGENTODOXA TO THE RANUNCULALES

The pollen of three monotypic genera, Circaeaster, Kingdonia, and Sargentodoxa has been examined by light and scanning electron microscopy and in the case of the last genus, also by transmission electron microscopy. The type of tectum found in Circaeaster and Kingdonia, derivations of a compound layer of striae, has a restricted distribution in the Order Ranunculales. Of 64 genera examined in this order only six had species with a similar tectum. They include Achlys, Epimedium, Jeffersonia, and Vancouveria of the Berberidaceae s.l., the controversial Hydrastis, and Trollius of the Ranunculaceae. Circaeaster and Kingdonia have been considered as related since both have rare and primitive vegetative characteristics, the most notable being open dichotomous leaf venation. They are probably best treated as a ditypic family, Circaeasteraceae. The pollen of Sargentodoxa, especially the structure of the exine, closely resembles that of the Lardizabalaceae. However, the fruits of Sargentodoxa have been considered to be distinct from those of the Lardizabalaceae, suggesting that it be treated as a separate, but closely allied, family.     

Pollen morphology within the Monodora clade,a diverse group of five African Annonaceae genera

Pollen morphology has played a major role in elucidating infrafamiliar‐level systematics and evolution within Annonaceae, especially within the African genera. The Monodora clade is composed of five genera, Asteranthe, Hexalobus, Isolona, Monodora and Uvariastrum, which are restricted to Africa and contain together c. 50 species. A molecular phylogeny of the family showed that the monophyly of the Monodora clade is strongly supported and that it is part of a larger clade of 11 African genera. In order to support classification a detailed survey was made of the pollen morphological variation within the Monodora clade, using scanning and transmission electron microsopy. For the two most species‐rich genera, Isolona and Monodora, a molecular species‐level phylogeny was used to assess the taxonomic usefulness of the pollen characters. The survey showed a wide range of pollen morphological diversity. The most conspicuous variation concerned the occurrence of monads without a thicker outer foliation in the basal exine layer in Isolona in contrast to tetrads with a thicker outer foliation in Asteranthe, Hexalobus, Monodora and Uvariastrum. At the infrageneric level, Hexalobus, Isolona and Monodora showed the largest diversity, with various pollen types based on tectum morphology. Hexalobus is exceptional with three types within only five species. The pollen types defined in this study are hardly useful in characterizing major groups identified within both Isolona and Monodora, but they do illustrate relationships within smaller groups.