Cycad Pollen Research Articles

Sedimentary Expressions of the Early Jurassic Jenkyns Event in an Inland Lacustrine System in the Yin'gen–Ejinaqi Basin, North China

AbstractThe Jenkyns Event, more widely known as the Toarcian Oceanic Anoxic Event (T‐OAE), is marked by globally distributed negative carbon‐isotope excursions, widespread oxygen depletion, and large‐scale organic carbon burial, which indicate major climate/environmental perturbations in Earth's surface systems during the Early Jurassic. Although extensive research has been conducted in European continental settings, particularly in the western peri‐Tethys regions, the impacts of this event beyond Europe remains largely unexplored. Here, a multiapproach study including investigations into the spore‐pollen assemblages, pyrite framboids, clay minerals, total organic carbon (TOC) levels, and organic carbon isotope (δ13Corg) levels in a lacustrine borehole section (MED1) from the Yin'gen–Ejinaqi Basin, North China, provides evidence of the occurrence of the Jenkyns Event and its extensive sedimentary responses in the eastern Tethys terrestrial systems. Two distinct spore‐pollen assemblages have been identified in MED1 (drilling depth: 982.4 m to 1267.5 m), with the Cycadopites–Protopinus–Osmundacidites assemblage in the lower part (1267.5 m to 1132.9 m) indicating a middle Early Jurassic age and the Classopollis assemblage in the upper part (1132.9 m to 985.7 m) suggesting a Toarcian age. Framboidal pyrite data suggest more anoxic conditions during the deposition of black mudstone and shale intercalations in the lower part of the Classopollis assemblage (1132.9 m to 1066.9 m), which combined with organic carbon enrichment and negative δ13Corg excursions, are considered the paleoenvironmental response to the Jenkyns Event in the study area. Furthermore, the evolution of vegetation groups changed from plant groups characterized by bisaccate and cycad pollen, as well as fern spores, to vegetation groups represented by Cheirolepidiaceae pollen across the Jenkyns Event, as evidenced by spore‐pollen data, together with the clay mineral assemblage change characterized by a notable increase in illite at the expense of kaolinite, suggests that while a subtropical‐temperate climate persisted, a change toward warmer and drier conditions most likely occurred in the early Toarcian in the study area. In contrast to the humidification evidenced in many coastal settings, this aridification trend in the Yin'gen–Ejinaqi Basin aligns with the conditions in many inland areas. It is hypothesized that the underlying cause of these divergent changes may be linked to certain patterns of spatially variable water availability on land, potentially driven by extremified hydrological conditions.

New stenurothripid thrips from mid-Cretaceous Kachin amber (Thysanoptera, Stenurothripidae).

Hitherto, only two species of the thysanopteran suborder Terebrantia have been reported from mid-Cretaceous Kachin amber (Myanmar). This is here expanded through the discovery of two new genera and species, described and figured as Parallelothripsseparatusgen. et sp. nov. and Didymothripsabdominalisgen. et sp. nov., both of the family Stenurothripidae. Both taxa have key apomorphies of the Stenurothripidae, allowing for a confident assignment as to family. Both species have characteristic comb-like anteromarginal setae, which are discussed along with structural differences between the two sexes. Cycad pollen was found on the thrips' bodies, providing further evidence that Thysanoptera were pollinators of gymnosperms during the mid-Cretaceous.

Palaeoecology and palaeoclimate of an Early Cretaceous peat mire in East Laurasia (Hailar Basin, Inner Mongolia, China)

In this contribution, we report the palynology, organic petrography and carbon isotopes for the #16 Seam in the Lower Cretaceous Yimin Formation of the Hailar Basin, Inner Mongolia, China. Eighteen palynological samples were collected from the coal seam as well as from intraseam partings and the immediate roof. Microfloras comprise a diverse assemblage of ferns, conifers, horsetails, lycopsids, bryophytes, ginkgoes and cycads. Conifer pollen is common, especially representative of Pinaceae (Pinuspollenites) as well as Cheirolepidiaceae (Classopollis) and Araucariaceae (Araucariacites, Chasmatosporites); however, this material is interpreted as mostly derived from an upland source rather than from the mire-forming vegetation. An overrepresentation of conifer pollen may explain the relatively poor abundance and diversity of ginkgo and cycad pollen, compared to what is found in the macrofossil record. The palynological assemblage found in Seam #16 indicates a Barremian to Aptian age. Maceral composition, ash yields, and δ13Corg signature of the coal suggest peat accumulation was controlled by a rising water table from flooding of the mire, coupled with significant ground (peat) fires and/or surface wildfires. These conditions resulted in high levels of detrital mineral matter and charcoal input, the latter represented by inertodetrinite in the coals. The top of the coal seam is characterised by a higher ash yield, lower δ13Corg values and increased sphagnacean spores, which suggest the gradual drowning of the palaeomire. Although angiosperms were present in the Hailar Basin during the time that Seam #16 was accumulating, they are absent from the pollen found in this study. This may indicate that they were not yet adapted to the extreme conditions of the mire (e.g., acidity, anoxia), and unable to compete with the established mire vegetation.

Pollen morphology of the Megamexican cycads reveals the potential of morphometrics to identify cycad genera

Background: Although pollen morphology has always been an important tool in plant systematics, the possibility of distinguishing between related taxa is still debated. A difficult case is the cycads, where their morphology and small size is very similar. Pollen morphology has been important in plant systematics and determination at the species level is possible depending on the group. Controversial cases are the cycads, where their small pollen and smooth sculpture when observed under light microscopy present few useful characters for separation at the species level.
 Question: Will cycad pollen morphometry be useful to distinguish taxa?
 Study species: 15 species within the genera Ceratozamia, Dioon and Zamia were investigated with five species of each genus.
 Methods: Pollen was obtained from both live specimens from the Clavijero Botanic Garden of the Instituto de Ecología, A.C., as well as herbarium specimens. Acetolyzed pollen was used for descriptions and morphometry, and non-acetolyzed pollen for SEM images.
 Results: The thickness of the exine and pollen sculpture of Dioon differed from that of Ceratozamia and Zamia thus allowing its identification. The pollen morphology of Ceratozamia and Zamia is very similar and therefore difficult to distinguish between species.
 Conclusions: The thicker exine of Dioon allows its pollen to be distinguished from the other two genera. Morphometric methods must be applied to distinguish pollen between species of Ceratozamia and Zamia.

Pollination of cycads in an urban environment

Research on urban systems has documented the contributions of cities as sites where insect pollinators undergo their life cycles, contributing to the reproduction of many groups of plants. However, reports of plants whose reproduction is assisted by specialist pollinators under conditions prevalent in cities are scarce. Cycads and insect pollinators are threatened mainly by loss and modification of their habitats. Here, we describe two cases in which cycad species from two genera distributed in Mexico (Ceratozamia tenuis (Dyer) D.W. Stev. & Vovides and Dioon edule Lindl.) reproduce and germinate successfully in urban areas, aided by insects, near their natural distribution areas. The plants examined were artificially planted in different gardens within the city of Xalapa-Enríquez. We found that specimens two genera of insect pollinators, Pharaxonotha sp. (Erotylidae: Pharaxonothinae) on C. tenuis, and Parallocorynus sp., (Belidae: Oxycoryninae) on D. edule, can survive in cycad pollen strobili, maintaining ecological interactions as they would occur in wild, conserved environments. In addition, we found beetles in ovulate strobili during the pollen receptivity phase, which suggests that they effectively reach ovulate strobili with pollen from pollen strobili. Characterization of this gymnosperm–beetle pollination system, which is unexpected in urban areas due the effects of human disturbance on insect communities, could promote new conservation biology research on flagship species in environments and landscapes shaped by anthropogenic impacts.

A Myanmar amber cockroach with protruding feces contains pollen and a rich microcenosis.

Early endosymbiotic interactions are recorded only from a Cretaceous termite and a cockroach. Mesoblatta maxi Hinkelman, gen. et sp. nov. is the second representative of the dominant, cosmopolitan Mesozoic family Mesoblattinidae known from Cenomanian northern Myanmar amber, and the fourteenth from both amber and sedimentary rocks. Unique characters are rare (n = 19), symplesiomorphies are frequent (n = 140), and foremost is a standard maxillary palp, an irregular area between forewing veins radius and media, central ocellus, and multisegmented styli, suggesting an ancestral position with respect to Blattidae. Autapomorphies of this otherwise conservative taxon are only its large size and a short probasitarsus. Two nymphs with fecal pellets protruding from their body, Blattocoprolites mesoblattamaxi Hinkelman, ichogen. et ichnosp. nov., represent the first cockroaches with formalized coprolites (along with Blattocoprolites blattulidae Hinkelman, ichnosp. nov. established herein from Lebanese amber) and provide evidence of burial defecation. Subhomogenic consistency of coprolites with mucous components, "pseudoinclusions," leaf, trichia, wood debris, cycad pollen, endosymbiotic protists, and epibiotic bacteria directly document pollen transfer through the digestive tract and the earliest coevolution with protists and bacteria. Other post-burial fecal bacteria at the surface are documented for the first time in the Mesozoic, directly indicating structured dung processing. Reference samples (as well as almost all Myanmar amber samples) contain numerous "pseudoinclusions," probably representing damaged or dead cysts of Chlamydomonas hanublikanus Vršanská et Hinkelman, sp. nov. established on the basis of its reproductive stages (with an origin within the resin inside the tree). These are documented together with green algae, including Spirogyra Nees, 1820; flagellates; and flagellate amoebae, promoting massive future microbiota studies.

Comparative Biology of Cycad Pollen, Seed and Tissue - A Plant Conservation Perspective

Cycads are the most endangered of plant groups based on IUCN Red List assessments; all are in Appendix I or II of CITES, about 40% are within biodiversity ‘hotspots,’ and the call for action to improve their protection is long-standing. We contend that progress in this direction will not be made until there is better understanding of cycad pollen, seed and tissue biology, which at the moment is limited to relatively few (<10%) species. We review what is known about germplasm (seed and pollen) storage and germination, together with recent developments in the application of contemporary technologies to tissues, such as isotype labelling, biomolecular markers and tissue culture. Whilst progress is being made, we conclude that an acceleration of comparative studies is needed to facilitate the integration of in situ and ex situ conservation programmes to better safeguard endangered cycads.

Pollen grains associated with gymnospermous mesofossils from the Jurassic of Uzbekistan

Pollen grains were studied which are associated with supposed ginkgoalean seeds and leaves from the Middle Jurassic locality of Angren in Uzbekistan. The fine morphology of the pollen grains was observed with the help of light, confocal laser scanning, scanning electron, and transmission electron microscopes. The morphology and ultrastructure of the pollen grains show that although some of them belong to ginkgoaleans, others belong to conifers and cycadaleans. Bisaccate conifer pollen grains were found in the pollen chambers of seeds of Allicospermum sp. and A. angrenicum. Non-saccate boat-shaped pollen grains that can be ascribed to the genus Cycadopites were found in association with a seed of Ginkgo gomolitzkyana, leaves of Sphenobaiera angrenica, and in pollen chambers of seeds of A. angrenicum. These Cycadopites pollen grains show several types of exine, which allowed us to draw conclusions about their botanical affinities. The Cycadopites pollen associated with a seed of Ginkgo gomolitzkyana shows a ginkgoalean exine ultrastructure. Of interest is that its ultrastructure shows some dissimilarities from that of pollen grains of the ginkgoalean morphology and ultrastructure, which were earlier studied from the pollen chamber of a seed of A. budantsevii from the same locality. We think on this basis that at least two members of the ginkgoaleans grew in the cenose. The Cycadopites pollen grains from leaves of S. angrenica and from pollen chambers of seeds of A. angrenicum show a cycadalean exine ultrastructure. In sum, the morphological studies revealed pollen grains of two members of ginkgoaleans associated with the ginkgoalean remains and pollen grains of conifers that are most probably alien to the enclosing seeds. As to the cycadalean pollen from the seeds of A. angrenicum, the seeds were either contaminated by cycad pollen or produced by a cycadalean parent plant rather than a ginkgoalean plant. The revealed pollen diversity is an argument to be more careful while reconstructing whole plants on the basis of such materials.

Exine ultrastructure of in situ pollen from the cycadalean cone Androstrobus manis Harris, 1941 from the Jurassic of England

Pollen grains extracted from the cycad pollen cone Androstrobus manis Harris, 1941 from the Bajocian of Yorkshire were studied by means of LM, SEM and TEM. Boat-shaped pollen grains of Cycadopites-type were found, with a distinct sulcus and a typically cycadalean foveolate-fossulate surface. The exine is two-layered, with a more electron-dense endexine. The tectum is continuous. There is no distinct boundary between the tectum and infratectum. The infratectum is alveolate, with numerous, irregularly distributed rounded alveolae in the majority of the sections. Occasionally, rounded alveolae are organized in several rows. Even more rarely, elongated alveolae are arranged perpendicular to the surface of the exine. Such a variable ectexine appearance has been already described in modern cycads and was explained by not strictly perpendicular orientation of pollen walls towards the plane of the sections. Distally, the exine becomes thinner at the expense of the ectexine. The alveolae disappear, and the ectexine becomes thin and homogeneous in the apertural region. Pollen grains of A. manis show many similarities to pollen of modern cycads: a distinct sulcus, a foveolate-fossulate surface pattern, and an infratectum consisting of elongated alveolae. They differ in the ultrastructure of the apertural region, since pollen grains of modern cycads retain the infratectal layer even in the apertural region. A. manis shows differences with earlier studied species of Androstrobus, that point to the heterogeneity of this cycadalean genus.

Carbohydrates, pollinators, and cycads

Cycad biology, ecology, and horticulture decisions are not supported by adequate research, and experiments in cycad physiology in particular have been deficient. Our recent report on free sugar content in a range of cycad taxa and tissues sets the stage for developing continued carbohydrate research. Growth and development of cycad pollen, mediation of the herbivory traits of specialist pollinators, and support of expensive strobilus behavioral traits are areas of cycad pollination biology that would benefit from a greater understanding of the role of carbohydrate relations.

The palynology of the Sonsela member (Late Triassic, Norian) at Petrified Forest National Park, Arizona, USA

Recent paleontological investigations and lithostratigraphic revisions reveal a marked biotic turnover zone within the continental deposits of the Sonsela member of the Chinle Formation (Late Triassic, Norian) at Petrified Forest National Park, USA.Within the Sonsela member we found three pollen assemblage biozones: Zone II (90.5–94m above the Mesa Redondo member) contains a relatively diverse palynological assemblage, with a mix of pteridosperms, voltzialean and some Mesozoic conifers. Following a 2.3m hiatus, Zone IIIa (96–97.5m) is characterized by a decrease in pteridosperms and Mesozoic conifers and a drop in voltzialean conifer diversity. The alleged voltzialean conifer pollen Klausipollenites gouldii was dominant in this part of the assemblage and a significant rise in spores and cycad pollen was also evident. In Zone IIIb (97.5–98.5m) diversity increases and several taxa, which were absent in Zone IIIa reappear, although K. gouldii remained the most abundant taxon. The transition between the palynological assemblages Zones II and IIIa coincide approximately (within a ~2.5m interval) with a documented faunal turnover.The floristic assemblages suggest that the climate of the south-western United States during the Norian was most likely semi-arid and highly seasonal, despite being located at tropical latitudes, with aridification occurring towards the end-Triassic as the continent drifted northwards and global volcanism increased. The gymnosperm-dominated palynofloral assemblage as opposed to the fern- and horsetail-dominated macrofossil record of the Sonsela member of the Chinle Formation, conforms to a semi-arid upland environment alternated by riparian, swampy lowland.

Exine ultrastructure of in situ pollen from the cycadalean cone Androstrobus prisma Thomas et Harris 1960 from the Jurassic of England

Pollen grains extracted from the cycad pollen cone Androstrobus prisma Thomas et Harris 1960 from the Bajocian of Yorkshire were studied by means of LM, SEM and TEM. The species is characterized by rounded-oval, inaperturate pollen with an indistinctly verrucate surface and predominantly homogeneous exine with occasional alveolate areas. Present and earlier published data on the fine morphology of pollen grains of fossil cycads are discussed in the light of the differentiation between Mesozoic non-saccate and presumably monosulcate pollen produced by a number of gymnosperm groups. Although the exine pattern in pollen grains of fossil cycads is usually indistinct in transmitted light, SEM reveals minute, but various sculpturing that can be used for taxonomic purposes, particularly to differentiate between species. Elongated ectexinal alveolae, situated mostly in one row and covered by a thin tectum, is an unequivocal cycadalean character. Sufficiently well preserved fossil cycad pollen clearly shows this type of exine ultrastructure. Poorly preserved cycad pollen grains commonly show an alternation of alveolate and homogeneous regions in the exine with predominance of homogeneous regions. This peculiar mode of preservation can be used as a hint to reveal cycadalean affinity. Inaperturate pollen grains are unknown in bennettites or ginkgophytes. Therefore, if a dispersed, non-saccate and boat-shaped pollen grain is proved by means of electron microscopy to lack an aperture, this would indicate a possible cycadalean affinity.

Does pollen aerodynamics correlate with pollination vector? Pollen settling velocity as a test for wind versus insect pollination among cycads (Gymnospermae: Cycadaceae: Zamiaceae)

Studies of cycad pollination globally have revealed obligate pollination mutualisms with insects, typically hostspecific beetles. Some cycads in the genus Cycas have floating seeds allowing oceanic dispersal, and have colonized remote islands such as Fiji, Guam, and Madagascar. This seems to contradict the expected requirement that specific beetle pollinators be present in order for cycads to establish in remote localities. We therefore postulated that these ‘island’ cycads are wind pollinated. We tested for adaptations associated with wind pollination in a laboratory study of pollen aerodynamics. The pollen from a range of (non-cycad) wind-pollinated plants was not relatively ‘buoyant’: the settling velocities of wind-pollinated species exceeded those of insect-pollinated cycads and some animal-pollinated angiosperms. We attributed this result to settling velocity being, in part, correlated with pollen size, as cycad pollen was consistently smaller. A better predictor for pollination vector was the tendency of pollen to aggregate during a vertical fall. Pollen of wind-pollinated plants separated into single grains, whereas cycad pollen and pollen of animal-pollinated angiosperms adhered in large aggregations, the settling velocity of which increased rapidly with aggregate size. The pollen of cycads known to be insect pollinated was similar in morphology, settling speed, and clumping behaviour to pollen of ‘island’ cycads of unknown pollination biology. Based on these aerodynamics, we predict that cycad species on oceanic islands have insect pollinators. More broadly, we propose that pollen aggregation during a vertical fall provides a sound test for wind versus animal pollination when the pollination vector is unknown.

Pollen of the Triassic cycad Delemaya spinulosa and implications on cycad evolution

The Cycadales are an order of gymnosperms that represent one of the oldest lineages of seed plants. Cycads are thought to have originated in the Carboniferous, and subsequently diversified and geographically expanded throughout the Mesozoic. Despite the geologic diversity of the group, the evolutionary history of cycads remains unresolved. To a large degree this is because the leaves of cycads and various other fossil groups (e.g., pteridosperms and Bennettitales) are morphologically similar. To date there are relatively few fossil cycad reproductive structures. Several are known from the Permian of China, but these compression specimens provide little detailed information useful in tracing the evolution of cycad cone morphology and anatomy. A permineralized cycad pollen cone from the Triassic of Antarctica, Delemaya spinulosa, contains in situ pollen. The objective of the current study is to further elaborate the structure of the pollen grains in this species, with particular emphasis directed at the structure and organization of the pollen wall. The elliptical shape, monosulcate aperture, and small size of the pollen grains are like those of extant Cycadales. The exine, although originally described as homogenous, appears to possess an alveolar organization. Pollen and cone features are compared to those in extant Cycadales.

Pollination ecology of the Australian cycad Lepidozamia peroffskyana (Zamiaceae)

Experiments carried out to investigate the reproductive ecology of the Australian cycad Lepidozamia peroffskyana (Regal, Bull. Soc. Imp. Nat. Mosc. 1857, 1: 184) revealed that this species is pollinated exclusively by host-specific Tranes weevils (Pascoe 1875). The weevils carry out their life cycle within the tissues of the male cones but also visit the female cones in large numbers. Female cones from which insects (but not wind) was excluded had a pollination rate that was essentially zero. In contrast, female cones from which wind (but not insects) were excluded had a pollination rate comparable with naturally pollinated cones. Assessment of Tranes weevil pollen load indicated that they are effective pollen-carriers. No other potential insect pollinators were observed on cones of L. peroffskyana. Sampling of airborne loads of cycad pollen indicated that wind-dispersed grains were not consistently recorded beyond a 2-m radius surrounding pollen-shedding male cones. The airborne load of cycad pollen in the vicinity of pollination-receptive female cones was minimal, and the spatial distribution of the coning population indicated that receptive female cones did not usually occur close enough to pollen-shedding male cones for airborne transfer of pollen to explain observed natural rates of seed set. These multiple lines of evidence suggest that wind–once considered the only pollination vector for cycads and other gymnosperms–plays only a minimal role in the pollination of L. peroffskyana, if any at all. The global diversity of insects associated with cycads suggests that some lineages of pollinating beetles may have been associated with cycad cones since Mesozoic times.

Gymnosperms from the Middle Triassic of Antarctica: The First Structurally Preserved Cycad Pollen Cone

The first permineralized cycad pollen cone is described from the lower Middle Triassic of Antarctica. The cone is characterized by helically arranged, wedge‐shaped microsporophylls, each with five or more spinelike projections extending from the rhomboid distal face. The vascular cylinder is dissected and produces paired traces to each microsporophyll. Three vascular bundles enter the base of the microsporophyll and divide to produce at least five vascular strands in the sporophyll lamina. Pollen sacs occur in two radial clusters near the lateral margins on the abaxial surface of the microsporophyll. Each cluster bears up to eight elongate pollen sacs that are fused for approximately half their length and display longitudinal dehiscence. Pollen sacs are sessile and attached to a vascularized, receptacle‐like pad of tissue that is raised from the surface of the microsporophyll. Pollen is ovoid, psilate, and monosulcate. Although the affinities of this cone with the Cycadales are obvious, the complement of characters in the fossil is unique and thus does not permit assignment to an extant family. Features of the cone are evaluated against reproductive aspects of living cycads.

The longevity of cycad pollen in storage

An investigation was made into the effects of temperature, headspace atmosphere and humidity on the longevity of stored pollen from several Encephalartos species and from Cycas thouarsii. Samples were assayed by in vitro germination tests and by in vivo pollination trials. Encephalartos pollen rapidly lost germinability at ambient temperatures but retained 50% germinability for approximately two years at 0°C and three years at − 15°C. Storage in ambient atmosphere resulted in better germinability than storage in a nitrogen atmosphere. Pre-drying for 24 h was beneficial but storage over a desiccant was disadvantageous. Pollen stored for 2 years at 0 – 4°C gave rise to a 58% seed germination when used to pollinate a female Encephalartos transvenosus cone. Pollen of Cycas thouarsii stored at reduced temperatures exhibited a pronounced seasonal variation in germinability with reduction in maxima over a 3-year period, implying the operation of a hitherto undocumented biological clock.

Development of the Pollen Tube of Zamia furfuracea (Zamiaceae) and Its Evolutionary Implications

Compared with pollen tubes of conifers, gnetophytes, and angiosperms, the pollen tube of cycads is exclusively a vegetative structure, uninvolved with the siphonogamous conduction of sperm to an egg. The cycad pollen tube appears to function primarily to obtain nutrients for the extensive growth and development of the male gametophyte. Previous workers have suggested that, similar to an haustorial fungus, the cycad pollen tube penetrates the reproductive tissues of the sporophyte by enzymatically destroying nucellar cells. These earlier studies did not document the precise structural relationship between the growing male gametophyte and its “host” tissue, the nucellus. Pollen tube growth, and its relation to the nucellus, was examined in Zamia furfuracea with light and transmission electron microscopy. Following germination, the pollen tube of Zamia furfuracea grows intercellularly through the subepidermal layers of the micropylar apex of the nucellus. Electron micrographs clearly show additional localized outgrowths of the pollen tube penetrating the walls of individual nucellar cells. Intracellular haustorial growth ultimately leads to the complete destruction of each penetrated cell, and appears to induce the degeneration of proximal unpenetrated nucellar cells. This pattern of intracellular penetration of the sporophyte by the male gametophyte in Zamia furfuracea is fundamentally different from what has been described in any other major group of seed plants (where intercellular growth of the male gametophyte is the rule), and suggests that the heterotrophic and tissue-specific relationships that male gametophytes of seed plants have with their host sporophytes are substantially more diverse than had previously been known.

DEVELOPMENT OF THE POLLEN TUBE OF ZAMIA FURFURACEA (ZAMIACEAE) AND ITS EVOLUTIONARY IMPLICATIONS

Compared with pollen tubes of conifers, gnetophytes, and angiosperms, the pollen tube of cycads is exclusively a vegetative structure, uninvolved with the siphonogamous conduction of sperm to an egg. The cycad pollen tube appears to function primarily to obtain nutrients for the extensive growth and development of the male gametophyte. Previous workers have suggested that, similar to an haustorial fungus, the cycad pollen tube penetrates the reproductive tissues of the sporophyte by enzymatically destroying nucellar cells. These earlier studies did not document the precise structural relationship between the growing male gametophyte and its “host” tissue, the nucellus. Pollen tube growth, and its relation to the nucellus, was examined in Zamia furfuracea with light and transmission electron microscopy. Following germination, the pollen tube of Zamia furfuracea grows intercellularly through the subepidermal layers of the micropylar apex of the nucellus. Electron micrographs clearly show additional localized outgrowths of the pollen tube penetrating the walls of individual nucellar cells. Intracellular haustorial growth ultimately leads to the complete destruction of each penetrated cell, and appears to induce the degeneration of proximal unpenetrated nucellar cells. This pattern of intracellular penetration of the sporophyte by the male gametophyte in Zamia furfuracea is fundamentally different from what has been described in any other major group of seed plants (where intercellular growth of the male gametophyte is the rule), and suggests that the heterotrophic and tissue‐specific relationships that male gametophytes of seed plants have with their host sporophytes are substantially more diverse than had previously been known.

Ultrastructure of in situ fossil cycad pollen from the English Jurassic, with a description of the male cone Androstrobus balmei sp. nov.

In situ pollen ultrastructure is described for four species of the male cycad cone Androstrobus (Nilssoniaceae), based on material from the Middle Jurassic of North Yorkshire. A new species of cone, Androstrobus balmei sp. nov., yields exceptionally well preserved pollen, with many of the grains preserved without compression in three dimensions. Its surface sculpture resebles that of modern Cycadaceae whereas the dimensions and exine infrastructure are more like those of Zamiaceae, in particular Zamia fischeri. Androstrobus wonnacottii pollen also appears to combine characters of the pollen of extant families. As recognized from previous optical studies, Androstrubus prisma is unique amongst Cycadales in having dominantly inaperturate pollen and in its characteristic sculpture of large, low lying verrucae and rugulae. Additionally, pollen of A. szei also proves to be unique amongst cycads; its surface sculpture of meshed rods resembles that of Ginkgo biloba pollen, a resemblance that although striking is considered to be homoplasious.