AERODYNAMICS OF EPHEDRA TRIFURCA: I. POLLEN GRAIN VELOCITY FIELDS AROUND STEMS BEARING OVULES

The behavior of pollen grains within the airspace around Ephedra trifurca is described. Vectoral analyses of pollen grains moving around stems and ovules indicate a complex pattern of directional and magnitudinal changes in trajectories that can be related to the geometries of surfaces that obstruct airflow. Pollen grains, passing around cylindrical stems oriented normal to the direction of ambient airflow, are deflected in circumrotating non-laminar flow-patterns. Stems tilted downwind deflect pollen grains into trajectories along leeward surfaces of stems. These trajectories travel acropetally in a spiraling pattern and may intersect airflow patterns created around and by ovules. Computer analyses of pollen motion in the vicinity of ovules indicate that pollen vector-direction is highly canalized and directed toward micropyles. Within the immediate vicinity of micropyles, which produce pollination droplets, analyses indicate that the magnitudinal variance of pollen grain vectors is high (spanning three orders of magnitude). This variance coincides with dramatic changes in the local Reynolds numbers, resulting in a localized region around the micropyle in which neither viscous nor inertial forces predominate. Based on additional aerodynamics parameters (vector curl, vector-divergence, and vector curl-differential) it is shown that the region around the pollination-droplet is characteristically a “pollen sink” (pollen grains collect in this airspace) in which abrupt changes can occur in the angular momenta of airborne pollen grains. These aerodynamic analyses suggest that the morphology of ovules and the stems to which they are attached facilitates pollen capture by creating an aerodynamic “singularity” (= a unique region) around the pollination-droplet.     

WIND POLLINATION OF TAXUS CUSPIDATA

The air disturbance patterns created by and around the ovules of Taxus cuspidata are quantified for various orientations to the direction of ambient airflow, and are shown to largely dictate the motion (vectoral trajectories) and mode of deposition of windborne pollen on ovule surfaces. Perpendicular orientation to the direction of airflow results in two regions characterized by high densities of adhering pollen — one on the windward surface of the ovule, resulting from direct inertial collision, and another on the leeward surface resulting from non-inertial sedimentation. Parallel and inclined orientations of the ovule to the direction of airflow produce quantitative and qualitative variations in the pattern of adhering pollen resulting from inertial and non-inertial deposition. Direct collision of windborne pollen grains with the micropylar ends of ovules occurs for all orientations to wind direction. The aerodynamics of the ovulate shoot complex of Taxus cuspidata is related to that previously described for conifer ovulate cones, cycad megastrobili, and simulated wind tunnel analyses of archaic Paleozoic ovules based on scale models. Water transport of pollen (adhering to integument and bract surfaces) to micropyles quantitatively alters the distribution of adhering pollen grains on ovule surfaces. Although there is no evidence that pollen grains of this species are osmotically ruptured, observations do not preclude the possibility that water transport of pollen may reduce the number of viable pollen grains reaching the micropyle.     

CONIFER OVULATE CONE MORPHOLOGY: IMPLICATIONS ON POLLEN IMPACTION PATTERNS

Empirically determined patterns of pollen impaction on the surfaces of pine ovulate cones are correlated with regions of nonlaminar flow created by the spatial arrangement and morphology (aspect ratios) of scale-bract complexes. Results from the serial discharge of pollen, upwind of ovulate cones, indicate that ovules on ovulate cones are preferentially impacted by pollen from their own species. Analyses indicate that while aerodynamic factors dominate the entrapment of pollen by ovulate cones, other factors such as pollen impaction-rebound and rebound-reentrainment are significant. Surface characteristics in addition to the settling velocities of pollen may play important roles in determining pollination efficiency. Wind tunnel analyses of the aerodynamic effects of scale-bract arrangement and aspect ratios indicate that each complex behaves as an aerofoil, deflecting air eddies toward the micropylar ends of ovules. The ovulate cone geometry, as a whole, deflects unidirectional wind into cyclonic vortices around the cone axis, each scale-bract deflecting nonimpacted pollen along orthostichies and parastichies. The morphology of the typical conifer ovulate cone is interpreted as a structure that optimizes anemophilous reproduction.     

AIRFLOW PATTERNS AROUND SOME EARLY SEED PLANT OVULES AND CUPULES: IMPLICATIONS CONCERNING EFFICIENCY IN WIND POLLINATION

Aerodynamic analyses showing characteristic airflow patterns and the potential for wind-mediated pollination are presented for models of Paleozoic (Carboniferous) ovules and ovulate cupules (i.e., Genomosperma kidstoni, G. latens, Salpingostoma dasu, Physostoma elegans, Eurystoma angulare, and Stamnostoma huttonense). Lobes on ovules and cupules are shown to produce localized regions of turbulent flow with a concomitant reduction in airflow velocity. Data based upon models that mimic the characteristics of windborne pollen (= pseudopollen) show that these regions of turbulent flow correspond to those in which suspended pseudopollen impact with ovule and/or cupule surfaces. These data have bearing on a sequence of ovule morphologies purported to show the evolution of the integument by the progressive reduction in length of “preintegumentary” lobes and their acropetal fusion. As the preintegumentary lobes of the models studied consolidate around the megasporangium, regions of turbulent flow and high pseudopollen impact become localized around the pollen chamber or salpinx. The general morphologic trend envisioned for the evolution of the ovule is seen to be associated with an aerodynamic streamlining and an increased potential for wind-mediated pollination. Data for hair-bearing ovules and for ovulate cupules are discussed within the context of possible selective pressures favouring streamlining.     

POLLINATION DROP IN RELATION TO CONE MORPHOLOGY IN PODOCARPACEAE: A NOVEL REPRODUCTIVE MECHANISM

Observation of ovulate cones at the time of pollination in the southern coniferous family Podocarpaceae demonstrates a distinctive method of pollen capture, involving an extended pollination drop. Ovules in all genera of the family are orthotropous and single within the axil of each fertile bract. In Microstrobus and Phyllocladus ovules are erect (i.e., the micropyle directed away from the cone axis) and are not associated with an ovule-supporting structure (epimatium). Pollen in these two genera must land directly on the pollination drop in the way usual for gymnosperms, as observed in Phyllocladus. In all other genera, the ovule is inverted (i.e., the micropyle is directed toward the cone axis) and supported by a specialized ovule-supporting structure (epimatium). In Saxegothaea there is no pollination drop and gametes are delivered to the ovule by pollen tube growth. Pollination drops were observed in seven of the remaining genera. In these genera the drop extends over the adjacent bract surface or cone axis and can retain pollen that has arrived prior to drop secretion (“pollen scavenging”). The pollen floats upward into the micropylar cavity. The configuration of the cone in other genera in which a pollination drop has not yet been observed directly suggests that pollen scavenging is general within the family and may increase pollination efficiency by extending pollination in space and time. Increased pollination efficiency may relate to the reduction of ovule number in each cone, often to one in many genera, a derived condition. A biological perspective suggests that animal dispersal of large seeds may be the ultimate adaptive driving force that has generated the need for greater pollination efficiency.     

AEROBIOLOGY AND POLLEN CAPTURE OF ORCHARD-GROWN PISTACIA VERA (ANACARDIACEAE)

The phenology of pollen release and pollen capture by Pistacia vera was studied in the field and laboratory respectively. Inflorescences of Pistacia vera were examined in a wind tunnel to determine whether the behavior of airborne conspecific pollen around receptive flowers differed as a result of changes in the shape and size of the inflorescence. In addition, the behavior of unclumped (single) and clumped pollen grains was studied to determine differences in the probability of their capture. Wind speeds within a commercial orchard during pollen shedding averaged 0.9–2.2 m/sec and atmospheric pollen concentrations were highest between 0900–1100 hr MST. Each of three stages in inflorescence development (defined on the basis of the number of exserted stigmas) was examined under identical ambient airflow conditions with equal concentrations of airborne pollen (1,000 grains/m³). The general pattern of pollen grain motion involves direct inertial collision by windward surfaces and by sedimentation of pollen onto leeward surfaces; clumped pollen rarely sedimented onto leeward surfaces. Small changes in ambient wind speed (0.5 m/sec to 1.0 m/sec) produced significant changes in the pattern of pollen motion around inflorescences and altered the number of pollen grains captured by leeward surfaces. Thus, wind pollination in P. vera is affected both by windspeed and by the shape or size of flower clusters. Differences in the behavior of clumped and unclumped pollen result from their inertial properties and responsiveness to local changes in the direction and speed of airflow. Unclumped pollen has a higher probability of being captured by leeward surfaces. The apparent insensitivity of pollen motion to differences in inflorescence size may ensure equitable pollination during the acropetal development of flowers.     

SILICIFIED PINUS REMAINS FROM THE MIOCENE OF WASHINGTON

Silicified leaves, dwarf shoots, pollen cones, and seed cones of Pinus from a Late Miocene chert bed within the Yakima Basalt Formation near Yakima, Washington are interpreted as coming from a single new species, P. foisyi. The needles and dwarf shoots are those of a three-needle pine. The needles contain two to four medial resin canals, a biform hypodermis, and endodermal cells with uniformly thickened walls. The pollen cones are ellipsoidal and about 1 cm long, and many contain bisaccate pollen grains. The seed cones are at least 6 cm long and are slightly asymmetrical. The cone axis has a broad sclerotic outer cortex, and the seed wing extends from a thick parenchymatous base. The scale apex bears a conspicuously swollen projection. The foliage and seed cones are identifiable with the Subgenus Pinus, Section Pinus, Subsection Oocarpae independently of one another, and together indicate a fossil species related to the modem Californian closed cone pines. Pinus foisyi represents one of the earliest occurrences of cone asymmetry associated with this group. However, cone serotiny characteristic of the modem species appears to have evolved after the Late Miocene.     

AERODYNAMICS OF WIND POLLINATION IN SIMMONDSIA CHINENSIS (LINK) SCHNEIDER

Wind tunnel analyses of Simmondsia chinensis (Link) Schneider or “jojoba” were conducted to quantify the behavior of airborne pollen grains around individual branches and leaves and near individual carpellate flowers. Field data (wind velocity) were used to ensure a correspondence between wind tunnel and natural conditions. Based upon the visualization of individual pollen grain trajectories, it is concluded that pollen deposition on stigmatic surfaces is influenced by large-scale aerodynamic patterns, generated by foliage leaves, and small-scale airflow patterns, formed around and by floral parts and stigmas. Leaves are seen to deflect airborne pollen grains into trajectories that can intersect ambient airflow at 90° angles, showering decumbent carpellate flowers with pollen. Similarly, flowers can deflect pollen upward and downwind, toward other flowers. The extent of floral bract and sepal recurvature is shown to influence the extent of pollen deposition by determining the characteristic airflow pattern around stigmas. Available evidence concerning the relatively recent evolutionary transition to anemophily in Simmondsia is interpreted within the context of morphological adaptations and exaptations favoring wind pollination.     

Pollination of Australian Macrozamia cycads (Zamiaceae): effectiveness and behavior of specialist vectors in a dependent mutualism

Complementary field and laboratory tests confirmed and quantified the pollination abilities of Tranes sp. weevils and Cycadothrips chadwicki thrips, specialist insects of their respective cycad hosts, Macrozamia machinii and M. lucida. No agamospermous seeds were produced when both wind and insects were excluded from female cones; and the exclusion of wind-vectored pollen alone did not eliminate seed set, because insects were able to reach the cone. Based on enclosure pollination tests, each weevil pollinates an average 26.2 ovules per cone and each thrips 2.4 ovules per cone. These pollinators visited similar numbers of ovules per cone in fluorescent dye tests that traced insect movement through cones. Fluorescent dye granules deposited by Cycadothrips were concentrated around the micropyle of each visited ovule, the site of pollen droplet release, where pollen must be deposited to achieve pollination. In contrast, Tranes weevils left dye scattered on different areas of each visited ovule, indicating that chance plays a greater role in this system. Each weevil and 25 thrips delivered 6.2 and 5.2 pollen grains, respectively, on average, to each visited ovule per cone, based on examination of dissected pollen canals. In sum, the pollination potential of 25 Cycadothrips approximates that of one Tranes weevil.     

MORE EVIDENCE FOR CONIFER DIVERSITY IN THE UPPER TRIASSIC OF NORTH CAROLINA

An ovulate strobilus from the Upper Triassic Deep River Basin, North Carolina, has helically arranged, loosely aggregated, elongated, spatulate bracts with axillary ovule-bearing appendages with about 8–10 ovules attached in two lateral rows, with outwardly directed micropyles. The axillary ovuliferous appendage is homologous with the voltzialean fertile dwarf shoot, but probably not directly evolved from it. More credible is a suggested origin from a completely fertile axillary appendage such as that of the Lower Permian Trichopitys. The occurrence of this cone, Metridiostrobus palissyaeoides, gen. and sp. nov., along with Compsostrobus neotericus and Voltzia andrewsii, reflects considerable diversity among conifer ovulate cones during the Upper Triassic.     

FERTILE CONIFEROPHYTE REMAINS FROM THE LATE TRIASSIC DEEP RIVER BASIN,NORTH CAROLINA

Seed cones (Compsostrobus neotericus gen. et sp. nov.), pollen cones, and vegetative remains of coniferophytes occur in Upper Triassic rocks of the Deep River Basin (Pekin Formation) of Central North Carolina. Seed cones have spatulate ovuliferous scales, each with two ovules and subtended by an elongated bract with an attenuate tip. Cuticle of seed cones resembles that of leaves on vegetative axes. Slender leaves are borne along two sides of the axis. Pollen cones have helically arranged microsporophylls, each with two abaxial sporangia bearing pollen grains of the Alisporites type. Seed cones, pollen cones, and vegetative remains suggest a coniferophyte very modern in aspect.     

The structure and phylogenetic significance of the conifer Pseudohirmerella delawarensis nov. comb. from the Upper Triassic of North America

The conifer genus Pseudohirmerella Arndt is based on ovulate cone scales of the type species, Pseudohirmerella platysperma (Mägdefrau) Arndt, from the Upper Triassic of Germany, which bear five distal lobes and two paired structures represented by bulges on the adaxial surface. The paired structures were first described as seeds, but have recently been re-interpreted as arils by Arndt. Similar scales from the Passaic Formation at Milford, NJ, USA were described as Glyptolepis delawarensis Bock, and the new combination Pseudohirmerella delawarensis is proposed. A relatively complete concept of Pseudohirmerella is presented, which includes ovulate cones, possible associated pollen cones, associated shoots with Pagiophyllum-Brachyphyllum morphotype leaves and associated, anatomically preserved wood. Based on the ovulate cone morphology and the presence of organic matter lining the concavities on the scale rather than the corresponding bulges on the counterparts, it is likely that the supposed seeds or arils of Pseudohirmerella are actually casts of empty, seed-bearing depressions. Cheirolepidiaceous affinities are likely based on ovulate cone scale morphology, persistent pollen cones, foliage type and details of wood anatomy. The derived ovulate cone scale morphology of Pseudohirmerella indicates a substantial, but mostly undocumented, Triassic diversification of the Cheirolepidiaceae.     

Ovulate cone,pollination drop,and pollen capture in Sequoiadendron (Taxodiaceae)

In Sequoiadendron ovules are borne inside the ovulate cone, and pollination drops secreted from these ovules collect pollen. We examined: (1) the relation between ovular position and pollen capture; (2) pollen behavior when in contact with a pollination drop; and (3) ultrastructure of ovules during pollination drop secretion. During wet periods a water sheet forms on the surface of the cone due to bract shape and wettability. Pollination drops persist inside the wetted cone, and pollen capture resumes immediately after drying. Pollen landing on a pollination drop is taken inside the drop and carried into the micropyle when the drop contracts. Several notable ultrastructural features appear in the nucellus, integument, chalaza, and bract lamina during pollination-drop secretion. The abaxial surface of the lamina is covered by a membrane that may contribute to the wettable nature of the surface.     

Western white pine (Pinus monticola Dougl.) reproduction: I. Gametophyte development

Male and female gametophyte development are described from light and transmission electron microscope preparations of ovules from first and second year Pinus monticola Dougl. seed cones. In the first year of development, pollen tubes penetrate about one-third the distance through the nucellus. The generative cell and tube nucleus move into the pollen tube. The megagametophyte undergoes early free nuclear division. First-year seed cones and pollen tubes become dormant in mid-July. In the second year, seed cones and pollen tubes resume development in April and the pollen tubes grow to the megagametophyte by mid-June. Early in June the generative cell undergoes mitosis, forming two equal-size sperm nuclei that remain within the generative cell cytoplasm. The generative cell has many extensions and abundant mitochondria and plastids. The megagametophyte resumes free nuclear division, then cell wall formation begins in early July. Cell wall formation and megagametophyte development follow the pattern found in other Pinaceae. Three to five archegonial initials form. The primary neck cell divides, forming one tier of neck cells. Jacket cells differentiate around each central cell. The central cell enlarges and becomes vacuolate; then vacuoles decrease in size and the cell divides, forming a small ventral canal cell and a large egg. Plastids in the central cell engulf large amounts of cytoplasm and enlarge. This process continues in the egg, and the peripheral cytoplasm of the egg becomes filled with transformed plastids. Mitochondria migrate around the nucleus, forming a perinuclear zone. The wide area of egg cytoplasm between these two zones has few organelles. A modified terminology for cells involved in microgametophyte development is recommended. Received: 9 December 1999 / Revision accepted: 30 April 2000     

PINUS AVONENSIS,A NEW SPECIES OF PETRIFIED CONES FROM THE OLIGOCENE OF WESTERN MONTANA

Fossils from the Oligocene of western Montana described in this treatment are the first structurally preserved ovulate cones of Pinus to be reported from the Tertiary of North America. They are about 5.5 cm long and have a maximum diam of 2.5 cm. Numerous scales are arranged spirally around the axis and each scale bears two winged seeds. The bract subtending the ovuli-ferous scale is 3-4 mm long and is free from the scale throughout its length. The pith and cortex of the axis are constructed of thick-walled parenchyma cells and 18-21 resin canals occur at the inner edge of the cortex. Resin canals entering the base of the ovuliferous scale are restricted to the abaxial side with vascular tissues occupying the adaxial side. Vascular strands near the tip of the scale are strongly rounded on the adaxial or phloem side. At the abaxial side of the tip of the ovuliferous scale is a broadly rhomboidal apophysis with a raised umbo that terminates in a short spine. The fossils differ from the several Recent cones examined in having fewer resin canals and biseriate rays in the secondary xylem of the cone axis. The shape of the cone, its anatomical features, and the morphology of the tip of the cone scale indicate affinity with the subgenus Diploxylon.     

Anatomically preserved Cycadeoidea (Cycadeoidaceae), with a reevaluation of systematic characters for the seed cones of Bennettitales

Four anatomically preserved ovulate cycadeoid cones have been recovered from three localities in Upper Cretaceous (Turonian/Coniacian-Late Campanian) sediments of Vancouver and Hornby Islands, British Columbia, Canada. All of the specimens are preserved by calcareous cellular permineralization and are quite similar to seed cones described as several species of Cycadeoidea and Bennettites. These cones, described as Cycadeoidea maccafferyi sp. nov., consist of tightly packed interseminal scales and ovulate sporophylls with terminal ovules. Two specimens also preserve remains of a small receptacle. Interseminal scales and ovulate sporophylls are oriented parallel to one another. Ovules are distinctly stellate at the base of the micropylar tube, and the sarcotesta consists of both longitudinally oriented tubular cells and large radially elongated cells attached to the sclerotesta. The vascular strand below each ovule is highly contorted in a pattern that is characteristic of contractile tissue in the roots of living plants. These specimens are the most recent anatomically preserved cycadeoid cones yet discovered, revealing details of the reproductive biology shortly before extinction of the clade. Superb preservation of the British Columbia cones confirms that Bennettitales lack a cupule, have radial seeds, and have a vascularized nucellus (but no integumentary tracheids), and that no pollen chamber is produced. Together with a new species of Williamsonia preserved at one of the same localities, these specimens reveal a clear set of contrasting systematic characters for differentiating between isolated seed cones of Williamsoniaceae and Cycadeoidaceae.     

EQUATIONS FOR THE MOTION OF AIRBORNE POLLEN GRAINS NEAR THE OVULATE ORGANS OF WIND-POLLINATED PLANTS

A technique is presented that is capable of predicting the motion of airborne pollen grains and the probability of pollen capture by wind-pollinated plants. Equations for the motion of rigid-walled particles (= pollen grains, spores, or Sephadex beads) in a supporting, compressible fluid (= air) are derived from the first principles of fluid dynamics. These equations are incorporated into a computer program (MODEL) which can be used with a desktop computer. The operation of MODEL requires empirical data on the pattern of airflow or the motion of a pollen species around the surfaces of the taxonomically relevant ovulate plant organ. With this information, MODEL can predict the behavior of any pollen species for which physical properties (size and density) are specified or empirically known. The significance of this procedure lies in the quantification of physical phenomena that influence the mechanics and fluid dynamics of pollen capture in wind pollination. The technique is illustrated and tested by its application to two grass species (Setaria geniculata and Agrostis hiemalis) for which velocity fields of pollen motion have been previously reported.     

PICEA WOLFEI,A NEW SPECIES OF PETRIFIED CONE FROM THE MIOCENE OF NORTHWESTERN NEVADA

Several silicified ovulate cones from the late middle Miocene (Barstovian) represent a new species, Picea wolfei Crabtree. This is the second species of Picea for which structurally preserved seed cones are known to be reported from the Tertiary. The cones are 5.0–8.0 cm long and 1.5–2.0 cm at their greatest diameter. Ovuliferous scales are inserted helically around the cone axis and are recurved at their point of divergence. Each scale is broadly obovate to spatulate with a rounded apex and bore two seeds adaxially. The bract subtending the scale is 4.5–7.3 mm long and is fused to the scale for 1.4–2.0 mm. Each bract has an inflated keel-like base which projects abaxially between the seeds of adjacent scales. The fossil cones superficially resemble those of the extant Picea breweriana, yet differ from them anatomically. The new species also resembles Picea lahontense, a fossil compression from the Miocene Trout Creek Flora of south-central Oregon, but the different modes of preservation preclude meaningful comparison. Picea diettertiana, the only structurally preserved fossil cone of this genus previously described, is quite dissimilar in that it lacks a sclerotic pith.     

Frenelopsis alata and its microsporangiate and ovuliferous reproductive structures from the Cenomanian of Bohemia (Czech Republic, Central Europe)

The conifer, Frenelopsis alata (K. Feistmantel) E. Knobloch (Cheirolepidiaceae), occurring mostly in the Cenomanian of Europe, is revised on the basis of the type material. Its comparison with relevant species of Frenelopsis is discussed.The ovuliferous cone associated with the genus Frenelopsis is recorded for the first time. For the associated ovuliferous cones of Frenelopsis, a new genus, Alvinia, is introduced in a new combination for the type: Alvinia bohemica (Velenovsky) comb. n. Its association with Frenelopsis alata is based on the presence of Classopollis pollen adhering to ovuliferous cone scales, and the same type of pollen found in the microsporangiate cone of F. alata, the same cuticle pattern present on ovuliferous cones, sterile twigs and microsporangiate cones of F. alata, and also the co-occurrence of ovuliferous cones or their scales and sterile twigs of F. alata.Large ovuliferous cones of Alvinia bohemica are formed by helically arranged ovuliferous scales subtended by bracts. Each ovuliferous cone scale displays one or two seeds covered by a covering flap, and three appendages, which form distally a funnel-like structure lined in its inner part by long trichomes. Numerous pollen grains of Classopollis adhere to the trichomes, and the structure is considered to function as a protostigmatic area.The ovuliferous cones of Alvinia differ from similar cones of the Cheirolepidiaceae, Hirmeriella and Tomaxellia, mainly in a high state of unification of the ovuliferous cone scale, reduction of appendages and in a presence of the protostigmatic funnel-like structure.The ovuliferous cones, Alvinia bohemica, rarely occur intact, so it is assumed that they disintegrate when mature. It seems likely that they were not woody. This assumption is supported by the flattened appearance of cones and their cone scales in the sediment, their flexibility and the absence of massive coaly matter known from cones of the Taxodiaceae and Cupressaceae. It is proposed that this type of ovuliferous cone scale indicates a specialized type of pollination. In addition, it is suggested that cone scales enclosing seeds play an important role in propagation.