Lens-shaped Structure Research Articles

Intrathermocline eddies observed in the northwestern subtropical Pacific Ocean

Two anticyclonic intrathermocline eddies (ITEs) were detected by an underwater glider in the northwestern subtropical Pacific Ocean during August-October 2019. They both exhibited a lens-shaped vertical structure within the thermocline with their cores located at ~170 m. The North Pacific Subtropical Mode Water (STMW) was found within the cores of these two ITEs. The lens-shaped structure of ITE1 observed by the glider was very clear since the glider seemed to have moved into its core during the observation. Further analysis reveals that ITE1 displayed no signals at the sea surface and lasted for about 20 days (26 August-14 September 2019). ITE1 was locally formed and the water inside it was a mixture of local water and the water in the northern adjacent area. The low-salinity water at 0-50 m from the northern adjacent area extended southwestward and mixed with the local water. As a result, the local salinity-forced restratification caused a potential vorticity (PV) decrease in the subsurface and finally resulted in the generation of ITE1. The baroclinic instability at 50-170 m may be the main energy source for ITE1 generation. On the other hand, the lens-shaped structure of ITE2 observed by the glider was less prominent since the glider did not move into its core. Further analysis reveals that the lens-shaped structure of ITE2 was also very clear near its core and ITE2 displayed clear signals at the surface as an anticyclonic eddy (AE2). AE2/ITE2 was remotely generated within the main formation region of STMW and then moved southwestward. The low PV STMW was trapped in AE2 and a lens-shaped structure developed in the subsurface. Subduction of the STMW caused the generation of ITE2.

Subsurface Anticyclonic Eddy Transited from Kuroshio Shedding Eddy in the Northern South China Sea

Abstract Subsurface eddies are a special type of oceanic eddy that display the maximum velocity in the subsurface layer. Based on field observations, a lens-shaped subsurface anticyclonic eddy (SAE) was detected in the northern South China Sea (SCS) in May 2021. The SAE was located between 20 and 200 m, with a shoaling of the seasonal thermocline and deepening of the main thermocline. Satellite images showed that the SAE exhibited positive sea level anomaly (SLA) and negative sea surface temperature (SST) anomaly. Eddy track indicated that this SAE originated from the Luzon Strait and was generated in the Kuroshio Loop Current (KLC) last winter. The evolution of the SAE was related to the anomalous water properties inside the eddy and the seasonal change of sea surface heat flux. In winter, the continuous surface cooling and Kuroshio intrusion led to a cold, salty core in the upper part of the anticyclonic eddy, which resulted in a subsurface-intensified structure through geostrophic adjustment. As the season changed from winter to spring, sea surface temperature increased. The lens-shaped structure was formed when the seasonal thermocline appeared near the surface that capped the winter well-mixed water inside the eddy. From 1993 to 2021, nearly half of the winter KLC shedding eddies (12/25) survived to late spring and evolved into subsurface lens-shaped structures. This result indicates that the transition of KLC shedding eddy to SAE is a common phenomenon in the northern SCS, which is potentially important for local air–sea interaction, heat–salt balance, and biogeochemical processes. Significance Statement Subsurface eddies are lens-shaped eddies with anomalous water properties in the subsurface layer. While such eddies have been reported in many regions of the World Ocean, they are poorly investigated in the SCS, especially the periodic subsurface eddies that appear in a fixed time frame with similar patterns and trajectories. This study reported a subsurface anticyclonic eddy (SAE) in the northern SCS and elucidated its generation and evolution processes. Statistical results confirm that this is a periodic SAE, which occurs nearly annually in late spring and evolves from the Kuroshio shedding eddy with seasonal changes. This study provides a new perspective on the evolution of subsurface eddies in the SCS and will benefit targeted observations in the future.

A lens-shaped, cold-core anticyclonic surface eddy in the northern South China Sea

Typically, compared with the normal ocean, an anticyclonic eddy has higher sea surface temperature (SST), greater surface mixed-layer depth (MLD), and a bowl-shaped structure under the action of geostrophy. This study is the first to report an abnormal anticyclonic eddy characterized by a lens-shaped structure, cold core, and shallower MLD, which were observed in situ in the northern South China Sea (SCS) in September 2021. The SST at core of the anticyclonic eddy was 0.4°C lower than that in its peripheral region. The MLD at the center of the eddy was about half of that outside the eddy. Below the surface mixed layer, a lens-shaped structure containing relatively well-mixed water was observed between the two high-gradient layers. Within this lens-shaped structure, the isothermal layers were stretched, but accompanied by water mixing that was about one third of that at the upper and lower bounds of the structure. This eddy originated from a Kuroshio Current intrusion in late October 2020 and died in November 2021, such that its lifespan exceeded 1 year. The shedding of the Kuroshio Loop into the SCS under strong air–sea interactions under continuous sea surface cooling in winter was considered a key mechanism for the generation of the cold core of the eddy. The lens-shaped structure formed below the surface mixed layer and was maintained through geostrophic balance by the subsurface maximum speeds of the Kuroshio Current intrusion (50–100 m), thereby forming a shallower MLD in the eddy center. The subsurface speed maximum within the eddy was also observed by a shipborne acoustic Doppler current profiler at 10 months after its formation, confirming the hypothesized mechanism. This type of abnormal anticyclonic may be a common phenomenon in the northern SCS and has potential implications for the local biogeochemistry, local heat budget, and regional oceanic models.

Topography and distribution of adenosine A2A and dopamine D2 receptors in the human Subthalamic Nucleus.

The human Subthalamic Nucleus (STh) is a diencephalic lens-shaped structure located ventrally to the thalamus and functionally implicated in the basal ganglia circuits. Despite recent efforts to characterize the neurochemical and functional anatomy of the STh, little to no information is available concerning the expression and distribution of receptors belonging to the dopaminergic and purinergic system in the human STh. Both systems are consistently implicated in basal ganglia physiology and pathology, especially in Parkinson’s Disease, and represent important targets for the pharmacological treatment of movement disorders. Here, we investigate the topography and distribution of A2A adenosine and D2 dopamine receptors in the human basal ganglia and subthalamic nucleus. Our findings indicate a peculiar topographical distribution of the two receptors throughout the subthalamic nucleus, while colocalization between the receptors opens the possibility for the presence of A2AR- D2R heterodimers within the dorsal and medial aspects of the structure. However, further investigation is required to confirm these findings.

Research of permafrost soil thawing under the structural foundation platform

The implementation of spatial foundation platforms on permafrost soils is justified in this paper. The stress-strain state of the spatial foundation platform under the building of a lens-shaped structure at different heights of the ventilated space is investigated. The structural solution of the spatial foundation platform is made in the form of a plate-and-rod structure. The calculation is carried out using the SCAD software package. The parameters of the rods and nodal joints of the spatial foundation platform are determined. With the help of the COMSOL Multiphysics software package, temperatures are investigated throughout the year in the central part of the platform under its lower belt.

Scope of inextensible frame hypothesis in local action analysis of spherical reservoirs

Spherical reservoirs, as objects perfect with respect to their weight, are used in spacecrafts, where thin-walled elements are joined by frames into multifunction structures. The junctions are local, which results in origination of stress concentration regions and the corresponding rigidity problems. The thin-walled elements are reinforced by frame to decrease the stresses in them. To simplify the analysis of the mathematical model of common deformation of the shell (which is a mathematical idealization of the reservoir) and the frame, the assumption that the frame axial line is inextensible is used widely (in particular, in the manual literature). The unjustified use of this assumption significantly distorts the concept of the stress-strain state. In this paper, an example of a lens-shaped structure formed as two spherical shell segments connected by a frame of square profile is used to carry out a numerical comparative analysis of the solutions with and without the inextensible frame hypothesis taken into account. The scope of the hypothesis is shown depending on the structure geometric parameters and the load location degree. The obtained results can be used to determine the stress-strain state of the thin-walled structure with an a priori prescribed error, for example, in research and experimental design of aerospace systems.

A peculiar lens-shaped structure observed in the South China Sea

Lens-shaped structures within thermocline potentially play a significant role in subsurface transport of mass, heat, and salt in the global ocean. Whilst such structures have been documented in many oceanic regions, none has been observed in the China Seas. This study reports on observations of a lens-shaped structure within thermocline in the southwestern South China Sea in September 2007. This structure had a maximum thickness of approximately 60 m and a horizontal extent exceeding 220 km. This lens was peculiar in that its size is larger than most similar structures documented in the literature. The lens core was characterized by well-mixed water with higher temperature (~28.8 °C), lower salinity (~33.3) and lower potential vorticity (PV) compared to the surrounding waters. Based on an ocean reanalysis, possible generation mechanism of the lens is explored by examining the evolution of surface and subsurface thermohaline properties, and an analysis of vertical PV flux. The lens was likely generated by a mixture of the local mixed-layer water and the water from the coastal jet separation site.

SUPPLEMENTARY DESCRIPTION OF MACANDREWELLA CHELIPES (GIESBRECHT, 1896) FROM THE GULF OF ELAT (COPEPODA: CALANOIDA: SCOLECITHRICIDAE), AND COMMENTS ON ITS RELATIONSHIPS WITH SCOTTOCALANUS SARS AND SCOLECOCALANUS FARRAN

ABSTRACT Specimens of the scolecithricid copepod Macandrewella chelipes (Giesbrecht, 1896) from the Gulf of Elat (Gulf of 'Aqaba), Red Sea, are examined so as to supplement the description of the species, and to compare it to specimens of Scottocalanus securifrons (T. Scott, 1894) collected off southeast Brazil. The genera Macandrewella A. Scott and Scolecocalanus Farran share between them and with Scottocalanus Sars most of the characters used so far in the generic diagnoses. The three genera are now tentatively separated by a combination of the following characters: crested or uncrested forehead with or without a lens-shaped structure, and presence or absence of female fifth legs.

Growth and demise of a Burdigalian coral bioconstruction on a granite rocky substrate (Bonifacio Basin, southeastern Corsica)

During the Early Miocene, coincident with the Sardinia–Corsica block rotation, mixed carbonate–siliciclastic sediments of the Cala di Labra Formation were deposited on the southern margin of the Bonifacio Basin (southeastern Corsica, France). The Burdigalian marine transgression is spectacularly represented by a peculiar coral bioconstruction, unconformably lying on the eroded Variscan granitic basement. Superb exposures allowed detailed, three-dimensional field mapping, lithofacies analysis, and characterization of the Cala di Labra coral bioconstruction. As a result of the extremely irregular and articulated substrate, the coral buildup appears as an organized lens-shaped structure, and its core is constituted by a relatively dense coral domestone with a moderate increase of platy corals in the upper part. A coral rubble associated with granitic cobbles and pebbles is locally present at the base. The inter-coral sediment consists of poorly sorted bioclastic wackestone to packstone. Results from this study clearly show evidence for the occurrence of a former submerged granitic substrate that, as very rarely documented in the geological record, is here interpreted as the subtidal substrate for growth of a small bioconstruction under relatively high energy and clear water conditions. The Cala di Labra bioconstruction is overlain by a fining-upward quartzose conglomerate and sandstone succession interpreted as deposited in a coastal setting near fluvial point sources. The demise of coral growth was caused by a regressive event and by the consequent quite-sudden burial and related changes of trophic conditions.

Seismic reflection imaging of a Warm Core Ring south of Hokkaido

A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200 m and the other with a shot interval of 50 m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (~2°) on the ocean side and gentler slopes (~1°) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.

Effects of the thickness of GaAs spacer layers on the structure of multilayer stacked InAs quantum dots

The effects of the thickness of GaAs spacer layers on the structure of multilayer stacked InAs quantum dots (QDs) grown by molecular-beam epitaxy were studied using transmission electron microscopy. To investigate QD structure depending on spacer layer growth, first uncapped free-standing QDs were grown and their structure compared with that of multilayer stacked QDs. In addition, vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The uncapped QDs were found to form a lens-shaped structure with side facets. Upon capping with a GaAs spacer, the apex of nonaligned QDs flattened by In diffusion. However, the aligned QDs maintained their lens-shaped structure with round apex after capping. It is believed that their apex did not flatten because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers. The results demonstrate the possibility of controlling QD structure by adjusting the thickness of spacer layers.

Microstructures and Growth Characteristics of Self-Assembled InAs/GaAs Quantum Dots Investigated by Transmission Electron Microscopy

The microstructure and strain characteristics of self-assembled InAs/GaAs quantum dots (QDs) were studied by using transmission electron microscopy. Compressive strain was induced to uncapped QDs from GaAs substrate and the misfit strain largely increased after the deposition of GaAs cap layer. Tensile strain outside QD was extended along the vertical growth direction; up to 15 nm above the wetting layer. Vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The QDs with a lens-shaped morphology were formed in the early stage of growth, and their apex was flattened by the out-diffusion of In atoms upon GaAs capping. However, aligned QDs maintained their lens-shaped structure with round apex after capping. It is believed that their apex did not flatten because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers.

Structure and thermal stability of InAs/GaAs quantum dots grown by atomic layer epitaxy and molecular beam epitaxy

The structure and thermal stability of self-assembled InAs/GaAs quantum dots (QDs) grown by atomic layer epitaxy (ALE) and molecular beam epitaxy (MBE) QDs were studied using high-resolution electron microscopy with in situ heating experiment capabilities. The ALE QDs were larger and more regular shaped than MBE QDs because of the higher diffusivity of In compared with that of InAs in MBE growth. The QDs were found to form a lens-shaped structure with side facets in the early stage of growth. Upon capping with a GaAs layer, however, the QD apexes flattened because of the diffusion of In and As from the QDs. The structural behavior of QDs at elevated temperatures was observed directly on the atomic scale by in situ heating experiments within TEM. The in situ high-resolution electron microscopy revealed that the uncapped ALE and MBE QDs remained stable up to 580 °C. However, above 600 °C, the QDs collapsed due to the diffusion and evaporation of In and As from the QDs.

The anatomy of the porcine subthalamic nucleus evaluated with immunohistochemistry and design-based stereology

This study provides a light-microscopic description of the organization, morphology and number of neurons in the subthalamic nucleus (STN) of the Göttingen minipig. It is based on histological material stained with Nissl, Golgi and autometallographic techniques, and employs design-based stereological estimation of the total neuron number. The organization of several neurotransmitters in the STN has been evaluated in histological preparations stained for acetylcholinesterase (AChE) and immunostained for choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), glutamic acid decarboxylase (GAD) and glutamate. In all of the stained preparations the STN appeared as a distinct lens-shaped structure located in the caudal diencephalon, medial to the internal capsule and ventrolateral to the zona incerta. Rostrally, the STN approached the globus pallidus pars interna, whereas caudally the ventromedial part of the STN was adjacent to the rostral part of the substantia nigra pars compacta (SNc), where some of the neurons of the two nuclei merged. The neurons in the STN had medium-sized (25-40 microm) ovoid or fusiform cell bodies, from which three to six large dendrites emanated in a direction predominantly parallel to the long axis of the STN. Immunohistochemistry revealed that most of the subthalamic neurons were glutamatergic and differed significantly in appearance from the large stellate TH-positive cells of the adjacent SNc. Numerous TH-positive bouton-rich fibers traversed the STN. The GAD-staining revealed a large number of terminals within the boundaries of the STN. The STN was highly AChE-positive, reflecting a prominent innervation by ChAT-positive terminals. The total number of subthalamic neurons in one hemisphere was estimated to be approximately 56,000. We conclude that the neuroarchitecture of the porcine STN is similar to primates, including humans, and appears well-suited for further studies examining the role of the STN in movement disorders.

Acoustic cavity modes in lens-shaped structures

A method for solving exactly the Helmholtz equation in parabolic rotational coordinates is presented using separability of the eigenfunctions and the Frobenius power series expansion technique. Two examples of interest in acoustics are considered and analyzed quasianalytically: The acoustic pressure in a cavity defined by two paraboloids (forming a lens-shaped structure) with (I) rigid wall boundary conditions and (II) pressure-release boundaries. The rigid-wall (pressure-release) acoustic enclosure problem is a Neumann (Dirichlet) boundary condition problem. In both cases, eigenfunctions and eigenmodes are calculated and the shape dependence of the eigenvalue for the ground state is examined.

Taxonomic significance of sporoderm structure in pollen of Euphorbiaceae: Tribes Plukenetieae and Euphorbieae

A pollen sporoderm study of tribe Plukenetieae - subtribe Tragiinae and tribe Euphorbieae was carried out. Most of the species of subtribe Tragiinae examined have a reduced exine, with a perforate tectum, short and irregular columellae, and a thin, undulated or absent foot layer. The endexine is homogeneous, fissured or spongy. The intine is well developed, thicker than the exine, and usually three-layered. In the apertural region both ectexine and endexine are present but are fragmented and disorganised. Only in the clavate-baculate pollen of Tragia volubilis is the exine well-developed and the intine thinner than the exine. The thicker intine is correlated with a thin and weakly developed exine that may provide support, compensating for the severe reduction of the foot layer and the poorly developed, fissured and spongy endexine. The pollen morphology of tribe Euphorbieae is concordant with the common pollen type in the genus Euphorbia . Nevertheless the pollen grains of this tribe have a particular apertural system, that includes modifications to the ectexine, endexine and intine at different aperture levels. The most important observation was the unusual intine with two thickenings that run along each side of the aperture. In general, pollen of the Euphorbiaceae has a thick and well-developed intine. This intine may be uniformly thickened around the grain or may form a lens-shaped structure (oncus) beneath the apertures. Intine thickenings such as those described for tribe Euphorbieae are unknown elsewhere in the family. The more specialised apertural structure, described for tribe Euphorbieae, supports the circumscription of this tribe.

Fine structure of spermatozoa of some labidognath spiders (Filistatidae, Segestriidae, Dysderidae, Oonopidae, Scytodidae, Pholcidae; Araneae; Arachnida) with remarks on spermiogenesis.

The present study reports on the spermiogenesis and spermatozoa of seven labidognath spiders: Filistata insidiatrix (Filistatidae), Segestria senoculata (Segestriidae), Dysdera sp., Harpactea hombergi (Dysderidae), Oonops domesticus (Oonopidae), Scytodes thoracica (Scytodidae), and Pholcus phalangioides (Pholcidae). Filistata insidiatrix is the first cribellate spider whose spermatology is described electron microscopically. A common characteristic of the spermatozoa of the cribellate spider and the remaining species, often referred to as haplogyne spiders, is the coiling process that occurs at the end of spermiogenesis. As a result of coiling, an elongated spermatid is converted into a lens-shaped structure with the flagellum bearing a 9 × 2 + 3 axoneme becoming incorporated into the cell body. Remarkable differences regarding the main components (shape of nucleus, acrosomal vacuole, implantation fossa, and centriolar complex) probably reflect systematic relationships. The formation of sperm capsules and sperm balls is described for the first time in detail. Sperm capsules occur in Filistata, in which numbers of individual spermatozoa are grouped together by a common secretory envelope established in the distal vas deferens. In contrast, in the sperm balls, two (Harpactea) or four (Segestria, Dysdera, Seytodes) spermatids fuse completely at the end of spermiogenesis. These sperm balls, considered unique in the animal kingdom, are also provided with an envelope. A further peculiarity not reported previously is the occurrence of a large vesicular area in the sperm balls of Dysdera and Harpactea; this area is also found in Oonops, which, however, possesses individual spermatozoa. Components of the spermatozoa such as the acrosomal vacuole, part of the nucleus, and the axoneme protrude into this area and are thus secondarily covered with a membrane. A detailed study of the individual spermatozoa of Pholcus phalangioides completes earlier investigations and stresses the exceptional position of the genus in comparison to that of other spiders.

Fertilization in Ginkgo biloba

1. At the time of shedding, the pollen grain of Ginkgo usually contains four cells: the first prothallial, second prothallial, generative, and tube cells. After germination of the pollen grain the first prothallial cell aborts, while the generative cell divides, forming the body and stalk cells. During a late stage of development of the body cell, two blepharoplasts are formed, and the nucleus changes from a spherical to a lens-shaped structure. While the nucleus is changing shape, two vacuole-like structures appear close beneath the blepharoplasts, and a conspicuous granule becomes visible beside the nucleus. 2. Chromatin in the nucleus of the body cell is diffuse but condenses into chromosomes during division. Each spermatozoid cell includes one of the blepharoplasts, which changes from a rounded to a crescent-shaped structure and then forms the cilial band. The vacuole-like structure separates from the blepharoplast and moves to the side opposite the cilial tail. Later it becomes impregnated with chromatin and apparently forms the head of the spermatozoid. One half of the granule near the nucleus of the body cell is included in each spermatozoid. The newly formed spermatozoid is flexible and flows out from the pollen tube. 3. Free-nuclear divisions occur simultaneously in early stages, and more than a thousand nuclei are formed before the initiation of wall formation in the megagametophyte. Wall formation proceeds centripetally. In the free-nuclear stage the cells of the spongy tissue of the nucellus surrounding the megagametophyte may be multinucleate, with extremely large polyploid nuclei. This tissue is absorbed by the developing megagametophyte. 4. Archegonial initials, usually two in number, appear superficially at the micropylar end of the megagametophyte, each containing a large central nucleus. Each initial divides periclinally to form a cover cell and an inner central cell. The cover cell divides anticlinally to form two neck cells. The central cell gradually enlarges, while adjacent cells form a well-organized jacket layer. Just before division of the central cell the two neck cells divide, forming four neck cells, which swell to form a hemispherical protuberance. The central cell divides unequally into the small ventral-canal cell and large egg. 5. When the spermatozoid is released from the pollen tube, the egg cell forms a small projection which extends between the neck cells. As soon as the spermatozoid becomes attached to the egg projection, the latter retreats, leaving a path through the neck. Only the head of the spermatozoid, which includes the chromatin, penetrates the egg cytoplasm, where it forms a globular mass. Simultaneously, the chromatin in the egg nucleus forms another group of granules. The union of the two sets of granules is considered to be the actual fertilization. 6. The first division of the zygote is perpendicular to the axis of the ovule. In telophase an obscure cell plate is visible but becomes less evident later. Free-nuclear division continues until 256 nuclei are formed. Wall formation in the proembryo is simultaneous. As the embryo continues to develop, cells differentiate into the cotyledons and the apical meristems of root and stem.

A contribution to the morphology and cytology ofMonochoria hastaefolia Presl.

The paper gives an account of the morphology, cytology and embryology ofMonochoria hastœfolia. 1. Monochoria hastœfolia is a marshy plant. It has a subterranean root-stock and bears hastate leaves. The flowers show trimerous symmetry. Of the six stamens, one is larger and the anther is differently coloured. The pollen grains produced in this anther show no morphological or size-difference when compared to those produced in the other anthers. All are equally viable. 2. Anthesis of flowers takes place generally before 8 a.m. On cloudy and humid days it is delayed. Bagging of inflorescences shows that self-pollination is the rule. Seed formation takes place under natural conditions and it takes about 15 days for the seeds to develop after syngamy. The germination of the seed is of the “Palm-type”. 3. The development of the microspore appears to be of the successive type. During heterotypic prophase the nucleolus becomes adpressed against the nuclear wall and assumes a lens-shaped structure. At this stage a distinct connection of the nucleolus with the spireme is first noted. At pachynema a small spherical body is seen to lie at the side of the nucleolus to which the spireme is connected. This has been referred to as the nucleolar body. At diakinesis a pair of bivalents are also seen to lie attached to the nucleolus. The haploid number of chromosome is 14. Cytokinesis takes place by cell plate formation. 4. The pollen grains have an elongated appearance when dry. The exine shows the presence of fine granulations all over the surface and has two furrows situated diametrically opposite to each other. The pollen grains are bi-nucleate at the time they are shed. The generative nucleus lies within a cytoplasmic sheath, the ends of which are drawn out. 5. The pollen grains germinate on the stigma within ten minutes after pollination. The pollen tubes pass through the stylar canals and the division of the generative nucleus takes place inside the pollen tube. The metaphase spindle is well organised. Furrows or cell plates have not been observed and it appears that the sperm-nuclei become separated by the disappearance of the spindle fibres.