SPORE WALL ULTRASTRUCTURE IN THE LIVERWORT ATHALAMIA HYALINA

Mature spores of Athalamia hyalina (Marchantiales, Cleveaceae) were examined with both scanning and transmission electron microscopes. Single, hollow, dome-like projections, sometimes having small pores and a coarsely granular surface texture, stud the spore surface, usually in a pattern of concentric circles. In section, the spore wall has an intine and two-layered exine. Intine-like material separates some lamellae of the inner exine, which is joined to the outer exine around the dome bases. Inner exine lamellae are composed of thin (5–6 nm), closely parallel membrane-like subunits. The outer exine is formed from a single large highly modified and doubly-coated lamella, the undulations of which form the surface domes. Dome cavities often are filled with a loose network of granular material.     

The cytoarchitecture of the medial layer in rat thoracic aorta: a scanning electron-microscopic study

Summary The cytoarchitecture of the medial layer of rat thoracic aorta was examined by scanning electron microscopy after removal of the connective tissue. The outermost lamella showed a lattice-like structure of muscle bundles of closely apposed smooth muscle cells (SMCs), whereas the inner lamellae consisted of more-or-less continuous muscle sheets of vaguely defined subgroups of parallel SMCs. Longitudinal rows of ridges ran along the adventitial surface of these muscle bundles and sheets. The SMCs of the outermost lamella, were 5.1 m wide, and varied in shape, whereas those of the inner lamellae, were 52.7 m long, 2.6 m wide and 4.1 m thick, and were elongated, spindle-shaped cells with serrated outlines. These latter SMCs extended obliquely, and partially overlapped each other. The surface of the SMCs in the outermost lamella exhibited a rugged texture, with nodular protrusions and oblique and longitudinal laminar folds, while the inner lamellar cells showed longitudinal laminar folds and finger-like processes on both sides of the ridges, pointing in opposite directions to the ridges. The angle of deviation from the transverse axis of the vessel, of the muscle bundles and subgroups in the outermost lamella, was 33.6°, in the second and third lamellae, 22.5°, and in the innermost lamellae, 12.8°. The mean angle of the muscle bundle and subgroup arrangement, with respect to the long axis of the vessel, however, was basically 90° in all lamellae.     

On the Ultrastructure of Pollen Exine in Metasequoia Hu and Cheng

Metasequoia is endemic to China. Present study deals with ultrastructure of pollen exine of M. glyptostroboides Hu et Cheng, and in comparision with other genera of the family. Pollen grains of Metasequoia are spheroidal or subsphoroidal and 27.8(24.3–32.3) μm in diameter. There is a papilla in the distal face. The papilla is wide at the base, 3.5–5.2 μm high, with pointed and circular end and the base crooked toward one side. Exine is about L5 μm thick, layers distinct, Nexine is as thick as sexine. Surface weakly granulate. According to observation by SEM, exine is covered with fine granules and rather coarse tuberculae. The former can be easily separated from the latter. The loose and uneven tuberculae are provided with minute spinules on the surface and generally fall off after acetolysis. The fine and dense granulae, however, remain intact after acetolysis. The study by TEM shows that ektexine is made of granules densely arranged and connected with each other. In addition, sparse Ubisch bodies are unevenly distributed on granular layer with geminate surface. The thick endexine, is composed of 10–15 lamellae. It is worthy to note that all lamellae possess tripartite structure. But lamellae of endexine in other genera of Taxodiaceae have no tripartite structure except the lamella near ektexine. Number of lamella and thickness of endexine in Metasequoia differ from those of other genera in Taxodiaceae; for example endexine with 8–10 lamellae in Taxodium, 8–9 lamellae in Sequoia, 6–7 lamellae in Glyptostrobus, 6–8 lamellae in Cunninghamia, about 16 lamellae in Cryptomeria etc.     

OCCURRENCE OF TRI-LAMELLAR BODIES IN ISOLATES OF NOSTOC (CYANOPHYCEAE)1

Tri-lamellar bodies were observed in eight of 29 isolates of Nostoc examined. They appeared identical to the previously described bodies in various species of Anabaena. The bodies consist of three discoid lamellae each ca. 0.3 μm diam and 8 nm thick. The outer lamella (closest to the plasma membrane) is separated from the middle lamella by a 12 nm space whereas the middle and inner lamellae are ca. 8 nm apart. Osmiophilic striations 3 nm wide were generally observed running between the lamellae. Osmiophilic β granules were usually associated with the inner lamella. The bodies were most always located close to the plasma membrane along the longitudinal wall near the junction of the cross and longitudinal walls. In three isolates the bodies located near the cross walls were associated with gas vesicles and possessed a slightly different morphology. These tri-lamellar bodies consisted of three discoid lamellae, each ca. 2 nm thick, ca. 25 nm apart with electron dense material between the inner and middle lamellae. Pores 20 nm diam and ca. 60 nm apart were observed in layer 2 of the cell wall adjacent to the tri-lamellar bodies. These wall pores were also observed in isolates lacking tri-lamellar bodies.     

CHLOROPLAST DEVELOPMENT AND ULTRESTRUCTURE IN THE FRESHWATER RED ALGA BATRACHOSPERMUM1

Chloroplast development and ultrastructure of the freshwater red alga Batrachospermum moniliforme are described. Chloroplasts develop from proplastids which have a double-membraned chloroplast envelope and a parallel double-membraned outer photo-synthetic lamella. Of these 2 double-membraned structures of the proplastid, only the outermost pho-tosynthetic lamella functions in production of further lamellae. The mature chloroplast consists of 2 or more concentric lamellae and a variable number of nonconcentric lamellae. These lamellae are not dense, uninterrupted sheets as described for other red algae, but are largely constructed of tubules, lying side by side, that form interrupted lamellar sheets. The possible physiological significance of lamellar interruptions in providing path-ways for movement of materials in the chloroplast stroma is discussed.     

ULTRASTRUCTURE OF SPOROGENESIS IN A MOSS,DITRICHUM PALLIDUM. III. SPORE WALL FORMATION

Ultrastructural evidence indicates that marked cytoplasmic polarity occurs during wall and aperture ontogeny in spores of the moss (Musci), Ditrchum pallidum (Hedw.) Hampe. Shortly after cytokinesis, an extensive system of microtubules underlies the entire distal spore surface where exine deposition is initiated. These microtubules appear to be focused on the plastid. The apposition of slips nearly of membrane dimension contributes to the forming exine. As the lamellate exine thickens and extends to the proximal surface, the plastid and associated nucleus migrate to the proximal surface where an elaborate system of microtubules involved in aperture development is generated. The exine gradually loses its stratiform character, becoming homogenous and eventually papillate. At maturity, the spore wall consists of four layers, the outermost perine, the exine, a separating layer, and the intine. The aperture is a complex, localized modification of these layers on the proximal surface. It consists of a pore containing a fibrillar material surrounded by a thin annulus.     

Aperture development in spores of the moss,Trematodon longicollis Mx.

Summary Young spores of the mossTrematodon longicollis Mx. are highly polar. Immediately after meiotic cytokinesis an extensive system of microtubules associated with the single plastid develops under the entire distal face. Following exine initiation on the distal surface a microtubule system is elaborated at the site of aperture development on the proximal surface. Both plastid and nucleus move from distal to proximal pole and are attached to microtubules of the proximal system. Microtubules underlie the plasma membrane as it withdraws from the exine in the initiation of both the surrounding annulus and central aperture pore. The central pore enlarges to form a bowl-shaped concavity in which a fibrillar plug develops basipetally. The annulus expands into a fibrillar-filled protrusion surrounding the central pore. The mature aperture consists of a central pore plug covered by a thin roof of exine and separated from the surrounding annulus by exine lamellae. The aperture of the mature spore is obscured by development of the ornate exine and is not a prominent feature of the mature spore surface.     

SPORE WALL FORMATION AND CHLOROPLAST DEVELOPMENT DURING SPOROGENESIS IN THE MOSS FISSIDENS LIMBATUS

The sequence of wall formation in spores of Fissidens limbatus Sullivant is as follows: The exine is formed around the protoplasts after the sporocyte has undergone meiosis. The fully enlarged spores then become coated by the perine; this is followed by intine formation. The source of the intine and exine appears to be from within the spore, but the perine is of an apparent exogenous origin. Ornamentation of the spore is due solely to deposition of the perine. Each spore originally has a single plastid. Plastids increase in number by fission, resulting in mature spores with numerous plastids with well differentiated lamellae.     

The fine structure of a tri-lamellar body in various species ofAnabaena

Summary A tri-lamellar body has been observed either near or adjacent to the crosswalls in 16 out of 20 different isolates ofAnabaena examined in thin sections. These bodies appear to consist of three discoid lamellae approximately 0.3 m in diameter. The outer lamella (closest to the plasma membrane) is separated from the middle lamella by a 12 nm space and is about 8 nm in thickness. The middle and inner lamellae, spaced about 8 nm apart, are approximately 8 nm in thickness. Electron dense granules, interpreted to be -granules, are associated with the inner lamella. In different species, osmiophilic lines 3 nm wide were observed. The osmiophilic lines run at right angles to the lamellae, either between the outer and middle lamellae, between the middle and inner lamellae or between all three lamellae. In some species, osmiophilic lines are absent. Up to six tri-lamellar bodies have been observed in median longitudinal sections. Pores 20 nm in diameter and 60 nm apart were observed in layer 2 of the cell wall of all the species ofAnabaena examined. All species which had tri-lamellar bodies also had wall pores closely associated with the bodies. Wall pores were also observed in four species lacking tri-lamellar bodies. The possible role of these structures is discussed.     

Preliminary Observations on the Fine Structure of the Pansporoblast of Thelohania bracteata (Strickland, 1913) (Microsporida, Nosematidae) as Revealed by Freeze-Etching Electron Microscopy

SYNOPSIS. The ultrastructure of a microsporidan pansporoblast was observed with freeze-etching electron microscopy. The cross-fractured face of ovoid mature spores, with the upper part of the spore coat fractured off, revealed the spore membrane; the convex face had many small depressions and the concave face bore fine particles. In cross-section the spore-coat was highly laminated and about 0.5 μ in diameter.
In the cytoplasm of the pansporoblast, fluid-filled and finger-print-life profiles of vesicles were observed. The vesicles were approximately 180 nm in diameter and laminated, each lamella being about 15–18 nm thick. In addition to these vesicles, a population of elevations, each with an average diameter of 40 nm, was evenly distributed in the pansporoblast among the spores. No other cytoplasmic organelles were observed within the pansporoblast. The pansporoblast wall was about 15–19 nm thick with particles 15–18 nm in diameter on its outer surface.     

ON THE ULTRASTRUCTURE OF CAMBIUM AND ITS VASCULAR DERIVATIVES. III. THE SECONDARY WALLS OF THE SIEVE ELEMENTS OF PINUS STROBUS

The sieve elements of Pinus strobus have thick, lamellate secondary walls, which are composed predominantly of cellulose and lesser amounts of polyuronides and pectins. Eight to 10 lamellae may be present in walls 2-3 μ in thickness. Each lamella represents a plane of high cellulose density which results from intersection of two parallel sets of fibrils. Polyuronides and pectins are more or less evenly distributed in the wall, possibly with a greater concentration near the middle lamella and the inner surface. Resemblance of these walls to the so-called nacré walls is indicated, and it is possible that the two represent the same structure.     

The ecological role of caudal lamellae loss in the larval damselfly,Ischnura posita (Hagen) (Odonata: Zygoptera)

Summary Damselfly larvae may autotomize and regenerate any of their 3 caudal lamellae. At least one missing or regenerating lamella was evident in 50.1% of field collected Ischnura posita larvae. Lamellae loss during molting is very infrequent (1 out of 117 recorded molts). Laboratory trials indicate that conspecifics remove lamellae and that this process is density dependent. The percentage of larvae losing lamellae during 24 h trials ranged from 73.5 at the highest density tested to 17.3 at the lowest density. I. posita larvae are cannibalistic. The presence of lamellae reduces an individual's chance of being cannibalized. More than twice as many final instar lamellae-less larvae were cannibalized during 24 h trials than analogous individuals having 3 lamellae at experimental initiation. Costs are also associated with lamellae autotomy. 1) Although individuals without lamellae can swim they are more reluctant to release from a wooden stalk and swim when threatened (9% release) than are larvae with lamellae (29% release). Since swimming is part of their repertoire of anti-predator behaviors this behavioral shift should be detrimental. 2) Caudal lamellae function in O₂ uptake. Trials were conducted with larvae having and not having lamellae in an experimental horizontal oxygen gradient system. Relative to larvae without lamellae, those with lamellae preferred deeper depths at PO₂ values greater than 70 torr. Many lamellae-less larvae distributed themselves at the water surface throughout the range of PO₂ values tested. Differential depth distribution between larvae with and without lamellae is highly significant (P < 0.01).     

Molecular requirements for actin-based lamella formation in Drosophila S2 cells

Cell migration occurs through the protrusion of the actin-enriched lamella. Here, we investigated the effects of RNAi depletion of approximately 90 proteins implicated in actin function on lamella formation in Drosophila S2 cells. Similar to in vitro reconstitution studies of actin-based Listeria movement, we find that lamellae formation requires a relatively small set of proteins that participate in actin nucleation (Arp2/3 and SCAR), barbed end capping (capping protein), filament depolymerization (cofilin and Aip1), and actin monomer binding (profilin and cyclase-associated protein). Lamellae are initiated by parallel and partially redundant signaling pathways involving Rac GTPases and the adaptor protein Nck, which stimulate SCAR, an Arp2/3 activator. We also show that RNAi of three proteins (kette, Abi, and Sra-1) known to copurify with and inhibit SCAR in vitro leads to SCAR degradation, revealing a novel function of this protein complex in SCAR stability. Our results have identified an essential set of proteins involved in actin dynamics during lamella formation in Drosophila S2 cells.     

Exine ultrastructure of in situ peltasperm pollen from the Rhaetian of Germany and its implications

Morphology and exine ultrastructure of pollen grains of Triassic peltasperms have been studied for the first time. Pollen grains of Antevsia zeilleri from the Rhaetian of Germany are of the Cycadopites-type and monosulcate; the sculpturing is the same in the apertural and non-apertural areas. The proximal exine includes a row of lacunae covered by a solid, thick tectum and underlined by a foot layer. Pillars are hanging from the tectum between the lacunae. The exine is thinning to a homogeneous layer in the apertural region. The latter is bordered by thicker alveolate areas of the exine, in places resembling a saccus-like ultrastructure. The endexine includes white-line-centred lamellae. The exine ultrastructure is compared with that of pollen of Permian peltasperms. Although pollen types ascribed to Permian peltasperms are completely different in their general morphology, a transformation can be hypothesized by ultrastructural data from Permian Vesicaspora into Triassic Cycadopites extracted from pollen sacs of Antevsia. Comparison with Cycadopites of non-peltaspermalean (Ginkgoalean, Cycadophyte) and unknown affinities has been accomplished. The exine ultrastructure is distinctive enough to differentiate among peltaspermalean, cycadalean and bennettitalean Cycadopites; some ultrastructural features are shared with pollen of modern Ginkgo biloba. More ultrastructural data are needed as well as numerous sections of pollen grains are necessary to reveal original unchanged ultrastructure.     

JUNCTIONAL PORES AND CELL WALL DEPRESSIONS OF HORMOSCILLA PRINGSHEIMII (CYANOPHYCEAE)1

Sassoon's isolate identified as Borzia trilocularis (but recently renamed Hormoscilla pringsheimii Anagnostidis and Komárek (1988) was studied with transmission electron microscopy because of its unusual combination of longitudinal wall features, described here in development for the first time. Junctional pores (linear rows of circumferential L-II layer pores near crosswalls) developed into multiple, parallel series, unlike the pores in many other species, which form only single rows. In dividing cells, two single pore rows appear opposite crosswalls after crosswall initiation, but an additional parallel row is usually added to each row by completion of fission. Elongating cells reveal 3–6 parallel and uniformly spaced pore rows developing on each side of crosswall pairs; these rows may end up toward the center of the cell wall after cell elongation. Pores are 18–24 nm wide with a center-to-center and row-to-row distance of 24–36 nm and occur in an especially thick L-II area. The second group of pit-like pores of longitudinal cell walls are 50–135 nm-wide depressions and have a center-to-center distance of 100–1000 nm. These depressions arise when the L-II layer fails to form and appear next to a row-pair of junctional pores soon after fission. Most depressions form single rows, but when they form several rows they may cover much of the surface of the cell. The L-III and L-IV wall layers line these L-II layer cavities; the outer surface of the L-IV layer around and within the depressions is covered with fibrous mucilage. Given their diversity, pores and depressions of longitudinal walls deserve further attention from functional and taxonomic points of view.     

Ultrastructure of spermatogenesis in the sea star,Asterina minor

The ultrastructural features of spermatogenesis were investigated in the hermaphroditic sea star Asterina minor. The primordial germ cells in the genital rachis contain small clusters of electron-dense material (nuage material) and a stack of annulate lamellae. They also have a flagellum and basal body complex situated close to the Golgi complex. After the development of the genital rachis into the ovotestis, spermatogenic cells increase in number and differentiation begins. Nuage material is observed in spermatogonia, but it gradually disappears in spermatocytes. The annulate lamellae do not exist beyond the early spermatogonial stage. By contrast, a flagellum and basal body complex are found throughout spermatogenesis. The Golgi-derived proacrosomal vesicles appear in the spermatocyte and coalesce to form an acrosomal vesicle in the early spermatid. The process of acrosome formation is as follows: (1) a lamella of endoplasmic reticulum (ER) continuous with the outer nuclear membrane encloses the posterior portion of the acrosomal vesicle; (2) the vesicle attaches to the cell membrane with its anterior portion; (3) periacrosomal material accumulates in the space between the acrosomal vesicle and the ER; (4) the nucleus proper changes its features to surround the acrosome; (5) amorphous, electron-dense material is deposited under the electron-dense disk; and (6) the nucleus forms a hollow opposite the electron-dense material.     

Electron-translucent angular areas in developing tapetum cell walls and pollen grains ofTilia platyphyllos

Summary InTilia platyphyllos, the anther tapetal cell walls undergo significant modifications from the tetrad stage onwards. During the tetrad stage the inner tangential and radial parts of the tapetal walls begin to dissolve, while the distal parts swell. After the tetrad stage, the distal and outer radial tapetal cell walls become covered by a thick, irregular, highly electron-dense, polysaccharide layer. Striking features of the maturing tapetal walls (microspore stage and later) are electron-translucent, structureless, unstainable angular areas of variable dimensions. Similar electron-translucent areas occur in the exine arcades and apertures, but also isolated in the locular fluid ofT. platyphyllos. Electron-translucent areas, that are also found in the exine arcades and tapetal cells of other angiosperms, can be interpreted as the products of poorly understood metabolic processes.     

The fine structure of mature and germinating chlamydospores of Fusarium oxysporum.

A number of features not described previously has been revealed in electron-microscope studies of mature chlamydospores of Fusarium oxysporum. On the maturation of one isolate, many spores formed a thick matrix-like layer containing electron-dense particles on the exterior surface of the spore wall. In thin sections of mature chlamydospores of the same isolate, cisternae of endoplasmic reticulum surrounding, and in close apposition to, the limiting boundary of the lipid bodies were revealed. The germination of chlamydospores was accompanied by (a) the rapid appearance of polysaccharide deposits and changes in the configuration of some subcellular organelles, (b) the formation of a new wall layer between the plasma membrane and the innermost layer of the spore wall, (c) the rupture of the outermost coats of the spore wall, and (d) the emergence of the germ tube as an extension of the new wall layer.     

Exine development in Nymphaea colorata (Nymphaeaceae)

We show a sequence of developmental events in microspores and tapetal cells in Nymphaea colorata based upon transmission and scanning electron microscopic observations. There are parallel cytoplasmic processes and surface coatings in microspores and tapetal cells. Uptake is indicated by the passage of lanthanum as a tracer from anther locule into the microspore cytoplasm and by the condition of the cytoplasmic surface of microspores. The callose envelope is not a barrier to transfer of lanthanum. During formation of the proexine glycocalyx tiny spiral elements, components of the exine substructural units, were oriented in different directions in the surface coating of microspores and tapetum. Lipoidal globules are associated with the spiral elements. After the uniform proexine stage, three regions of different exine structure and their gradations become differentiated in the sporoderm: 1) a proximal region with thick tectum and foot layer, thin columellae and a compact layer of lamellated endexine; 2) a distal pole region with separately disposed endexine lamellae; and 3) an equatorial encircling-sulcate aperture region which consists of infratectal layer, foot layer, and endexine lamellae. Based upon the presence of structurally comparable surface coats in microspores and tapetal cells, experimental uptake of lanthanum nitrate, and the co-ordinated processes in tapetum and microspores, we conclude that there is probably a reciprocal controlling influence between the microspores and the tapetum and other sporophytic tissues.     

The ultrastructure of fossil dispersed monosulcate pollen from the Early Cretaceous of Transbaikalia,Russia

The general morphology, surface sculpturing, and exine ultrastructure have been studied in dispersed monosulcate pollen from the Early Cretaceous of Transbaikalia, Russia. The pollen grains dominate the palynological assemblage extracted from coal deposits of the Khilok Formation in the Buryat Republic, which also contain ginkgoalean leaves of Baierella averianovii as the only constituent of the assemblage of plant megafossils. The relationship between the pollen grains and ginkgoalean leaves from this autochthonous burial is hypothesised on the basis of taphonomical analysis and palaeobiogeographical data. It is shown that the ectexine of the pollen grains includes a thick solid tectum, a thin granular infratectum and a thin foot layer; the endexine is fine-grained, slightly more electron-dense than the ectexine, and is preserved only in places. The distal aperture is formed by a thinning of the exine. No analogous ultrastructure has been described so far in fossil pollen grains of this morphotype studied ultrastructurally from in situ material. For comparison, we also studied the exine ultrastructure of pollen grains Ginkgo biloba. The fossil pollen is not identical to pollen of extant G. biloba, but shows several significant similarities in the exine ultrastructure, which does not contradict the presumable ginkgoalean affinity of the fossil pollen.