Reticulate Layer Research Articles

Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

SYNTHESES, Structures and Fluorescent Properties of Two Zn(II)-Diphosphonate Coordination Polymers

With the introduction of 2,2′-bipyridine (2,2′-bipy) agent, two new zinc coordination polymers (CPs), {[Zn6(HL-H)4(2,2′-bipy)4]·¼H2O}n1 and {[Zn3(HL-H)2(H2O)4]·6H2O}n2, have been hydrothermally obtained derived from diphosphonic acid, 1-hydroxy-2-(3-pyridyl)ethylidene-1,1-diphosphonic acid (H5L = (H4C5N)CH2C(OH)(PO3H2)2, HL-H = (H4C5N–H)CH2C(OH)(PO3)2), and determined by single-crystal X-ray diffractions, elemental analysis, IR, powder X-ray diffractions (PXRD). The single-crystal X-ray diffractions reveal that compound 1 owns a novel 1-D fishbone-like inorganic chain structure built from binuclear building blocks [Zn2(O–P–O)2]n via [ZnO4]n tetrahedron linkers. It is the first example that N-heterlcyclic amine 2,2′-bipy is used as the second ligand to construct the compound 1 with the diphosphonic acid ligand H5L. Both hydrogen bonding and π–π stacking interactions further extend the 1-D chain to the 3-D supramolecular network. Compound 2 exhibits a 2-D reticulate layer structure containing 4-, 8- and 16-membered rings created from 1-D ladder-like inorganic chain, and the organic pyridine rings in the structure point to voids defined by the 16-membered rings. Moreover, the room-temperature fluorescence measurements show that both the compounds display two fluorescent emissions centered at 379, 424 nm for 1 and 380, 424 nm for 2, respectively, caused by intraligand π*–π emission state of organic pyridine ring (λex = 235 nm). With the introduction of 2,2′-bipyridine (2,2′-bipy) agent, two new Zn(II)-phosphonate CPs, 1-D {[Zn6(HL-H)4(2,2′-bipy)4]·¼H2O}n1 and 2-D {[Zn3(HL-H)2(H2O)4]·6H2O}n2 (H5L = (H4C5N)CH2C(OH)(PO3H2)2, HL-H = (H4C5N–H)CH2C(OH)(PO3)2), have been hydrothermally obtained and characterized. The thermal stabilities and fluorescent properties of them have been investigated.

THE EFFECT OF CHLORAMPHENICOL AND COPPER ON THE CELL ULTRASTRUCTURE OF SCENEDESMUS SP.

The present study aimed to evaluating the effect of chloramphenicol (CAP) and copper (CuSO4) on the cell ultra-structure of Scenedesmus sp. Three different concentrations of CuSO4 (10-3, 10-4, 10-5 M) and two of CAP (0.25 and 0.5%) were added to exponentially growing algal culture (107 cells ml-1) for 24 hrs. Cells treated with 10-3 M CuSO4 and 0.5% chloramphenicol which gave high amount of cell lyses were chosen for examination by electron microscopy. Transmission electron microscopy photographs of Scenedesmus cells treated with CuSO4 showed deformation of the chloroplasts, while numerous vesicles were observed in the cytoplasm. The three–laminar sheath (TLS) and the reticulate layer (RL) had no differences in comparison with control. On the other hand, tubular spikelets had no distinct shape compared with the control. The inner layer of cell wall (cellulose layer) showed shrinkage in thickness. Moreover, cells treated with 0.5% CAP showed that chloroplast thylakoids were reduced in size and number, disorganization, shrinkage in all protoplasmic components and increasing in the number of vacuoles and cellulose layer thickness. These results may help in understanding the effect of some toxins such as antibiotics and heavy metals on the cell structure of microalgae.

Sporoderm and tapetum ontogeny in Juniperus communis (Cupressaceae). Connective structures between tapetum and microspores

The purpose of this work was to study in detail the successive stages of pollen wall development in Juniperus communis and intimate connection between the microspores and nutritive tapetum with TEM. Another goal was to clear up the mechanisms underlying the developmental processes. The key stages observed are: the appearance of the initial outer glycocalyx layer as a row of spherical units at the early tetrad stage, followed by the emergence of the underlying radially oriented string-like units, resulting in the formation of a reticulate layer. After sporopollenin accumulation, spherical units turn into granules. String-like units start from the microspore plasma membrane, pierce the glycocalyx layer, pass between the primexine granules and through callose jacket and come out to the anther loculus, reaching the tapetal cells. Their substructure and size are similar to viscin threads, but not their function and chemical composition. All the structures, observed in sporoderm development, correspond to subsequent mesophases of a micellar system. The latter develops by self-assembly, under genomic control, regulating increasing concentration of glycoprotein surfactants in the periplasmic space. A thick network of filaments, connecting microspores and tapetum, supplies microspores with necessary nutrients and testifies the opinion that, in the process of development, exine is a living, dynamic system, arranged from plasmodesmata-like units.

Sensitivity to fuel diesel oil and cell wall structure of some Scenedesmus (Chlorococcales) strains

Sensitivity of three <i>Scenedesmus</i> strains exposed to aqueous fuel-oil extract (AFOE) is strongly strain-dependent <i>S. quadricauda</i> is the most resistant, <i>S. armatus</i> moderately tolerant whereas the most sensitive appears to be <i>S. microspina</i>. The sensitivity of tested species increases parallel with decreasing of cell size and cell number in coenobium. The values of the cell surface/cell volumes ratios only partly explain the above relationships. Electron microscope investigations reveal that the sensitivity may depend on cell wall structure of the strains. Cell wall of all here investigated strains is built of two layers: the inner so-called cellulosic layer and the outer one showing a three-laminar structure (TLS). The latter contains an acetolysis-resistant biopolymer (ARB). These two layers are similar in thickness in the three strains tested, but the surface of <i>Scenedesmus</i> is covered with various epistructures that are characteristic of strains. Some of them as the tightly fitting warty layer of <i>S. armatus</i> and especially the loosely fitting reticulate layer of <i>S. quadricauda</i> may contribute to lower permeability of cell wall. The structure of the rosettes also appears to be correlated with the sensitivity of strains. Presence of invaginations of plasmalemma in areas under rosettes indicates their role in transport processes inside/outside the cells.

Comparative anatomy of the floral elaiophore in representatives of the newly re-circumscribed Gomesa and Oncidium clades (Orchidaceae: Oncidiinae)

Recently, molecular approaches have been used to investigate the phylogeny of Oncidiinae. This has resulted in the transfer of taxa previously considered to be species of Oncidium Sw. into Gomesa R. Br. and the re-circumscription of both genera. In this study, the structure of the floral elaiophore (oil gland) is described and compared for Gomesa echinata (Barb. Rodr.) M.W. Chase & N.H. Williams, G. ranifera (Lindl.) M.W. Chase & N.H. Williams, Oncidium amazonicum (Schltr.) M.W. Chase & N.H. Williams and O. oxyceras (Königer & J.G. Weinm.) M.W. Chase & N.H. Williams in order to determine whether phylogenetic revision is supported by differences in its anatomy. The floral elaiophore structure was examined and compared at three developmental stages (closed bud, first day of anthesis and final stage of anthesis) for all four species using light microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry. In all species investigated, the floral elaiophore occurs on the labellar callus and is of the epithelial type, comprising cuboidal to palisade-like, secretory epidermal cells and a layer of sub-epidermal cells, both tissues enclosing ground parenchyma supplied with collateral vascular bundles and containing idioblasts, often with raphides or phenolic contents. A bi-layered cuticle comprising an outer, lamellate and an inner, reticulate layer is present, and sub-cuticular accumulation of secreted material results in distension of the cuticle. Secretion-filled cavities are present at anthesis in the elaiophore cell walls and, in most species, the outer, tangential walls of the elaiophore have small, peg-like projections that protrude into the cytoplasm. In all taxa examined, the elaiophore organelle complement, especially the smooth endoplasmic reticulum (SER), is typical of lipid-secreting cells. In terms of location, morphology, anatomy and ultrastructure, the floral elaiophores of both Gomesa and Oncidium species examined are very similar, and distinction between these genera is not possible based on elaiophore features alone. Furthermore, many of these elaiophore characters are shared with representatives of other clades of Oncidiinae, including the Ornithocephalus clade. Consequently, elaiophores are considered homoplasious and of limited value in investigating the phylogeny of this subtribe.

Ultrastructure and Molecular Phylogeny of the Cryptomonad Goniomonas avonlea sp. nov.

We describe a new species of cryptomonad, Goniomonas avonlea sp. nov., using molecular phylogeny and comprehensive microscopic investigation. G. avonlea is a marine bacterivorous flagellate, measuring 8-11 μm long and 6-7 μm wide, with two subequal flagella that are directed anteriorly and posteriorly. G. avonlea is morphologically and genetically distinct from three other Goniomonas species that have been described to date. SEM and TEM show that G. avonlea shares ultrastructural features with other Goniomonas and cryptomonads, including the presence of bipartite ejectisomes, double septa in the transition region, flat mitochondrial cristae, a furrow complex, a rhizostyle, rectangular periplast plates, and the infundibulum. The discharged large ejectisome is straight and has a unique loose, reticulate layer. The flagellar apparatus includes non-tubular roots, microtubular roots, and a compound root that is reminiscent of the multilayered structure (MLS) observed in the flagellate cells of streptophytes and a few other eukaryotes. Molecular phylogenies based on 18S and 28S rRNA genes suggest a specific affiliation of G. avonlea to marine Goniomonas species, and support the monophyly of Goniomonas to the exclusion of plastid-bearing cryptomonads. Our study adds to a growing body of evidence for the high level of diversity and antiquity of the genus Goniomonas.

Floral elaiophore structure in four representatives of the Ornithocephalus clade (Orchidaceae: Oncidiinae)

A significant number of species assigned to the Neotropical orchid sub-tribe Oncidiinae reward insect pollinators with oil produced in floral glands termed elaiophores. The latter may be glabrous (epithelial elaiophores) or hirsute (trichomal elaiophores). Although the detailed anatomy and ultrastructure of epithelial elaiophores have been studied for a number of genera, such as Oncidium Sw., Gomesa R. Br. and Trichocentrum Poepp. & Endl., hitherto, trichomal elaiophores have been investigated only for a single species of Oncidiinae, Ornithocephalus ciliatus Lindl. Furthermore, this is the only representative of the Ornithocephalus clade to be investigated to date. Here, an examination is made of the elaiophore anatomy and ultrastructure of a further four species currently assigned to this clade (Ornithocephalus gladiatus Hook., Phymatidium falcifolium Lindl., Zygostates grandiflora (Lindl.) Mansf. and Zygostates lunata Lindl.) and the results compared with those obtained for other Oncidiinae. Elaiophore structure was examined for all species at three stages of flower development: closed bud, first day of anthesis and final stage of anthesis, using light microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry. Elaiophores of O. gladiatus occur upon the lateral lobes of the labellum and display characters intermediate between those of typical epithelial and trichomal elaiophores, in that they are largely glabrous, consisting mainly of cuboidal epidermal cells, but bear short, unicellular hairs proximally. By contrast, the elaiophores of all the other species investigated occur on the callus and are of the trichomal type. In P. falcifolium, these unicellular hairs are capitate. In all species, oil secretion commenced at the closed floral bud stage. Ultrastructurally, the mainly trichomal elaiophores of the four representatives of the Ornithocephalus clade closely resembled the epithelial elaiophores of other Oncidiinae, in that their cells displayed an organelle complement typical of lipid-secreting cells. However, in some taxa, a number of noteworthy characters were present. For example, the elaiophore cuticle of O. gladiatus and P. falcifolium was bi-layered, the outer layer being lamellate, the inner reticulate. The cuticle of Z. grandiflora and Z. lunata was also lamellate, but here, a reticulate layer was absent. Accumulation of secreted oil resulted in the localized distension of the cuticle. Cuticular cracks and pores, however, were absent from all species. The walls of the secretory cells of Z. grandiflora were also atypical in that they had short protuberances or ingrowths, and contained cavities which are thought to be involved in the secretory process. Of the species investigated, most displayed similar anatomical organization, their trichomal elaiophores occurring on the labellar callus. They, thus, differ from many other members of the Oncidiinae, where epithelial elaiophores are found either on the callus, or on the lateral lobes of the labellum. However, ultrastructurally, all elaiophores, whether those of representatives of the Ornithocephalus clade, or those of other oil-secreting Oncidiinae, possessed a similar complement of organelles, regardless of whether the elaiophores were trichomal or epithelial. In view of the latter, and the similar chemical composition of oils derived from all Oncidiinae investigated to date, it is probable that position and type of elaiophore, and possibly the structure of the overlying cuticle, play an important role in pollinator selection in these oil-secreting orchids.

Atomic Force Microscopy of the Intervessel Pit Membrane in the Stem of Sapium Sebiferum (Euphorbiaceae)

Sapwood and juvenile wood of Sapium sebiferum (Euphorbiaceae) was collected during 2000 –2002. In air-dried vessel elements, the surface of pit membranes (PMs) in the outermost growth ring was coated with plaque-like or interstitial material that was 2–5 nm thick. This coating was phase dark and overlaid a phase bright layer of globules and reticulately arranged microfibrils (MFs) that was 25–50 nm thick. Beneath the reticulate layer there was another surface exposed during sectioning/fracturing. It had parallel MFs which appeared to be continuous with the middle lamella, and were also coated. The total thickness of the dried PM appeared to be in the range of 50–100 nm. Overwintering and heartwood PMs were encrusted with a non-microfibrillar layer that differed from the above mentioned coating. Prior to chemical treatment, specific dried, untreated PMs were located and then the sample was dismounted, treated with acidic H2O2, and observed after treatment so that before and after images could be compared. Treatment with acidic H2O2 removed some of the coating and greatly modified the fibrillar nature of the surface layer, but did not reduce its overall thickness. The native structure of sapwood PMs was observed in water. Non-dried PMs displayed two layers, each with a different type of surface. The outer layer was non-microfibrillar and covered the entire surface of the PM. The non-microfibrillar layer was extremely sensitive to mechanical perturbation by the AFM tip, and had phase characteristics similar to the coating of dried PMs. The underlying layer was thick and microfibrillar. The MFs in non-dried PMs were, like the dried MFs, phase bright but they were much more loosely intermeshed compared with those seen in dried materials. The measurable thickness (which does not represent the total thickness) of non-dried PMs frequently ranged from 90–225 nm, although a few 500 nm vertical features were measured.

Immunoreactivity for β/A4 protein, but not for its precursor, in human chromaffin cells

The present study was designed to establish a) whether chromaffin cells of the human adrenal medulla express immunoreactivity for β-amyloid precursor protein (βAPP) and/or β-amyloid protein (β/A4); and b) whether cells expressing one or both of the above-mentioned proteins display immunoreactivity for the low- (gp75) and/or the high-affinity (gp140- trkA) nerve growth factor receptor. To identify chromaffin cells and their supporting cells, chromogranin A, neurofilament proteins, and S-100 protein were studied in parallel. βAPP and β/A4 immunoreactivity (IR) was observed primarily labeling two different cell populations, without colocalization: βAPP IR was found in the adrenal cortical cells, which were mainly localized in the reticulate layer and in the blood vessel walls, whereas β/A4 IR was observed in the chromeffin cells. Furthermore, supporting cells were also immunoreactive for β/A4, and sympathetic gangionic cells were immunoreactive for both βAPP and β/A4. Interestingly, clusters of cells expressing β/A4 IR also displayed gp75 IR and/or gp140- trkA IR. Finally, all chromaffin cells (identified by chromogranin A IR) were immunolsbeled for the 200 kDa neurofilament subunit, but not for a phosphorylated epitope of this protein. These results demonstrate the occurrence of β/A4 IR, but not of βAPP, in the chromaffin cells of the human adrenal gland. The complementary distribution of amyloid-related proteins, and the possible involvement of neurotrophins in β/A4 metabolism are discussed.

Effects of ozone and acidic fog on red spruce needle epicuticular wax production, chemical composition, cuticular membrane ultrastructure and needle wettability.

One-year-old red spruce (Picea rubens Sarg.) seedlings were exposed, from bud break, to ozone (O3 ) and acidic fog for 14, 42 or 77 days. Ozone was more damaging than acidic fog to epicuticular wax. Wax quantity on needles exposed to charcoal-filtered air (CFA) and pH 4.2 fog (control) increased from 14 to 42 days, but decreased by 77 days. Exposure of elongating needles to 70 ppb O3 and pH 4.2 fog reduced the rate of de novo wax synthesis. Red spruce wax is composed of secondary alcohols (50%), nonacosane diols (25%), alkyl esters (12%), primary alcohols (6 %), estolides (2 %), fatty acids (3 %), and hydroxy-fatty acids (2 %). Wax chemical composition varied temporally. Needles exposed to O3 had significantly less secondary alcohols, diols, alkyl esters, fatty acids, and hydroxy-fatty acids than needles exposed to CFA. Production of secondary alcohols was significantly reduced following needle exposure to fog at pH 3.0. Cuticular membrane thickness increased significantly following needle exposure to O3 . The increase in thickness at 250 ppb O3 was due to a new amorphous layer which appeared above the reticulate layer. Contact angles on needles exposed to CFA and pH 4.2 fog increased from 92° after 14 days to 96° after 42 days, then declined to 77° after 77 days. Needle exposure to O3 for 42 or 77 days significantly decreased contact angles.

SPERMATOPHORE FORMATION IN THE WHITE SHRIMPS PENAEUS SETIFERUS AND P. VANNAMEI

The medial and distal vasa deferentia and terminal ampullae of Penaeus setiferus and P. vannamei were studied by light and electron microscopy to assess their roles in spermatophore formation. The ascending medial vas deferens of each species consisted of 2 parallel epitheliumlined ducts, referred to as the spermatophoric and accessory ducts, with the accessory duct fitting into a groove along the spermatophoric duct. In the spermatophoric duct, the sperm mass was surrounded by a thick primary spermatophore layer secreted by the epithelial lining. Two secretions forming accessory layers 1 and 2 were deposited by the epithelial cells of the accessory duct. The lumina of the two ducts partially merged at the flexure between the ascending and descending portions of the medial vas deferens. Upon confluence, accessory layer 1 flowed into the spermatophoric duct and formed an additional layer around the primary spermatophore layer. Additional spermatophore components were deposited in the terminal ampulla, which consisted of 5 interconnecting chambers or lumina in P. setiferus and 4 in P. vannamei, respectively. The spermatophoric and accessory ducts terminated in chambers I and II, respectively. New secretions from chamber I were a thick dorsal plate and a thin adhesive layer. In chamber II, structural alteration of accessory layer 2 into corky reticulate and collapsed or fibrous reticulate portions occurred. Chamber III was a branching duct and contained glutinous material. Chamber IV seemed to be an extension of chamber II, but formed a distinct large lumen located in the proximal to medial regions of the terminal ampulla. This chamber contained a large amount offibrous reticulate substance similar to that observed in chamber II. Chamber V of P. setiferus contained the wing portion of the spermatophore. The terminal ampulla of P. vannamei possessed neither this chamber nor wing material. Upon ejaculation, each spermatophore joined mesially along the adhesive layer and formed a compound spermatophore. Accessory layer 1 and the corky reticulate layer were hard and functioned as a sheath for the sperm mass. These layers also supported the structure of the compound spermatophore. The dorsal plate, glutinous material, fibrous reticulate layer, and wing served to attach the compound spermatophore to the open thelycum.

PHENOTYPIC PLASTICITY IN SCENEDESMUS COMMUNIS (CHLOROPHYCEAE). I. RELATIONSHIP OF S. COMMUNIS TO S. KOMAREKII1

ABSTRACTWhen scenedesmus communis Hegew. (UTEX 76) was transferred daily in dilute media and a low cell density was maintained (ca. 1000 cells · mL−1), up to 30% unicells were produced in that population. Unlike previously described uncell‐coenobium‐unicell transformation with other species, these unicells never produced S. communis coenobia (large coenobium type, LCT) but rather small coenobium type (SCT) resembling S. komarekii Hegew. Growth and morphological development of the paratype strain of S. komarekii (UTEX 1236) was compared with an isolated SCT strain (SCT 76–8). SCT 76–8 never produced LCTs and grew significantly faster than UTEX 1236. Both SCT 76–8 and UTEX 1236 produced uncells at low cell densities. Coenobia formed when cell densities increased over time in batch cultures. SCT 76–8 and UTEX 1236 did not differ morphologically when viewed with the light microscope. Under scanning electron microscopy, an outer opaque layer covered an inner warty layer on unicells. The outer layer was reduced or absent in coenobia from batch cultures in stationary growth. In addition, long spikelets, not present on the walls of unicells, were prominent on coenobial walls. The spikelets of UTEX 1236 appeared smaller and more uniformly distributed than in strain 76–8. In contrast, the surface wall morphology of LCT S. communis was composed of an outer reticulate layer supported by spikelets and appeared as a pentagonal meshwork covering the cell walls. This phenotypic plasticity, as demonstrated by SEM and light microscopy, provides further evidence needed for an understanding of Scenedesmus evolution.

THE ULTRASTRUCTURE OFSCENEDESMUS(CHLOROPHYCEAE). II. CELL DIVISION AND COLONY FORMATION1

SUMMARYCell division in Scenedesmus is fairly typical of other chlorococcalean genera. The closed spindle has centrioles at polar fenestrae and apparently a series of nuclear divisions precedes cytokinesis. The phycoplast system of cytokinetic microtubules predicts the path of cleavage furrows whose mode of formation is obscure. Before and during cell division, the endoplasmic reticulum invariably accumulates granular material which later, during cytokinesis, appears to he secreted via the golgi bodies. Similar dense granular material then at accumulates outside the forming daughter cells but inside the parental wall, as the latter begins eroding away. By the end of colony formation, the cellulosic parental wall has disappeared, leaving its outer sheath and attached ornamentative features (spines, combs, reticulate or warty layer, etc.) intact as a “ghost.”The spines and combs of new colonies appear to condense out of the extracellular aggregate; their precise mode of formation is obscure. As they form, the daughter cells, having become rearranged within the parental wall, stick to one another apparently at specific sites on their outer surface. A trilaminar (sporopollenin‐containing) layer arises first in each cell at these adhesive sites and immediately afterwards, dense material aggregates between the adjacent layers to give rise to the coenobial adhesive. Plaques of the trilaminar layer later appear over the rest of the cell's surface; they grow and fuse so that eventually each cell is enclosed by one continuous Trilaminar Sheath (TLS). While the plaques are forming, another dense layer materializes around the whole coenobium. Depending on the species, this layer turns into either the warty layer, in which instance it is applied directly on to the surface of the TLS except near the coenobial adhesive, or else it becomes the reticulate layer, in which instance it remains entirely separate from the TLS, soon acquiring the complex system of propping spikelets which suspend it from the coenobial surface.When fully farmed, the daughter coenobium is tightly compressed within the parental TLS, with its spines folded lengthwise along the daughter cells. Release of the colony follows a quite explosive rupturing of the parental TLS, and immediately upon release, the daughter colony flattens out and erects its spines.

THE ULTRASTRUCTURE OFSCENEDESMUS(CHLOROPHYCEAE). I. SPECIES WITH THE “RETICULATE” OR “WARTY” TYPE OF ORNAMENTAL LAYER1

SUMMARYThe ultrastructure of the wall layers and ornamentative features ofScenedesmus pannonicusandS. longusare described using carefully correlated freeze‐etched replicas, thin sections and scanning electron micrographs. The two species arc enclosed by different types of ornamented layers,S. pannonicusby the tightly filling, “warty” layer andS. longusby the loosely fitting, “reticulate” layer, held off the coenobium by 2 types of tubular propping spikelets and rosettes. The reticulate layer has an intricate substructure, especially when studied with freeze‐etching. Its inner and outer surfaces appear different, as is its attachment to the 2 types of spikelets. Whole cells ofS. longussubjected to acetolysis lack the cellulose wall and cytoplasm, but all other surface features survive, including the Trilaminar Sheath (TLS); this ornamentation cannot be “pectic.” The cellulose wall and ornamentation is unaffected by boiling water alone. Boiling in 6nNaOH removes the surface ornamentation, but the TLS and wall remain; the possibility that these features contain silica is discussed.The terminal spines of both species consist of closely packed spikelets enclosed within a skin of hexagonally‐packed subunits. Similar subunits are seen in the propping spikelets of S. longus, and in the rows or “combs” of laterally fused spikelets ofS. pannonicus.The warty layer ofS. pannonicusis tightly appressed to the TLS except close to where the cells are joined, where it is suspended free. It is composed of a layer of globular subunits, and small indentations form the warts. Single, evenly distributed warts characterize the freely suspended sections of the warty layer, and the layer that encloses young coenobia soon after they have been formed: in contrast, the warts are clumped over the surface of older and larger colonies. Some of the single warts form characteristic double rows, but these latter remain single even on older cells. The surface structure of the warty layer, TLS, and plasmalemma are revealed by the freeze‐etching process.