The Structure of Normal and High-Lysine Barley Grains

The structure of the starchy endosperm of normal and high-lysinebarleys has been examined by light and scanning electron microscopy.The small granules (1-5 µm) seen using the latter methodhave been demonstrated to be small starch granules rather thanprotein bodies. Certain of the high-lysine mutations (Risø1508, Notch-2, lys 449 and lys 95) cause dramatic shrinkageof the endosperm structure whilst others (Risø 7 and56) cause little change. In particular Risø 1508 causesa decrease in small starch grains whereas those grains presentin Notch-2 are all much smaller than normal. The endosperm oflys 449 shows considerable disruption of the cells.     

Studies on Mature Seeds of Cuscuta pedicellata and C. campestris by Electron Microscopy

The seeds of Cuscuta pedicellata have been investigated by transmissionand scanning electron microscopy. Additional observations havebeen made on seeds of C. campestris by SEM only. The seed coatconsists of an outer single epidermis, two different palisadelayers, and an inner multiparenchyma layer. The outer epidermalwall in C. pedicellata has a thick cuticle and zones rich inpectic substances. The thicker ‘U-shaped’ cell wallsin the outer palisade layer are strengthened by a wall layerof hemicellulose. The inner palisade layer has thick walledcells with a ‘light line’. The inner cell wall ofthe compressed multiparenchyma layer has a thin cuticle. A fairlythick cuticle is positioned directly on the endosperm surface.The aleurone cell walls are different from the remaining endospermwalls. The latter are thick and believed to be of galactomannans.There is a ‘clear’ zone between the plasmalemmaand the cell wall in the aleurone cells. The embryo cells arepacked with lipids and proteins. In Cuscuta campestris mostendosperm has been absorbed during the seed development. Theembryo apex has two minute leaf primordia. The features of theCuscuta seeds are discussed in relation to functional and environmentalconditions. Cuscuta pedicellata, Cuscuta campestris, seed, seed coat, cuticle, cell walls, endosperm, aleurone cells, galactomannan, embryo, TEM, SEM     

Nodulation of Oilseed Rape (Brassica napus) by Rhizobia

Nodules were induced on the non-legume oilseed rape, followingenzyme treatment of seedling roots and inoculation with Rhizobiumlegnminosarum, Bradyrhizobtum 32H1 or a mixture of R. lott withBradyrhizobium 32H1 in the presence of PEG. A Nod– strainof R leguminosarum also induced nodules, but a Nod– strainfailed to elicit this response Nodules induced on oilseed rapewere morphologically similar, when examined by light microscopyand cryo-scanning electron microscopy, to those induced on rootsof white clover by R trifolu. Transmission electron microscopyshowed rhizobia within cells of the nodules These observationsare discussed with respect to the extension of Rhizobium symbiosisto non-legumes. Key words: Brassica napus, Bradyrhizobium, Rhizobium, non-legumes, nodulation, transmission electron microscopy, cryo-scanning electron microscopy, acetylene reduction, cell wall degrading enzymes     

Polyphosphate metabolism in the blue-green alga Microcystis aeru-ginosa

The uptake of inorganic phosphorus was studied in an axenicstrain of phosphorus-starved cells of the blue-green alga Microcystisaeruginosa, an organism often causing blooms in freshwater bodies.Rates of growth and of cellular polyphosphate content as a functionof initial orthophosphate in the medium indicate the operationof the ‘phosphorus overplus’ phenomenon in M. aeruginosa,accompanied by formation of volutin granules. The granules wereisolated by a non-aqueous centrifugation method, and identifiedas polyphosphate bodies.     

Effect of High Temperature During Grain-filling on the Structure of Developing and Malted Barley Grains

High temperatures (up to 35 °C) were applied to plants ofmalting barley,Hordeum vulgareL. (‘Schooner’) fora period of 5 d during grain-filling. Heat treatment had a profoundeffect on the structure of the mature barley grain. There wasevidence of degradation of endosperm storage products in heat-treatedgrain. Starch granule development was reduced in sub-aleuronecells following heat treatment and alterations to starch granuledistribution and growth were observed in the endosperms of thesegrains. Endosperm cell wall and crushed cell layer (CCL) developmentwere sensitive to high temperatures, with the reduced thicknessof the CCL and generally patchy Calcofluor fluorescence of endospermcell walls indicative of partial hydrolysis of ß-glucans.Increased growth of the embryo took place in heat-treated grainscompared with control grains. Endosperm texture was generallymore friable in heat-treated grains than in control grains,and these grains overmodified during malting, with considerabledegradation of starch in the form of extensive pitting of A-typestarch granules. Evidence is presented for developmental andgermination events occurring simultaneously within the developinggrain.Copyright 1998 Annals of Botany Company Barley,Hordeum vulgareL., starch granules, crushed cell layer, scutellum, embryo, fluorescence microscopy, scanning electron microscopy, confocal microscopy, malting quality.     

Physiological Color Changes in Reptiles

SYNOPSIS. The physiological regulation of color changes in reptilesas studied in the lizard, Anolis carolinensis, is discussed.In Anolis, the ability to adapt to a background is dependentupon the level of circulating MSH, therelease of which is dependenton information received through the eyes. Blinded (or intact)lizards are brown under conditions of strong illumination andgreen under conditions of lower light intensities, and, again,these color changes are regulated by MSH. According to Kleinholz,color changes in the blinded lizard are regulated by dermalphotoreceptors. High or low temperatures directly affect thecolor of Anolis skins and alter the rate at which skins respondto hormones. Aggregationof melanin granules within Anolis melanophoresin response to sympathomimetic stimulation is regulated throughalpha adrenergic receptors whereas dispersion of melanin granulesin response to such stimulation is controlled through beta adrenergicreceptorspossessed by the melanophores. Most Anolis melanophores possessboth alpha and beta adrenergic receptors, but some melanophorespossess only beta adrenergic receptors. In the normal physiologyof the lizard, under conditions of stress, stimulation of alphaadrenergic receptors by catecholamines leads to an "excitement—pallor"followedby an "excitement—darkening" resulting from stimulationof beta adrenergic receptors which causes dispersion of melaningranules within localized populations of melanophores. Thus,in Anolis, dispersion of melanin granules within melanophoresis regulated by both MSH and by catecholamines. Evidence ispresented that the intracellular level of cyclic 3', 5'-AMPwithin melanophores may be responsible for the regulation ofmovement of melanin granules.     

Arrays of Granules Associated with the Plasmalemma in Swarmers of Cladophora

The surface region of swarmers of the green alga, Cladophorarupestris, has been studied in both chemically fixed and freeze-etchedmaterial. Swarmers prepared by each of these methods show arraysof granules outside the plasmalemma, covered by a layer of fibrousmaterial of unknown composition which completely surrounds thecell. In the case of freeze-etched swarmers, the granules areformed into a lattice closely similar to that predicted earlierin the ‘ordered granule hypothesis’ of cellulosebiosynthesis. No microfibrils have yet been observed in associationwith the granules, but their location in the region of wallformation, at a time when this process is in its early stages,lends support to the possibility that the granules may be theorganelles responsible for the production and orientation ofthe cellulose microfibrillar component of the cell wall. Noevidence was observed during this work to suggest that any otherorganelle is responsible for cellulose formation.     

The Establishments of Tissue 4

Callus cultures of 12 temperate grasses were established, somefor the first time, by incubating detached roots or whole seedlingson a Linsmaier and Skoog basal medium which contained 2, 4-dichlorophenoxyaceticacid (2, 4-D) at 1.0 mg 1^–1 as the only growth hormone.The callus, which developed in the pericycle of the roots andon the embryo of the seeds, was subcultured on the same medium;further growth of the callus varied from good in the case ofDactylis glomeraia, Agrostis tennis, Cynosurus cristatus, andPoa trivialis, to poor in several Lolium species and varieties.Most cultures developed root primordia which sometimes grewinto visible roots, but shoot primordia, none of which grewinto shoots, were found only in the callus of Lolium multiflorumvar. westerwoldicum. Cell suspension cultures were also readily established and maintainedusing the same culture medium. Most cultures contained a highproportion of round or oval cells, which ranged from 18 to 77µm in diameter or length, while many also had a significantproportion of larger, more elongated cells which varied in lengthfrom 46 to 182 µm. The cells of Dactylis glomerata werecharacteristically larger and more convoluted than the cellsof other grass species that were examined. The addition of kinetinat 0.1 mg 1^–1 to the 2, 4-D-containing culture mediumincreased the proportion of irregular-shaped cells and reducedthe dispersion of the cells, perhaps by improving cellular contactand adhesion; in some species, such as Agrostis tenuis and Phleumpratense, the presence of kinetin promoted the deposition ofstarch granules in cells.     

Variations in the Lengths of Fusiform Cambial Cells and Vessel Elements in Kalopanax pictus

Samples of a mature specimen of Kalopanax pictus, a ring-poroushardwood, were studied to compare the respective lengths offusiform cambial cells and vessel elements in the stem. Thelengths of dormant and reactivated fusiform cambial cells weremeasured with a confocal laser scanning microscope in tissuethat had been macerated by digestion with pectinase and in thicktangential sections. The lengths of early wood and late woodvessel elements were measured in tissues that had been maceratedby Franklin's method. The vessel elements and fusiform cambialcells varied considerably in length within individual samples.The mean length of early wood vessel elements corresponded tothat of fusiform cells in the dormant cambium but not in thereactivated cambium. Significant differences were observed betweenthe mean lengths of dormant and reactivated fusiform cambialcells, between those of reactivated fusiform cambial cells andearly wood vessel elements, between those of reactivated fusiformcambial cells and late wood vessel elements, and between thoseof early wood and late wood vessel elements. The frequency distributionsof lengths of cambial cells were bimodal and differed from thoseof vessel elements, which more closely resembled a normal distribution.The proportion of shorter lengths was higher in the reactivatedcambium than in the dormant cambium, the early wood and thelate wood vessel elements. Our results do not suppot the hypothesisthat the lengths of early wood vessel elements in ring-poroushardwoods change during differentiation. The similar rangesof recorded lengths suggest that short and long vessel elementsmight be derived directly from short and long cambial cells,respectively. Copyright 1999 Annals of Botany Company Kalopanax pictus, cambium, vessel element, confocal laser scanning microscopy, maceration, cell length.     

Structural analysis of the digestive gland of the queen conch Strombus gigas Linnaeus, 1758 and its intracellular parasites

This study describes the structure of the digestive gland ofStrombus gigas in individuals from Guadeloupe and discussesthe function of its cell types and their relationship with intracellularApicomplexa-like parasites. Three cellular types were foundin the epithelium of the blind-ending tubules of the digestivegland according to histological and transmission electron microscopy(TEM) observations; these were: digestive cells, pyramidal cryptcells and vacuolated cells. Columnar digestive cells were characterizedby large Alcian blue-positive granules, which have not beenpreviously described in digestive cells of other caenogastropods.Such granules contain large quantities of proteoglycans thatare exported to the stomach through the physiological destructionof the digestive cells, which undergo a holocrine secretion.Their cytoplasm appears vacuolar due to lipid extraction bysolvents used for tissue preparation. Vacuolated cells alsoappear to be lipid-storage cells. Small triangular-shaped cryptcells, on the other hand, appear to be metabolically activeas suggested by a strong positive in situ hybridization of eukaryoticribosomes, which was confirmed by their large content of ribosomesand rough endoplasmic reticulum compared to the other cell types.These observations suggest that crypt cells may be immaturecells that are involved in the replacement of eliminated digestivecells. However, their spherocrystal inclusions indicate thatthey may be excretory cells or calcium cells. Large brown inclusionswere frequently observed in vacuolated cells; these were identifiedas parasitic protozoans and were present in the digestive glandof all sampled specimens. These protozoans have previously beendescribed from a queen conch population in the San Andres Archipelago(Colombia). Several life cycle stages of the parasite were identifiedby scanning electron microscopy and TEM; trophozoites were characterizedby their conoid-like structure, sporocysts by their thick walls,and gamonts by their thin walls. These observations suggestthat this parasite completes its entire life cycle within thesame host and type of tissue. Although previous investigationsplace this parasite within the Apicomplexa group, further investigationsare necessary in order to confirm the identification of theparasite. (Received 13 May 2008; accepted 3 October 2008)     

Further Observations on Root Contraction

Further observations, by light and electron microscopy, on theanatomy of root contraction in Hyacinthus orientalis cvs. arereported, together with some comparisons with Narcissus andGladiolus. It is maintained that root contraction in these andsome other species is essentially a growth process, as previousinvestigators have shown, and that an understanding of the re-directedgrowth of the cortical cells which brings it about may be basedon the specialized structural features of their cell walls,here described. Physical, anatomical and comparative reasonsare given for our belief that the alternative explanation ofthe contractile process in Gladiolus which relates it to cellcollapse resulting from transpirational ‘pull’ isuntenable. anatomy, contraction, Hyacinthus, Narcissus, Gladiolus, root     

Floral Development of Three Maesa Species, with Special Emphasis on the Position of the Genus within Primulales

Floral development of three Maesa Forssk. species, M. argenteaWall., M. japonica(Thunb.) Moritzi ex Zoll. and M. perlarius(Lour.)Merr., was studied with a scanning electron microscope. Thedistinctive floral features of the genus, such as presence oftwo persistent bracteoles, perigynous flowers and many-seededfruits, have been used to discuss the systematic position ofthe genus within Primulales. In addition, the development ofcommon stamen-petal primordia, the number, arrangement and positionof ovules and the shape of the anthers provide evidence in favourof placing Maesa in a separate family, Maesaceae, as suggestedby Anderberg and Ståhl (Canadian Journal of Botany, 73:1699–1730, 1995). Copyright 2000 Annals of Botany Company Primulales, Maesa argentea Wall., Maesa japonica(Thunb.) Moritzi ex Zoll., Maesa perlarius(Lour.) Merr., floral ontogeny, SEM     

Regeneration of the Root Cap of Zea mays L. and Pisum sativum L.: A Study with the Scanning Electron Microscope

Removal of the root cap from a root apex initiates regenerationof a new cap. The process has been followed using scanning electronmicroscopy. Quantitative data have been obtained for the growthin area of the exposed acroscopic surface of the quiescent centre(QC) and the increase in volume of the regenerating cap tissue.In Zea the surface of the QC shows an initial rapid increasein area followed by a slower increase. In Pisum the surfacearea increases uniformly, a rapid initial phase being absent.Together with observations on the behaviour of an incision atthe exposed surface, the results indicate that in Zea the capnormally imposes a constraint upon radial growth at the acroscopicsurface of the QC; in Pisum the QC appears not to be so constrained.The different responses may be related to the different arrangementsof cells at the apex of the meristem of these two species. Zea mays, Pisum sativum, maize, peao, scanning electron microscopy, root apex, regeneration     

Stomata and Structure of Tetraploid Apple Leaves cultured in Vitro

Leaves of anther-derived tetraploid apple (Malus pumila Mill.)shoots were examined by low-temperature scanning electron microscopy(LT-SEM). Leaves were serrate and wide with an undulating adaxialsurface due to convex epidermal cells, apparently without crystallineepicuticular wax. Stomata were absent from the adaxial surface,except for the marginal teeth which exhibited 40-60 stomataper leaf; they probably originated from residual mitotic activity.One third of abaxial stomata was occluded by the residual cuticleof the mother guard cell across the stomatal pore which rupturedwhen the stomata became functional. The stomatal index was 7·2(± 1·6) with 60-75 stomata mm^-2, i.e. abaxialstomata of tetraploid leaves expanded in vitro were less frequentthan those in triploid leaves either cultured in vitro (475-575stomata mm^-2) or grown on the tree (320-390 stomata mm^-2) wherethe stomatal index was 21 (± 4). Freeze-fracture transsectionsshowed that the tetraploid in vitro leaves were composed ofa layer of adaxial epidermal cells, followed by a single layerof palisade cells and four to five layers of spongy mesophyllcells and the abaxial layer of epidermal cells, in contrastto juvenile seedling-grown apple leaves in which the two layersof palisade cells comprised the majority (52-60%) of the leafvolume. The same morphological features, such as wide and lesspointed leaves, reduced stomatal density and stomatal index,and increased stomatal size that were previously reported fortree-grown tetraploid leaves were also expressed in vitro. Thus,causes of the stomatal deformation in tissue-cultured Rosaceaeare interpreted to be in part genetic and not purely environmental.Copyright1994, 1999 Academic Press Malus pumila Mill., apple, biotechnology, breeding, cryo-preservation, CO₂, juvenile, low temperature-scanning electron microscopy (LT-SEM), micropropagation, ploidy, stomata, tissue-culture, transpiration     

Intracellular localization of capsaicin and its analogues, capsaicinoid, in Capsicum fruit 1. Microscopic investigation of the structure of the placenta of Capsicum annuum var. annuum cv. Karayatsubusa

Transverse sections of the placenta of hot pepper, Capsicumannuum var. annuum cv. Karayatsubusa, at different stages afterflowering were examined microscopically. Examination of thecellular structure of the placenta using a light microscoperevealed that some morphological changes took place mainly inthe epidermal tissue of the placenta during maturation. Elongationof the epidermal cells and many osmiophilic granules were recognizedin the epidermal cells of the placenta in which capsaicinoidwas being formed and actively accumulated. Moreover, a granule-likestructure having an absorption at 280 nm was also recognizedby ultraviolet microscopy around the same region where the osmiophilicgranules were observed. By electron microscopy, many electron-densegranules stained with glutaraldehyde and osmium tetroxide wereobserved both in the small vesicles and vacuoles of epidermalcells of the placenta. The electron-dense granules varied insize from smaller than 1 µm to larger than 2 µmin diameter. They were thought to be capsaicinoid and were observedonly in the epidermal cells. Therefore, the epidermal tissueappeared to be the site of capsaicinoid accumulation. ¹Formation and metabolism of pungent principle of Capsicum fruits.Part V. (Received December 27, 1979; )     

HowStriga Parasitizes its Host: a TEM and SEM Study

The cellular contact betweenStriga hermonthica andStriga asiaticaand their hosts,Zea mays andSorghum bicolor , was investigatedby light, transmission electron and scanning electron microscopy.The xylem connections between parasites and hosts involve veryspecific, clustered intrusions into the host's water conductingelements, predominantly into the large vessel elements. A singlehaustorial cell can penetrate a host vessel element with morethan one intrusion. All intrusions become covered by an additionalelectron-opaque wall layer. During subsequent differentiation,a dissolution of specific wall parts of the cell intrusionsoccurs so that open, cup- or trunk-like structures result. Thevessel-like host contact can comprise up to five openings withina single intrusion. Concomitantly, the intrusions and the haustorialcells to which they belong lose their protoplasts and transforminto elements which take up water. The walls of the haustorialcells and both wall parts of their appendages become stronglylignified. The water and nutrient absorbing structures insertedinto the host vessel are named ‘oscula’. Withinthe whole haustorial complex of bothStriga species no phloemelements were detected. Translocation of substances from hostto parasite are briefly discussed. Striga hermonthica ; Striga asiatica ; haustorial anatomy; xylem contact; osculum     

Natural Peptide antibiotics from tunicates: structures, functions and potential uses

Because tunicates rely on innate immunity, their hemocytes areimportant contributors to host defense. Styela clava, a solitaryascidian, have eight hemocyte subtypes. Extracts of their totalhemocyte population contained multiple small (2–4 kDa)antimicrobial peptides. When purified, these fell into two distinctfamilies that were named styelins and clavanins. Styelins A-E are phenylalanine-rich, 32 residue peptides withactivity against marine bacteria and human pathogens. They showconsiderable sequence homology to pleurocidins, antimicrobialpeptides of the flounder, Pseudopleuronectes americanus. StyelinD, one of the five styelins identified by peptide isolationand cDNA cloning, was remarkable in containing 12 post-translationallymodified residues, including a 6-bromotryptophan, two monohydroxylysines,four 3,4-dihydroxyphenylalanines (DOPA), four dihydroxylysinesand one dihydroxyarginine. These modifications enhanced StyelinD's bactericidal ability at acidic pH and high salinity. A novelhistochemical stain for DOPA suggested that Styelin D was restrictedto granulocytes. Clavanins A-E are histidine-rich, 23 residue peptides that areC-terminally amidated and most effective at acidic pH. Clavaspirinis a newly described family member that also has potent cytotoxicproperties. By immunocytochemistry, clavanins were identifiedin the granules of five eosinophilic granulocyte subtypes andin macrophage cytoplasm. Transmission and scanning electron micrographs of methicillin-resistantStaphylococcus aureus (MRSA) and E. coli that had been treatedwith Styelin D and clavaspirin suggested that both peptidesinduced osmotic disregulation. Treated bacteria manifested cytoplasmicswelling and extrusion of cytoplasmic contents through theirpeptidoglycan cell wall. The diverse array of antimicrobialpeptides in S. clava hemocytes constitutes an effective hostdefense mechanism.     

Bicapitate Nitzschia species: abundant nanoplankton in aggregates during November-December (l992) in the equatorial Pacific

From 12°S to 9°N-140°W on the benthic cruise ofthe US Joint Global Ocean Flux Studies (JGOFS), horizontal near-surfacenet tows (n = 18) and vertical net hauls (n = 15, 200-0 m) yieldedhigh relative abundances of small, bicapitate, intact Nitzschiavalves (average 59.0 and 56.0% of total diatoms, respectively),although >90% of them were too small to have been caughteffectively with the net used. This complex was much less common(average 3.2% of diatoms) in the surface sediment samples (n= 7, 12°S-9°N, below water depths of 4269-4991 m). Examinationunder the light microscope showed concentrations of cells associatedwith organic detritus, planar and cylindrical membranous structures,fecal pellets, and occasionally on other diatoms (e.g. Asterolampra),i.e. on or within particles large enough to be collected inthe net. Although Nitzschio bicapitato Cleve has been consideredto have a characteristic range of shapes, and has often beencited from open-ocean habitats in nano- and microplanktonicstudies, a scanning electron microscopic study revealed severalspecies in our material. Examples of larger and smaller speciesare given, and Nitzschia ikeanae G.Fryx. & H.Lee, sp. nov.,is described here. Ecologically, these observations indicatethat although some are grazed and have been seen aggregatedin fecal pellets, these small pennate diatoms can affix to andgrow on substrates, termed ‘pseudo-benthic habitats’in the open ocean, resulting in unexpected concentrations thathave implications for their life histories and sexual cycles,selection or avoidance by grazers, sinking on substrates, androles in the recycling of nutrients in near-surface waters.     

Seasonal Production of Secondary Phloem in the Twigs of Certain Tropical Timber Trees

Secondary phloem production in four deciduous (Albizzia lebbeck,Dalbergia sissoo, Tectona grandis and Terminalia crenulata)and three evergreen plants (Calophyllum inophyllum, Mangiferaindica and Morinda tinctoria) is briefly described. The totalduration of phloem production for each year was worked out forall these plants. In three of the four deciduous trees therewere two instalments of phloem production in correspondencewith the presence of two flushes of cambial activity while inTectona grandis and in all the three evergreen trees there wasonly one instalment. The time of initiation and cessation ofphloem tissue production was found to be variable in the differentplants studied. Periodicity in the production of different componentsof phloem tissue as well as the difference in the dimensionsof the different phloic elements produced during each flushof cambial activity resulted in detectable growth increments(or ‘rings’) within the phloem. There was a distinctshortening of the different phloem elements during the approachof dormancy/least activity. Conspicuous changes were found inthe ergastic contents of phloem parenchyma and ray cells adjacentto the cambial zone during the initiation of cambial activityand during the approach of dormancy/least activity. Seasonal growth, secondary phloem, deciduous and evergreen trees, cambial activity     

Scanning Electron Microscopy of Silica Deposits in the Culms, Floral Bracts and Awns of Barley (Hordeum sativum Jess.)

Deposits of silica in the culm internodes, floral bracts andawns of Hordeum sativum Jess (cv Deba Abed) have been investigatedusing the scanning electron microscope The deposits were isolatedfrom all organic matter by digestion with nitric and perchloricacids Two basic types of deposits were recognized, lumen andwall silicification, the latter with or without lumen infillings In the culm internodes, lumen deposits are derived from idioblasts(‘hats’), sclerenchyma and xylem vessels In thefloral bracts they are derived from idioblasts (‘hats’and astenform opals), epidermal long cells (dendriform opals),sclerenchyma and xylem vessels The shape of these deposits doesnot generally resemble the outline of the cell itself, but depositsderived from cell walls do closely resemble the infact cell.In the culm, the walls of stomatal cells, trichomes and, largelythe outer tangential walls of mature long epidermal cells, becomesilicified, together with some cork cells In the floral bracts,silica is found in most epidermal cell walls but is confinedto the trichomes, scutiform cells and costal epidermal cellsearly in their development At maturity concentrations of silicaare much higher in the floral bracts and awns than in the culmsand leaves The results are discussed in relation to the pattern of depositionand its possible functions. Hordeum sativum Jess, barley, silica deposits, opals, scanning electron microscopy