Surface Growths Research Articles

Rough neutron fields and nuclear reactor noise.

Nuclear reactor cores achieve sustained fission chain reactions through the so-called "critical state"-a subtle equilibrium between their material properties and their geometries. Observed at macroscopic scales during operations, the resulting stationary neutron field is tainted by a noise term that hinders various fluctuations occurring at smaller scales. These fluctuations are either of a stochastic nature (whenever the core is operated at low power) or related to various perturbations and vibrations within the core, even operated in its power regime. For reasons that are only partially understood using linear noise theory, incidental events have been reported, characterized by an increase of the power noise. Such events of power noise growth, sometimes up to seemingly unbounded levels, have already led in the past to voluntary scramming of reactors. In this paper, we will use a statistical field theory of critical processes to model the effects of neutron power noise. We will show that the evolution of the neutron field in a reactor is intimately connected to the dynamic of surface growths given by the Kardar-Parisi-Zhang equation. Recent numerical results emerging from renormalization-group approaches will be used to calculate a threshold in the amplitude of the reactor noise above which the core enters a new criticality state, and to estimate the critical exponents characterizing this phase transition to rough neutron fields. The theoretical model of nonlinear noise built in this paper from ab initio statistical mechanics principles will be correlated and compared to data of misunderstood reactor noise levels and reactor instabilities and will be shown to provide both qualitative and quantitative insights into this long-standing issue of reactor physics.

Clinical and Pathologic Presentation of Primary Ocular Surface Tumors among Zambians

Aim: This study aimed to characterize the clinical and pathologic presentation of ocular surface tumors (OSTs) and to more precisely differentiate the grades of ocular surface squamous neoplasia (OSSN) and benign lesions among Zambians. Methods: Two-hundred sixty-five Zambian patients presenting with ocular surface growths, suspicious for OSSN, were recruited between November 2017 and November 2019 to a cross-sectional study to investigate their lesions. Sociodemographic data were collected, HIV infection status and vision tests were performed, and lesions were measured and documented. Lesions >2 mm in diameter were excised and sent for pathology analysis. In addition to the biopsies, tears, blood, and buccal swabs were collected. CD4+ T-cell counts were measured by flow cytometry. Lesions were classified according to the WHO guidelines. χ² and bivariate correlations were used to analyze variable associations and strengths with phi/Cramer’s V and correlation coefficients, respectively. Binary logistics was used to adjust for covariance. Results: In this study, 68.3% of the participants were found to be HIV positive. The most frequent diagnoses were invasive OSSN (45.3%), preinvasive OSSN (29.1%), and pterygium (22.6%). Invasive OSSN comprised keratinizing squamous cell carcinoma (SCC) (87.5%), basaloid SCC (3.3%), and spindle cell carcinoma (3.3%). Unusual carcinomas, not described previously, included hybrid SCC (5.0%) and acantholytic SCC (0.8%). Invasive OSSN had advanced tumor (T3/T4) staging (93.3%) at diagnosis. Lymphadenopathy was rare (2.3%), and metastasis was absent. Patients were mostly female (59.2%). Median age was 36 (interquartile ranges 33–41) years (ranges 18–81). Patients with invasive OSSN were more likely to present with pain (p = 0.007), redness (p = 0.034), excessive tearing (p = 0.0001), discharge (p = 0.011), bleeding (p = 0.007), reduced vision (p = 0.0001), fungating lesion (p = 0.001), and blindness (p = 0.005); location at temporal limbus (p = 0.0001), inferior limbus (p = 0.0001), or circumlimbal (p = 0.001); and extension to cornea (p = 0.006) and forniceal palpebral conjunctiva (p = 0.001). Invasive OSSN was associated with any smoking habit and alcohol consumption (p = 0.04 and 0.03, respectively). HIV positivity was strongly associated with OSSN (74.6% OSSN vs. 49.3% benign lesions; p = 0.0001; phi: 0.237 [p = 0.0001]). Conclusion: OSTs are very common in Zambia and are strongly associated with HIV coinfection. Patients with OSSN were more likely to be HIV positive than those with pterygia. Despite the commonality of OSTs in sub-Saharan Africa, these cancers have historically been poorly characterized.

Usage of masterplots in kinetic analysis of complex surface/volume crystallization processes in Se-Te glasses

Utilization of masterplots in the kinetic analysis of complex crystallization data was analyzed. The analysis was focused on the overlapping surface and volume-located crystal growth processes following the nucleation-growth kinetic concept. Particular attention was paid to the overlaps resulting in a pseudo-single-peak behavior. Comparatory method was introduced, based on the comparison of theoretically simulated and experimentally achieved masterplots; applicability of the method was confirmed for the selected Se-Te glasses. The comparatory analysis has universal usage for nucleation-growth crystallization processes with respect to the majority of kinetic parameters, but it is restricted only to cases where the “kinetically true” surface and volume crystal growths occur. To overcome this restriction, a new non-linear optimization method was introduced, based on the simultaneous optimization of the kinetic data-curve and its corresponding masterplot functions. Reliable physically meaningful description of any kinetic complexity can be achieved for overlaps of independent sub-processes with similar activation energy.

Current-Dependent Growth Mechanisms of Sodium Metal Anode in Diglyme-Based Electrolytes

Sodium-ion batteries are economical substitute for lithium-ion batteries due to the earth abundance of sodium element [1, 2]. Similar to lithium metal anodes, while sodium metal anodes hold the promise to improve the energy density of next-generation sodium-ion batteries, the nonuniform electrodeposition of sodium metal that can lead to internal shorts remains a major obstacle toward its practical applications [3]. Here, we systematically investigated the electrodeposition of sodium in diglyme-based electrolytes in our unique capillary cells and realistic split cells. While a similar transition of electrodeposition mode to dendritic growth at Sand’s time was observed for sodium [4], only the smooth surface growths were observed at under-limiting current densities, suggesting a possible much higher cycling efficiency due to much less surface areas to be covered by the solid electrolyte interphase (SEI) layers. By examining the limiting current and Sand’s time obtained from operando experiments with the capillary cells, the diffusion coefficient (1.13×10-6 cm2 s-1) and transference number (0.45) of sodium ions in 1M NaClO4 in diglyme were accurately extracted, which were further verified by post-mortem analyses of linear sweep voltammetry experiments with split cells. The unique two current plateaus, shown in Figure 1, surprisingly can be explained by a single limiting current density in a self-consistent manner. At such plateaus, deposits of sodium metal were discovered to grow along the surface, rather than through the pores, of the ceramic-coated separator and form equiaxed and columnar dendrites with orthogonal branches similar to that found in solidification [5].Our work provides a platform to not only evaluate the current-dependent growth mechanisms of sodium, but also reveal the roles of separators toward designing safe sodium metal batteries.

Monitoring changes in the impervious surfaces of urban functional zones using multisource remote sensing data: a case study of Tianjin, China

Previous research on urban impervious surface monitoring has ignored the differences, spatial patterns, and driving factors of impervious surfaces among the various functional zones in cities. In this paper, we extracted the impervious surface coverage of Tianjin in 1990, 1995, 2000, 2005, 2010, and 2015 based on the Classification and Regression Tree model. Then, using the methods of quantitative geography, we systematically analyzed the spatiotemporal variations of impervious surface coverage among the functional zones in Tianjin from 1990 to 2015. Additionally, the driving mechanisms of impervious surface growths among different functional zones were investigated. The results showed that (1) the growth of impervious surfaces shows two spatially separate core areas of growth. The core functional zone (CFZ) had the highest impervious surface coverage, and the ecological conservation zone (ECZ) had the lowest impervious surface coverage. (2) High-high aggregation regions of impervious surface coverage were mainly located in CFZ, while low-low aggregation regions of impervious surface coverage area were mainly located in ECZ. (3) Influences of social-economic factors on area growths of impervious surface are stronger in urban function expanded zone, coastal city key development zone, Beijing–Tianjin coordinated development zone, and urban-rural coordinated development zone than those in CFZ. Our results can provide a point of reference for decision-making regarding urban planning management.

Spatial Distribution of Abdominal Aortic Aneurysm Surface Expansion and Correlation With Maximum Diameter and Volume Growth

Abdominal aortic aneurysm (AAA) growth rate, measured as maximum diameter (Dmax) change over time, is used as a surrogate marker of rupture risk. However, AAA expansion presents significant spatial variability. We aim to record the spatial distribution of regional wall surface expansion. Thirty AAAs were retrospectively studied. Each AAA had one baseline and at least one follow-up computed tomography scan. Three-dimensional AAA models were reconstructed, and change in Dmax and total aneurysm volume was recorded to calculate annual growth rates. Regional surface growth was quantified using the VascForm algorithm, which is based on nonrigid point cloud registration and iterative closest point analysis. Maximum and average surface growths were calculated and correlated with the diameter/volume growth rates. Furthermore, to identify potential correlation between maximum thrombus (intraluminal thrombus) thickness and maximum surface growth, as well as between peak wall stress (PWS) and surface growth, their colocalization was examined. The median average annual surface growth was 6% (0%-28%), and the maximum surface growth 24% (11%-238%). There was strong evidence of a moderate correlation between Dmax and average as well as maximum surface growth. Regarding volumes, there was strong evidence of a very strong association with average surface growth rate and a moderate association with maximum surface growth rate (rho: 0.91, P<0.001; rho: 0.7, P<0.001, respectively). In 51.6% of the follow-ups, maximum surface growth occurred away from Dmax site. Sixteen cases presented maximum surface growth away and fifteen at the region of maximum initial intraluminal thrombus thickness. AAAs in the former group had significantly thinner initial intraluminal thrombus thickness (11.3 vs 19.5mm, P<0.001) than those in the latter. Apart from a single case, maximum surface growth did not occur at the PWS region. More than half of the lesions display maximum growth away from Dmax, suggesting that a more accurate method of analyzing AAA growth needs to be established in clinical practice that will take into account local surface growth.

Threshold of oscillation of a vocal fold replica with unilateral surface growths.

Among vocal fold diseases, the presence of a surface growth is often encountered and can be considered a public health issue. While more energy is required to achieve phonation than in healthy cases, this situation can lead to a wide range of voice perturbations, from a change of voice quality to aphonia. The present study aims at providing finer comprehension of the physical phenomena underlying this type of pathological phonation process. A vocal fold replica is used to perform measurements of mechanical responses of each vocal fold as well as of the subglottal pressure in both healthy and pathological configurations. Besides these physical measurements, a theoretical model is derived, using the one-mass-delayed model involving asymmetry of mass and geometry in order to simulate pressure signals. The theoretical model parameters are determined according to mechanical measurements on the replica. Results from measurements and simulations show that this unique vocal fold replica behaves in a manner comparable to clinical observations. The energy required to produce sound increases in the presence of a growth as well as with the size of the growth. Further investigation tends to show that the contact of the growth on the opposite vocal fold, considered as additional damping, plays a critical role.

Changing trends in pterygium management

Pterygia are progressive unilateral or bilateral ocular surface growths that develop within palpebral aperture area that leads to contour disruption with symptoms ranging from foreign body sensation to cosmetic blemish. Commonly, they appear during the active period of human life, generally between third and fourth decade. Fate of pterygium spans from progressive growth in its earlier stages that undergo diversity of changes such as inflammation, infection, and degeneration ultimately ending in regressive stage during the sixth to eighth decade of life. Pterygium recurrence is the main complication encountered by surgically treating it with or without adjuvant. Not uncommonly pterygium occurrence had been described during childhood that resolved by conservative treatment. Untreated pterygia might grow exuberantly with the invasion of corneal layers, which might result in significant visual loss due to visual axis obstruction and high astigmatism. Therefore, it is justifiable to treat pterygia at any clinical stage of presentation to prevent ocular morbidity and recurrences. Common misbelief in letting the pterygia until corneal encroachment develops should be made understandable to public by conducting awareness programs. Primary objectives of the present review article are to search for the most successful method in terms of nil to low recurrence rates by various modality of treatment and to arrive at promising surgical technique that provided maximum relief in terms of survival rates on the basis of follow-up duration. Secondary objectives are related to intra- and post-operative complications, which have an effect on the ocular surface and final visual gain or loss.

Highly loaded aerodynamic design and three dimensional performance enhancement of a HTGR helium compressor

Abstract A design study of multistage axial helium compressor of a 300 MWe high temperature gas-cooled reactor is presented in this paper. Helium compressor is characterized by shorter blades, narrow flow channels, numerous stages and longer slim rotor, which result in losses due to blade surface and end wall boundary layers growths, secondary flows and clearance leakage flows, any occurrence of flow separation and stage mismatch. Therefore, the purpose of this paper is to improve and optimize the aerodynamic design of helium compressor. The property of helium is different from that of air, so how to choose the design parameters of a helium compressor is discussed first. And then how to shorten the axial length of the helium compressor or how to decrease the number of stages for a certain pressure ratio by increasing the stage loading are investigated. The new highly loaded helium compressor of larger flow coefficient and high reaction is designed and optimized. Three-dimensional flow patterns in a helium stage are simulated with CFD software (NUMECA). Adjusting the position of blade maximum camber deflection position; redistributing radial compression work; 3D blading techniques, such as distribution spanwise relative airfoil thickness, custom tailoring airfoils and bowed stator vane to mitigate end wall boundary layers and corner separation have improved the aerodynamic performance of the first stage of helium compressor.

Climate change: a catalyst for global expansion of harmful cyanobacterial blooms

Cyanobacteria are the Earth's oldest known oxygen-evolving photosynthetic microorganisms, and they have had major impacts on shaping our current atmosphere and biosphere. Their long evolutionary history has enabled cyanobacteria to develop survival strategies and persist as important primary producers during numerous geochemical and climatic changes that have taken place on Earth during the past 3.5 billion years. Today, some cyanobacterial species form massive surface growths or 'blooms' that produce toxins, cause oxygen depletion and alter food webs, posing a major threat to drinking and irrigation water supplies, fishing and recreational use of surface waters worldwide. These harmful cyanobacteria can take advantage of anthropogenically induced nutrient over-enrichment (eutrophication), and hydrologic modifications (water withdrawal, reservoir construction). Here, we review recent studies revealing that regional and global climatic change may benefit various species of harmful cyanobacteria by increasing their growth rates, dominance, persistence, geographic distributions and activity. Future climatic change scenarios predict rising temperatures, enhanced vertical stratification of aquatic ecosystems, and alterations in seasonal and interannual weather patterns (including droughts, storms, floods); these changes all favour harmful cyanobacterial blooms in eutrophic waters. Therefore, current mitigation and water management strategies, which are largely based on nutrient input and hydrologic controls, must also accommodate the environmental effects of global warming.

Divalent Cd and Pb uptake on calcite [formula omitted] cleavage faces: An XPS and AFM study

The interaction of divalent Cd and Pb with the { 10 1 ¯ 4 } cleavage faces of calcite has been investigated with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Analysis of the { 10 1 ¯ 4 } cleavage planes of calcite was carried out with X-ray photoelectron spectroscopy (XPS) after exposure to divalent metal-bearing solutions in the 0.1–100 μM concentration range for times ranging from 1 to 24 h. The uptake of Cd 2+ by calcite was determined to be greater than that of Pb 2+ under similar experimental conditions (1 μM, pH 8.2, 24 h exposure time). In both cases, the majority of the divalent metal was postulated to exist in a surface precipitate. AFM results showed that the exposure of calcite to a 1 μM Pb 2+ solution resulted in ellipsoidal surface growths that were attributed to the nucleation of a PbCO 3 bulk phase. In the Cd circumstance, AFM showed comparatively flat growth features forming on the calcite surface even at concentrations down to 0.1 μM, where the solution would be expected to be undersaturated with respect to Cd bulk phases. These features were attributed to a (Ca,Cd)CO 3 solid solution. The individual exposure of these Cd/CaCO 3 and Pb/CaCO 3 samples to water pre-equilibrated with calcite (metal free) for 1 h led to the removal of no more than 20% of the divalent metal, suggesting that if there was an adsorbed Pb or Cd complex initially on the calcite surface, it was an minority species compared to the precipitate phase. Exposure of calcite to 100 μM Cd and Pb resulted in the accumulation of precipitate on the calcite surface presumably due to the divalent metal initial solution concentrations exceeding the solubility products of CdCO 3 and PbCO 3, respectively.

Surface growth models with a random-walk-like nonlocality.

To understand the effects of a random-walk-like nonlocality on the dynamical scaling properties of surface growths, a stochastic growth model in which the height difference triangle up h((i,i+1))=|h(i)-h(i+1)| of a chosen nearest neighbor column pair (i,i+1) is decreased by one unit is introduced and studied by simulations. The probability P((i,i+1)) of choosing a column pair (i,i+1) on a one-dimensional substrate is assigned as P((i,i+1))=e(kappa triangle up h((i,i+1)))/ summation operator (L)(j=1)e(kappa triangle up h((j,j+1))). On a substrate of given size L, the dynamical scaling property satisfies a normal scaling behavior as W=L(alpha)f(t/L(z)), when kappa is very small. If kappa becomes moderately large, the scaling property with the dynamic exponent z=1 as in diffusion-limited erosion appears. If kappa becomes very large, no surface roughening is found.

Anomaly of the height-height correlation functions in self-flattening surface growth.

By Monte Carlo simulations and scaling theories, we consider the height-height correlation function G(r,t;L) of the one-dimensional equilibrium self-flattening surface growths, where the deposition (evaporation) attempt only at the globally highest (lowest) site is suppressed. G(r,t:L) is shown to satisfy the anomalous scaling behavior G(r,t;L)=L(2alpha)g(1)(r/L(delta),t/L(z)) or G(r,t;L)=t(2beta)g(2)(r/t(1/z(')),L/t(1/z)). Here alpha, beta, and z are the roughness, growth, and dynamic exponents, respectively, for the surface width, with alpha=1/3 and z=alpha/beta=3/2. Anomalous exponents z(') and delta are found to satisfy z(')=9/4 and delta=z/z('). We also show that anomalous behavior of G(r,t;L) can be understood from a scaling theory based on the competition between local random-walk-like behavior and the global-length-scale suppression.

Dynamical surface structures in multiparticle-correlated surface growths.

We investigate the scaling properties of the interface fluctuation width for the Q-mer and Q-particle-correlated deposition-evaporation models. These models are constrained with a global conservation law that the particle number at each height is conserved modulo Q. In equilibrium, the stationary roughness is anomalous but universal with the roughness exponent alpha=1/3, while the early time evolution shows nonuniversal behavior with the growth exponent beta varying with models and Q. Nonequilibrium surfaces display diverse growing and stationary behaviors. The Q-mer model shows a faceted structure, while the Q-particle-correlated model shows a macroscopically grooved structure.

Evidence for magmatic vapor deposition of anhydrite prior to the 1991 climactic eruption of Mount Pinatubo, Philippines

Anhydrite (CaSO4) phenocrysts from Mount Pinatubo pumices show evidence of having responded dynamically to changing conditions prior to the June 15, 1991 climactic eruption. Micrometer-sized and smaller pyramidal surface growth features and lesser numbers of etch pits on anhydrite surfaces are documented by scanning electron microscopy. Chemical analyses indicate that the pyramids are a CaSO4 polymorph and electron backscatter diffraction patterns show conclusively that the pyramids are indeed orthorhombic anhydrite and not another Ca-sulfate phase. Unit-cell measurements of volcanic anhydrite are identical with evaporitic anhydrite, as determined from single-crystal X-ray diffraction patterns. The computer program SOLVGAS was used to identify conditions under which the pyramids may have precipitated. Thermodynamic modeling of a cooling magmatic gas (H2O-CO2-SO2) at 500 bars (maximum model pressure) and NNO +1.7 was performed. Assuming that the gas contained >10−9 mol% Ca and 4 mol% SO2, the program indicates that anhydrite will precipitate homogeneously at approximately 780 °C, whereas an isothermal drop in pressure would likely lead to dissolution. Pyramids located between a phenocryst and adjacent glass provide physical evidence that at least a portion of the pyramids nucleated and grew before the melt quenched. We propose a mechanism to account for these previously unrecognized surface growths, which is that the anhydrite pyramids precipitated from a fluid or vapor phase that had separated from the magma at depth. At least a portion of the Pinatubo anhydrite phenocrysts provided substrates for nucleation and epitaxial growth of anhydrite. Because the anhydrite pyramids resemble products of chemical vapor deposition of metals and ceramics, we propose that this previously unrecognized process be termed magmatic vapor deposition. Evidence of high-temperature, magmatic vapor deposition of anhydrite could be used as an indicator that a pre-eruptive gas phase was accumulating or stored at depth, especially at arc volcanoes where excess amounts of sulfur are vented into the atmosphere.

Anomalous height fluctuation width in crossover from random to coherent surface growths

We study the anomalous behavior of the height fluctuation width in the crossover from random to coherent growth of a surface for a stochastic model. In the model, random numbers are assigned on perimeter sites of a surface, representing pinning strengths of disordered media. At each time step the surface is advanced at the site having a minimum pinning strength in a random subset of the system rather than at a global minimum. The subset is composed of a randomly selected site and its (l-1) neighbors. The height fluctuation width ${\mathrm{W}}^{2}$(L;l) exhibits nonmonotonic behavior with l and has a minimum at ${\mathrm{l}}^{\mathrm{*}}$. It is found numerically that ${\mathrm{l}}^{\mathrm{*}}$ scales as ${\mathrm{l}}^{\mathrm{*}}$\ensuremath{\sim}${\mathrm{L}}^{0.59}$ and the height fluctuation width at that minimum, ${\mathrm{W}}^{2}$(L;${\mathrm{l}}^{\mathrm{*}}$), scales as \ensuremath{\sim}${\mathrm{L}}^{0.85}$ in 1+1 dimensions. It is also found that the subset size ${\mathrm{l}}^{\mathrm{*}}$(L) is the characteristic size of the crossover from the random surface growth in the Kardar-Parisi-Zhang universality to the coherent surface growth in the directed percolation universality.

Influence of cryptobiotic soil crusts on elemental content of tissue of two desert seed plants

Soil surface growths dominated by cyanobacteria and the lichen Collema in southeastern Utah are shown to be associated with greater tissue content of several bio‐essential elements in two co‐occurring seed plants (Festuca octoflora, Poaceae, and Mentzelia multiflora, Loasaceae). The elements N, P, K, Ca, Mg, and Fe were present in significantly greater concentrations in Festuca growing on soils heavily encrusted with cyanobacteria and cyanolichens than in plants on the same soil where foot traffic had destroyed the cryptobiotic crusts. With Mentzelia, N, Mg, and Fe were present in significantly greater concentrations in plants from sites with encrusted soil surfaces than on blow‐sand sites. The cryptobiota appeared to compete vigorously with Mentzelia for available P: Mentzelia plants from crusted sites contained significantly smaller concentrations of P than plants grown on soils where wind action precluded development of surface crusts. These cryptobiotic crusts fix considerable amounts of N, which apparently becomes available to associated seed plants via decomposition and cellular secretion processes. Other macronutrients are apparently accumulated in forms available to seed plants as the crusts develop in interspaces between higher plants. The trace element Fe appears to be rendered more available to higher plants by the cryptobiotic growth. That effect may be related to chelating compounds known to be present in the mucilaginous sheaths of cyanobacteria. Other possible reasons are discussed for the enhanced nutrient uptake of seed plants growing in cryptobiotic crusts.

Heterogrowth of Ge on the Si(001)2 × 1 reconstructed surface

The step configuration on the Si(001)2 × 1 surface and heteroepitaxial growths on that surface were investigated in situ by ultrahigh-vacuum electron microscopy (UHV-EM). First, the shapes of monolayer-high steps and the terrace widths were examined by using the UHV-EM in the reflection mode. Previous results are summarized and compared with the results presented here. Second, heterogrowths of germanium were observed and are discussed from a point of view that relates with surface reconstructions or with stabilities of steps on a clean Si(001) surface. It was observed that step growths occurred selectively at the S A segments (Chadi's notation), and that island growths resulted in an introduction of out-of-phase boundaries. The out-of-phase boundaries are observed to work as preferential nucleation sites in heterogrowths of germanium on Si(001)2 × 1 surfaces. The high defect densities and surface roughening are caused by the out-of-phase boundaries which are formed during the heterogrowths.

Cyanobacteria and eukaryotic algae in sports turf and amenity grasslands: a review

A review is presented on the occurrence and role of cyanobacteria and eukaryotic algae in sports turf, especially golf curses, and on the problems arising when these organisms become abundant. The literature depends largely on observations in only a few countries, mainly Canada, New Zealand, U.K. and U.S.A., but problem growths are probably widespread. The genera reported to be conspicuous at times include Nostoc, Phormidium, Coccomyxa, Cosmarium, Cylindrocystis, Klebsormidium, Mesotaenium and Zygogonium. Conspicuous surface growths are probably most often related to unsatisfactory drainage or irrigation practices, but other factors such as fertilizer treatment (especially excess ammonium sulphate) have been implicated. These surface growths sometimes incorporate copious mucilaginous ‘slime’, which can be a serious hazard by causing people to slip. U.K. observations suggest that this is especially likely under acidic conditions, where Coccomyxa, Cylindrocystis and Mesotaenium are among the probable culprits.

Mycoplasma pneumoniae and Mycoplasma salivarium: electron microscopy of colony growth in agar.

Colonies of Mycoplasma pneumoniae and Mycoplasma salivarium grown in PPLO agar were examined by light and electron microscopy. The main objective of the investigation was to attempt in situ fixation and minimize tonic changes in the organisms. Microscopy revealed that both organisms grew both in and upon the agar. The agar and surface growths of M. pneumoniae exhibited similar profiles, whereas those of M. salivarium differed strikingly. Both organisms are highly pleomorphic, but their matrix was denser and appeared more intact than in previously reported profiles. Cells which resemble the commonly reported mycoplasma were occasionally observed. The significance of these discrepant profiles remains unanswered. It is suggested that they may represent aged or osmotically damaged cells.