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The latitudinal diversity gradient (LDG) dominates global patterns of diversity1,2, but the factors that underlie the LDG remain elusive. Here we use a unique global dataset3to show that vascular plants on oceanic islands exhibit a weakened LDG and explore potential mechanisms for this effect. Our results show that traditional physical drivers of island biogeography4—namely area and isolation—contribute to the difference between island and mainland diversity at a given latitude (that is, the island species deficit), as smaller and more distant islands experience reduced colonization. However, plant species with mutualists are underrepresented on islands, and we find that this plant mutualism filter explains more variation in the island species deficit than abiotic factors. In particular, plant species that require animal pollinators or microbial mutualists such as arbuscular mycorrhizal fungi contribute disproportionately to the island species deficit near the Equator, with contributions decreasing with distance from the Equator. Plant mutualist filters on species richness are particularly strong at low absolute latitudes where mainland richness is highest, weakening the LDG of oceanic islands. These results provide empirical evidence that mutualisms, habitat heterogeneity and dispersal are key to the maintenance of high tropical plant diversity and mediate the biogeographic patterns of plant diversity on Earth.

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces—a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60)∞by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence spectroscopy reveals the existence of moiré-like superlattices3. More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

Forests are a substantial terrestrial carbon sink, but anthropogenic changes in land use and climate have considerably reduced the scale of this system1. Remote-sensing estimates to quantify carbon losses from global forests2–5are characterized by considerable uncertainty and we lack a comprehensive ground-sourced evaluation to benchmark these estimates. Here we combine several ground-sourced6and satellite-derived approaches2,7,8to evaluate the scale of the global forest carbon potential outside agricultural and urban lands. Despite regional variation, the predictions demonstrated remarkable consistency at a global scale, with only a 12% difference between the ground-sourced and satellite-derived estimates. At present, global forest carbon storage is markedly under the natural potential, with a total deficit of 226 Gt (model range = 151–363 Gt) in areas with low human footprint. Most (61%, 139 Gt C) of this potential is in areas with existing forests, in which ecosystem protection can allow forests to recover to maturity. The remaining 39% (87 Gt C) of potential lies in regions in which forests have been removed or fragmented. Although forests cannot be a substitute for emissions reductions, our results support the idea2,3,9that the conservation, restoration and sustainable management of diverse forests offer valuable contributions to meeting global climate and biodiversity targets.

Determining the drivers of non-native plant invasions is critical for managing native ecosystems and limiting the spread of invasive species1,2. Tree invasions in particular have been relatively overlooked, even though they have the potential to transform ecosystems and economies3,4. Here, leveraging global tree databases5–7, we explore how the phylogenetic and functional diversity of native tree communities, human pressure and the environment influence the establishment of non-native tree species and the subsequent invasion severity. We find that anthropogenic factors are key to predicting whether a location is invaded, but that invasion severity is underpinned by native diversity, with higher diversity predicting lower invasion severity. Temperature and precipitation emerge as strong predictors of invasion strategy, with non-native species invading successfully when they are similar to the native community in cold or dry extremes. Yet, despite the influence of these ecological forces in determining invasion strategy, we find evidence that these patterns can be obscured by human activity, with lower ecological signal in areas with higher proximity to shipping ports. Our global perspective of non-native tree invasion highlights that human drivers influence non-native tree presence, and that native phylogenetic and functional diversity have a critical role in the establishment and spread of subsequent invasions.

The origin and evolution of hominin mortuary practices are topics of intense interest and debate1–3. Human burials dated to the Middle Stone Age (MSA) are exceedingly rare in Africa and unknown in East Africa1–6. Here we describe the partial skeleton of a roughly 2.5- to 3.0-year-old child dating to 78.3 ± 4.1 thousand years ago, which was recovered in the MSA layers of Panga ya Saidi (PYS), a cave site in the tropical upland coast of Kenya7,8. Recent excavations have revealed a pit feature containing a child in a flexed position. Geochemical, granulometric and micromorphological analyses of the burial pit content and encasing archaeological layers indicate that the pit was deliberately excavated. Taphonomical evidence, such as the strict articulation or good anatomical association of the skeletal elements and histological evidence of putrefaction, support the in-place decomposition of the fresh body. The presence of little or no displacement of the unstable joints during decomposition points to an interment in a filled space (grave earth), making the PYS finding the oldest known human burial in Africa. The morphological assessment of the partial skeleton is consistent with its assignment to Homo sapiens, although the preservation of some primitive features in the dentition supports increasing evidence for non-gradual assembly of modern traits during the emergence of our species. The PYS burial sheds light on how MSA populations interacted with the dead.

To constrain global warming, we must strongly curtail greenhouse gas emissions and capture excess atmospheric carbon dioxide1,2. Regrowing natural forests is a prominent strategy for capturing additional carbon3, but accurate assessments of its potential are limited by uncertainty and variability in carbon accumulation rates2,3. To assess why and where rates differ, here we compile 13,112 georeferenced measurements of carbon accumulation. Climatic factors explain variation in rates better than land-use history, so we combine the field measurements with 66 environmental covariate layers to create a global, one-kilometre-resolution map of potential aboveground carbon accumulation rates for the first 30 years of natural forest regrowth. This map shows over 100-fold variation in rates across the globe, and indicates that default rates from the Intergovernmental Panel on Climate Change (IPCC)4,5may underestimate aboveground carbon accumulation rates by 32 per cent on average and do not capture eight-fold variation within ecozones. Conversely, we conclude that maximum climate mitigation potential from natural forest regrowth is 11 per cent lower than previously reported3owing to the use of overly high rates for the location of potential new forest. Although our data compilation includes more studies and sites than previous efforts, our results depend on data availability, which is concentrated in ten countries, and data quality, which varies across studies. However, the plots cover most of the environmental conditions across the areas for which we predicted carbon accumulation rates (except for northern Africa and northeast Asia). We therefore provide a robust and globally consistent tool for assessing natural forest regrowth as a climate mitigation strategy.

Urban heat islands (UHIs) exacerbate the risk of heat-related mortality associated with global climate change. The intensity of UHIs varies with population size and mean annual precipitation, but a unifying explanation for this variation is lacking, and there are no geographically targeted guidelines for heat mitigation. Here we analyse summertime differences between urban and rural surface temperatures (ΔTs) worldwide and find a nonlinear increase in ΔTswith precipitation that is controlled by water or energy limitations on evapotranspiration and that modulates the scaling of ΔTswith city size. We introduce a coarse-grained model that links population, background climate, and UHI intensity, and show that urban–rural differences in evapotranspiration and convection efficiency are the main determinants of warming. The direct implication of these nonlinearities is that mitigation strategies aimed at increasing green cover and albedo are more efficient in dry regions, whereas the challenge of cooling tropical cities will require innovative solutions. The effect of cities on urban climate (often warmer but sometimes cooler than their surroundings) is largely explained by local hydroclimate and patterns of city development.

Soil organisms are a crucial part of the terrestrial biosphere. Despite their importance for ecosystem functioning, few quantitative, spatially explicit models of the active belowground community currently exist. In particular, nematodes are the most abundant animals on Earth, filling all trophic levels in the soil food web. Here we use 6,759 georeferenced samples to generate a mechanistic understanding of the patterns of the global abundance of nematodes in the soil and the composition of their functional groups. The resulting maps show that 4.4 ± 0.64 × 1020nematodes (with a total biomass of approximately 0.3 gigatonnes) inhabit surface soils across the world, with higher abundances in sub-Arctic regions (38% of total) than in temperate (24%) or tropical (21%) regions. Regional variations in these global trends also provide insights into local patterns of soil fertility and functioning. These high-resolution models provide the first steps towards representing soil ecological processes in global biogeochemical models and will enable the prediction of elemental cycling under current and future climate scenarios.

The identity of the dominant root-associated microbial symbionts in a forest determines the ability of trees to access limiting nutrients from atmospheric or soil pools1,2, sequester carbon3,4and withstand the effects of climate change5,6. Characterizing the global distribution of these symbioses and identifying the factors that control this distribution are thus integral to understanding the present and future functioning of forest ecosystems. Here we generate a spatially explicit global map of the symbiotic status of forests, using a database of over 1.1 million forest inventory plots that collectively contain over 28,000 tree species. Our analyses indicate that climate variables—in particular, climatically controlled variation in the rate of decomposition—are the primary drivers of the global distribution of major symbioses. We estimate that ectomycorrhizal trees, which represent only 2% of all plant species7, constitute approximately 60% of tree stems on Earth. Ectomycorrhizal symbiosis dominates forests in which seasonally cold and dry climates inhibit decomposition, and is the predominant form of symbiosis at high latitudes and elevation. By contrast, arbuscular mycorrhizal trees dominate in aseasonal, warm tropical forests, and occur with ectomycorrhizal trees in temperate biomes in which seasonally warm-and-wet climates enhance decomposition. Continental transitions between forests dominated by ectomycorrhizal or arbuscular mycorrhizal trees occur relatively abruptly along climate-driven decomposition gradients; these transitions are probably caused by positive feedback effects between plants and microorganisms. Symbiotic nitrogen fixers—which are insensitive to climatic controls on decomposition (compared with mycorrhizal fungi)—are most abundant in arid biomes with alkaline soils and high maximum temperatures. The climatically driven global symbiosis gradient that we document provides a spatially explicit quantitative understanding of microbial symbioses at the global scale, and demonstrates the critical role of microbial mutualisms in shaping the distribution of plant species. A spatially explicit global map of tree symbioses with nitrogen-fixing bacteria and mycorrhizal fungi reveals that climate variables are the primary drivers of the distribution of different types of symbiosis.

Chronic traumatic encephalopathy (CTE) is a neurodegenerative tauopathy that is associated with repetitive head impacts or exposure to blast waves. First described as punch-drunk syndrome and dementia pugilistica in retired boxers1–3, CTE has since been identified in former participants of other contact sports, ex-military personnel and after physical abuse4–7. No disease-modifying therapies currently exist, and diagnosis requires an autopsy. CTE is defined by an abundance of hyperphosphorylated tau protein in neurons, astrocytes and cell processes around blood vessels8,9. This, together with the accumulation of tau inclusions in cortical layers II and III, distinguishes CTE from Alzheimer’s disease and other tauopathies10,11. However, the morphologies of tau filaments in CTE and the mechanisms by which brain trauma can lead to their formation are unknown. Here we determine the structures of tau filaments from the brains of three individuals with CTE at resolutions down to 2.3 Å, using cryo-electron microscopy. We show that filament structures are identical in the three cases but are distinct from those of Alzheimer’s and Pick’s diseases, and from those formed in vitro12–15. Similar to Alzheimer’s disease12,14,16–18, all six brain tau isoforms assemble into filaments in CTE, and residues K274–R379 of three-repeat tau and S305–R379 of four-repeat tau form the ordered core of two identical C-shaped protofilaments. However, a different conformation of the β-helix region creates a hydrophobic cavity that is absent in tau filaments from the brains of patients with Alzheimer’s disease. This cavity encloses an additional density that is not connected to tau, which suggests that the incorporation of cofactors may have a role in tau aggregation in CTE. Moreover, filaments in CTE have distinct protofilament interfaces to those of Alzheimer’s disease. Our structures provide a unifying neuropathological criterion for CTE, and support the hypothesis that the formation and propagation of distinct conformers of assembled tau underlie different neurodegenerative diseases.

The ordered assembly of tau protein into abnormal filamentous inclusions underlies many human neurodegenerative diseases1. Tau assemblies seem to spread through specific neural networks in each disease2, with short filaments having the greatest seeding activity3. The abundance of tau inclusions strongly correlates with disease symptoms4. Six tau isoforms are expressed in the normal adult human brain—three isoforms with four microtubule-binding repeats each (4R tau) and three isoforms that lack the second repeat (3R tau)1. In various diseases, tau filaments can be composed of either 3R or 4R tau, or of both. Tau filaments have distinct cellular and neuroanatomical distributions5, with morphological and biochemical differences suggesting that they may be able to adopt disease-specific molecular conformations6,7. Such conformers may give rise to different neuropathological phenotypes8,9, reminiscent of prion strains10. However, the underlying structures are not known. Using electron cryo-microscopy, we recently reported the structures of tau filaments from patients with Alzheimer’s disease, which contain both 3R and 4R tau11. Here we determine the structures of tau filaments from patients with Pick’s disease, a neurodegenerative disorder characterized by frontotemporal dementia. The filaments consist of residues Lys254–Phe378 of 3R tau, which are folded differently from the tau filaments in Alzheimer’s disease, establishing the existence of conformers of assembled tau. The observed tau fold in the filaments of patients with Pick’s disease explains the selective incorporation of 3R tau in Pick bodies, and the differences in phosphorylation relative to the tau filaments of Alzheimer’s disease. Our findings show how tau can adopt distinct folds in the human brain in different diseases, an essential step for understanding the formation and propagation of molecular conformers. The structures of tau filaments from patients with the neurodegenerative disorder Pick’s disease show that the filament fold is different from that of the tau filaments found in Alzheimer’s disease.

Alzheimer’s disease is the most common neurodegenerative disease, and there are no mechanism-based therapies. The disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. Neurofibrillary lesions comprise paired helical and straight tau filaments, whereas tau filaments with different morphologies characterize other neurodegenerative diseases. No high-resolution structures of tau filaments are available. Here we present cryo-electron microscopy (cryo-EM) maps at 3.4–3.5 Å resolution and corresponding atomic models of paired helical and straight filaments from the brain of an individual with Alzheimer’s disease. Filament cores are made of two identical protofilaments comprising residues 306–378 of tau protein, which adopt a combined cross-β/β-helix structure and define the seed for tau aggregation. Paired helical and straight filaments differ in their inter-protofilament packing, showing that they are ultrastructural polymorphs. These findings demonstrate that cryo-EM allows atomic characterization of amyloid filaments from patient-derived material, and pave the way for investigation of a range of neurodegenerative diseases.

Viruses isolated from leukaemic and preleukaemic tissues of the AKR mouse differ in their ability to induce thymic lymphomas. The Gross passage A virus, which originates from the thymus of a leukaemic AKR mouse1, and the MCF viruses, isolated from preleukaemic and leukaemic AKR tissues2,3, accelerate the onset of disease on injection into newborn AKR mice3,4whereas the endogenous ecotropic Akv virus, which is expressed in AKR mice from mid-gestation onward, is nonleukaemogenic3,5,6. What are the structural features of the genome that account for the leukaemogenic activity of these viruses? The observation that the gp70 envelope glycoprotein of the leukaemogenic MCF virus contain ecotropic and xenotropic regions7–9raised the possibility that the structure of the gp70 gene was the major determinant of leukaemogenic activity2. However, a cloned, in vitro passaged, leukaemogenic isolate of the Gross A virus very closely resembles that of the nonleukaemogenic Akv virus in the gag–pol and gp70 coding regions of the genome10. Most of the differences in structure of the Gross A and Akv viruses are located within the 3′-terminal 1,000 nucleotides—this raises the possibility that sequences outside the gp70 gene which are proximal to the 3′ end of the genome encode determinants of leukaemogenic activity. Due to the complex passage history of the Gross A virus we sought to isolate further leukaemogenic AKR viruses that had not been passaged in other animals to determine whether or not they resembled the Gross A virus. Here we report the isolation and characterization of new leukaemogenic viruses from a lymphoid cell line derived from a spontaneous AKR tumour. These isolates resemble the Gross A virus in the 3′ region of the genome. Our results suggest that this region encodes determinants of leukaemogenic potency and that major changes in the envelope glycoprotein are not necessary for the leukaemogenic activity of these viruses.

Although the murine retrovirus SL3-3 is highly leukaemo-genic1,2, in both the structure of its genome and in its properties of replication in tissue culture it closely resembles the non-leukaemogenic retrovirus Akv (refs 3, 4). An.earlier investigation of the properties of recombinant SL3-3-Akv viruses localized the major determinant of leukaemogenicity outside the env gene, in a region of the viral genome that includes the gag gene and the noncoding long terminal repeat (LTR)4. To localize the determinant of SL3-3's leukaemogenicity more precisely we have now construced a recombinant provirus containing the LTR of SL3-3 and the coding region of Akv. The leukaemogenicity of these recombinants demonstrates that the determinant of leukaemogenicity ties within the SL3-3 LTR. Nucleotide sequencing of the LTRs of SL3-3 and Akv shows that they differ by a set of changes in the region thought to contain a transcriptional enhancer element. We suggest that enhancer region sequences are the major determinants of leukaemogenicity in these viruses.

The enzyme L-asparaginase, which breaks down asparagine, causes certain tumours to regress. This property has been put to use in the treatment of malignant disease in man.

Criteria are given for obtaining maximum information from a set of projections of a general object while preventing the introduction of spurious detail by effects which are beyond resolution.

Methods are developed for computing three dimensional density maps from transmission electron micrographs using Fourier transforms. The reconstructions objectively combine data from several different views of one or more particles. When applied to negatively stained spherical viruses, the technique gives clear reconstructions of their surface structures.

The imidazole groups of the C-terminal histidines of the β-chains, together with the α-amino groups of the α-chains, are responsible for most of the Bohr effect. In oxyhaemoglobin these groups are free, while in deoxyhaemoglobin their pKs are raised, probably by linkage to carboxyl groups.

WE present twenty-four identifications of radio sources with neutral coloured objects of stellar appearance. We consider these objects to be of special interest because they do not fall into the two normally accepted categories for identification; that is, galaxies or blue stellar objects. Two of them have recently been found to have exceptionally high redshifts1,2, and it is hoped that further optical measurements on the others will produce equally interesting results.

WE describe an improved alogrithm (EFIRT) for solving iteratively the linear equations relating a three-dimensional density to a given set of its projections, when the unknown density is expressed as samples on a grid. The need to solve such equations arises in electron microscopy, medical radiography, radio and X-ray astronomy and other fields. The projection equations comprise a very large, sparse and often under-determined set. If the number of unknowns is not too large, the equations may be solved by a least squares procedure, using filtering to combat the ill-conditioning1. The number of unknowns is often too great for such a procedure to be computationally feasible and iterative techniques2,3must be used. Alternatively the Fourier transform provides a stable and computationally efficient means of solution1. Sometimes use of the Fourier transform is not convenient; for example, if point rather than plane projections are given or if the projection data are weighted by a non-uniform but linear response of the measuring device. Such problems are much more readily expressed in terms of projection equations and the development of stable methods for their solution is therefore important.

Fish muscle provides a particularly suitable specimen for studying the organization of thick filaments in vertebrate striated muscle because the filaments are arranged with a single orientation on a well ordered hexagonal lattice1. The M-band consists of sets of cross-links, which join the myosin filaments in the middle of the A-band. Previous work1concluded that the fish muscle M-band had the local symmetry of the dihedral point group 32. We present here a quantitative analysis of transverse sections of the M-band, using general methods2developed for crystalline layers, to combine various tilted views. The three-dimensional map computed to a resolution of about 70 Å confirms previous findings1; it shows new features of the thick filament structure in the M-band region and provides new information on the use of three-dimensional reconstruction from sectioned biological material. The accompanying paper3describes a novel technique of three-dimensional reconstruction of the fish M-band from a single view of a slightly oblique plastic section.

THIS international congress of physicists, chemists and biologists was conceived and organized within ninety days, chiefly through the initiative of M. Frederic Joliot-Curie, who was the chairman of the executive committee. It was attended by eleven hundred men of science and four hundred other members. Its aims iincluded the intellectual celebration of the success of the Palais de la Dècouverte, the unique quality of which was described in NATURE of August 21, p. 328, and which was visited by two million persons between May 25 and October 7.

THE possible importance of X-rays in the medical world was recognised so clearly by their discoverer, Röntgen, that the first communication on the subject was made by him to a medical society, and was published to the world in a medical journal. Nor were medical men slow to appreciate the potency of the weapon which had thus been placed in their hands. The medical profession, be it spoken to its praise, has been unremitting in its search for new weapons in the fight against disease. Dr. Gilbert, himself no mean physician, and author of the first treatise on magnetism, records, with perhaps undue scorn, how, in the days when magnetism was the latest scientific marvel, patients were dosed with decoctions of lodestone as a possible panacea for all ills. It was not likely that so startling a discovery as X-rays would be overlooked, and we find medical men among the pioneers of X-ray work in nearly all countries. Further, the economically effective demand of medical radiology for more power, and still more power, has persuaded engineers and manufacturers to produce the modern high-power X-ray plant which has made possible the recent advances in the subject.

THERE is now considerable evidence that there are in animals tumour specific antigens of a transplantation type, that the host reacts against its own tumour and that this reaction is in some cases immunological, mediated by lymphocytes and similar to the delayed hypersensitivity reaction and homograft rejection1. The situation in man is not so clear because of the difficulty of research. We therefore decided to study the morphology and DNA synthesis of lymphocytes in Hodgkin's disease for two reasons. First, in this disease cellular immunity is depressed more than in any other human malignant disease; and second, lymphocytic and/or histiocytic proliferation is associated with localized disease and a relatively good prognosis, whereas lymphocytic depletion is associated with a poor prognosis and a generalized disease2,3.

IT is difficult to establish the identification of a radio source with an object of stellar appearance and neutral colour. We show in this letter that such identifications can be made by comparing radio and optical positions of accuracy better than 1 arc sec.

In this Letter, a middle initial and additional affiliation have been added for author G. J. Nabuurs; two statements have been added to the Supplementary Acknowledgements; and a citation to the French National Institute has been added to the Methods; see accompanying Author Correction for further details.

Pseudogenes—sequences which are homologous to functional genes but which contain mutational changes precluding the formation of a functional product—seem to be a common feature of eukaryotic genomes. Such sequences were first described in the 5S gene of Xenopus laevis1and have since been found in several gene families including those of the α- and β-globins of several species2–10, immunoglobulin Vκgenes11, the actin genes of Dictyostelium12and the small nuclear RNA genes of man13. Among the globin pseudogenes, there appear to be two kinds of structural organization: genes that have retained their intervening sequences4–7and those that have lost them completely2,3. A possible explanation for the generation of such intron-lacking genes involves the reverse transcription of a processed mRNA to form cDNA, and the introduction of this cDNA copy into the genome either by recombinant heteroduplex formation2, insertion into a staggered chromosomal break3, or via a retrovirus intermediate14. Here we report the complete sequence of a human β-tubulin pseudogene. The sequence data reveal the absence of any intervening sequences, and the presence, in the genome, of an uninterrupted 17 base-pair (bp) tract of A residues 14 base-pairs 3′ to a poly(A) signal (AATAAA)15. This sequence organization corresponds closely to the sequence organization of the poly(A) signal and poly (A) tract in a β-tubulin mRNA16.

THE culture of normal haemopoietic cells was accomplished1by obtaining colonies of granulocytic and monocytic cells from mouse bone marrow grown in a semisolid agar system. The method was modified for human bone marrow and subsequent studies showed that leukaemia cells from the peripheral blood and marrow of patients with acute myelogenous leukaemia (AML) produced small colonies or clusters1,2. Using this technique, serial sampling and morphological or biochemical studies of the growing cells was difficult. A liquid culture technique was then developed to study the growth of granulocytic or monocytic cells from normal bone marrow3. This in vitro diffusion chamber technique had the advantage of easy sampling but involved the use of large culture vessels. The method has subsequently been used to cultivate AML cells4.

A new method of three-dimensional image reconstruction from electron micrographs is presented here, which enables a three-dimensional image to be produced from a single oblique section of a two- or three-dimensional crystal. The method, which is most powerful when the section is thin and the angle of obliquity small, has several potential advantages over the conventional method of reconstruction1using tilted views of the section. In particular the accumulated electron dose on the specimen is much smaller and the reconstruction is not affected by changes in the thickness of the section during exposure to the electron beam. The method involves the solution of a generally almost singular set of linear projection equations2, relating through the sectioning geometry the three-dimensional crystal density to the two-dimensional projected image density. This can be achieved most conveniently by linear least squares filtering3. An application of the method to determine the structure of the M-band of fish muscle is described. The resulting map agrees well with that produced by the more conventional approach involving tilted views4.

DURING work involving sterilization by autoclaving of groundnut meals containing aflatoxin, the hepatotoxic metabolites of Aspergillus flavus, Link, ex-Fries1, it was noted that an apparent reduction in the aflatoxin B1 content was induced when the normal methods of fluorescence assay2were afterwards applied to the sterile meal.