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The Scientist has asked a group of experts to comment periodically upon recent articles that they have found noteworthy. Their selections, presented herein every issue, are neither endorsements of content nor the result of systematic searching. Rather, the list represents personal choices of articles the columnists believe the scientific community as a whole may also find interesting. Reprints of any articles cited here may be ordered through The Genuine Article, 3501 Market St., Philadelphia, Pa. 19104, or by telephoning (215) 386-4399. Author: PETER D. MOORE Department of Biology King's College London, U.K. Variability in biological systems has often been regarded as an unfortunate but inevitable feature. Yet modeling studies of host-parasite associations have now shown that their variability in a patchy environment may contribute to their long-term stability. The patchiness ensures survival of both host and parasite, and thus contributes to the stability of the relationship. S.W. Pacala, M.P. Hassell, R.M. May, "Host-parasitoid associations in patchy environments," Nature, 344, 150-3, 8 March 1990. (University of Connecticut, Storrs; Imperial College, London) Sitka spruce is one of the most important trees for commercial forestry of oceanic temperate areas, and a knowledge of its natural ecology is valuable for its management. In the Pacific Northwest it occupies a gap-filling role - colonizing areas opened by disturbance in hemlock forests. The fast growth rate it needs to fill this temporal niche is precisely the feature that makes it valuable to foresters. A.H. Taylor, "Disturbance and persistence of sitka spruce (Picea sitchensis [Bong] Carr.) in coastal forests of the Pacific Northwest, North America," Journal of Biogeography, 17, 47-58, January 1990. (University of Maryland, Baltimore) The control of the water fern salvinia - an aquatic pest - with herbivorous weevils from its native Brazil has been one of the most remarkable tales of biological control since the prickly pear. The simple life cycle and genetic uniformity of salvinia have made it particularly susceptible to herbivore attack, but the history of the early attempts at control shows that accurate taxonomy, though currently an unfashionable subject, is vital for the success of such work. P.M. Room, "Ecology of a simple plant-herbivore system: biological control of Salvinia," Trends In Ecology and Evolution, 5, 74-9, March 1990. (CSIRO, Brisbane, Australia) The past may hold some important messages for the present, as in the case of the extinction of the megafauna at the end of the last glacial episode. The elimination of many species of large (more than 1,000 kilograms) herbivores, possibly as a consequence of human predation, precipitated a series of extinctions of other species because of the habitat modifications that resulted. If modern large herbivores, such as elephant and rhinoceros, are lost from the savanna environments of Africa, the subsequent changes in habitat could bring about the extinction of many other animals. N. Owen-Smith, "Megafaunal extinctions: the conservation message from 11,000 years B.P.," Conservation Biology, 3, 405-12, December 1989. (University of Witwatersrand, Johannesburg, South Africa) Plants that use the C4 photosynthetic strategy tend to be associated with high-temperature conditions; thus, very few are found in high latitudes. Studies on physiological ocotypes of the C4 grass Echinochloa crus-galli from contrasting climatic situations, however, have revealed that the C4 system can be coupled with a degree of cold tolerance. C. Potvin, J.-P. Simon, "The evolution of cold temperature adaptation among populations of a widely distributed C4 weed: barnyard grass," Evolutionary Trends In Plants, 3, 98-105, December 1989. (McGill University, Montreal; Universit‚ de Montréal) The capacity of the albatross to forage over vast distances in the open ocean has long been suspected but has been difficult to measure precisely. Male Wandering albatrosses have now been equipped with transmitters and tracked by satellite, and the results have proved more remarkable than could have been imagined. These birds can travel up to 900 kilometers per day and may cover as much as 15,000 kilometers on a single foraging trip. P. Jouventin, H. Weimerskirch, "Satellite tracking of Wandering albatrosses," Nature, 343, 746-8, 22 February 1990. (Centre National de la Recherche Scientifique, Beauvoir, France) Author: THEODORE DAVIDSON Institute of Materials Science University of Connecticut Storrs Determining the detailed motion of a macromolecule in the solid state is a major step beyond establishing the static crystal structure. A group at the Du Pont Experimental Station has made a tour de force computer simulation of molecular dynamics and compared the predictions with experimental 2H NMR line shapes. At room temperature, methylene sequences in the adipoyl units are seen to undergo larger amplitude librational motions than the -CH2 units in hexamethylenediamine portions of the polymer. Experimental and model values of the distribution of librational angles compare well. J.J. Wendoloski, K.H. Gardner, J. Hirschinger, H. Miura, A.D. English, "Molecular dynamics in ordered structures: computer simulation and experimental results for nylon 66 crystals, Science, 247, 431-6, 26 January 1990. (E.I. Du Pont de Nemours and Co., Wilmington, Del.) A recent report demonstrates atomic level imaging of a metal crystal in contact with the electrolyte solution from which it was grown. In a nontunneling mode, lead was electrodeposited onto graphite using the scanning tunneling microscopy (STM) tip as an electrode. One to a few hundred monolayers were built up and then imaged in the scanning tunneling mode. The lead deposit could be electrolytically etched with the effect of electropolishing the surface and the original graphite underlayer could be exposed and re-imaged. M. Szklarczyk, J.O'M. Bockris, "In situ STM studies of lead electrodeposition on graphite substrate," Journal of the Electrochemical Society, 137, 452-7, February 1990. Texas A&M University, College Station) It is paradoxical that crystalline diamonds can be grown from the vapor phase, since graphite is the thermodynamically stable polymorph. It is now suggested that a high concentration of lattice vacancies near the face of the growing crystal favors diamond over graphite. A corollary is that growth of diamond with few defects by chemical vapor deposition will be difficult. An increase in nucleation density and the possibility of growing continuous films is promoted by electron bombardment during growth. N-type or p-type diamonds may be made by doping during growth, and such doping may produce semiconducting films with a lower concentration of vacancies. Y. Bar-Yam, T.D. Moustakas, "Defect-induced stabilization of diamond films," Nature, 342, 786-7, 14 December 1989. (Weizmann Institute for Science, Rehovot, Israel; Boston University) A more general discussion of progress and pitfalls in diamond growth is presented by Yarbrough and Messier. W.A. Yarbrough, R. Messier, "Current issues and problems in the chemical vapor deposition of diamond," Science, 247, 688-96, 9 February 1990. (Pennsylvania State University, University Park) Interaction of fast ions with solids is the basis for phenomena such as Rutherford backscattering. Methods that originated as nuclear physics are providing useful tools for modifying and analyzing solids. Beams of dopant ions modify semiconductor, alloy, and ceramic solids, and the resulting depth distribution profile can be probed by the scattering of other ion beams, such as He+. A.D. Marwick, "Ion beam characterization and treatment," Metallurgical Transactions, 20A, 2627-36, December 1989. (IBM Watson Research Laboratory, Yorktown Heights, N.Y.) A description of the origins of brittleness in oxide glasses goes on to cover strengthening mechanisms. Surface modifications reduce the influence of superficial defects, while bulk composition, phase separation, and fiber or particulate reinforcement may increase both strength and fracture toughness. Successes (and limitations) are evenly presented. I.W. Donald, "Methods for improving the mechanical properties of oxide glasses," Journal of Materials Science, 24,4177-208, December 1989. (Atomic Weapons Establishment, Berkshire, England) Author: BRUCE G. BUCHANAN Department of Computer Science University of Pittsburgh Pittsburgh, Pa. Scheduling tasks, or transactions, in a computer system involves finding ways to maintain or increase throughput. A special case for a transaction processing system is a task that takes a very long time, a "long-living transaction." One such task is creating a back-up copy of a database. An algorithm is presented for handling such transactions and identifies relevant parameters for consideration of this class of algorithms. K. Brahmadathan, K.V.S. Ramarao, "On the management of long-living transactions," Journal of Systems and Software, 11, 45-52, January 1990. (University of Pittsburgh) Hashing is ubiquitous in computing. Yet the behavior of different hashing algorithms is not as well studied or understood as the volume of literature on the topic would suggest. Several hashing algorithms are examined, and suggestions are made for some criteria and parameter settings for selecting a good hashing function. B.J. McKenzie, R. Harries, T. Bell, "Selecting a hashing algorithm," Software - Practice and Experience, 20, 209-24, February 1990. (University of Canterbury, Christchurch, New Zealand) Collection-oriented languages are based on data structures that represent collections of items and operations for manipulating the collections. Some of these languages allow nested collections and nested operations, which have not been allowed in most parallel systems. Compiler techniques are presented that allow execution of nested parallel operations on flat parallel hardware. G.E. Blelloch, G.W. Sabot, "Compiling collection-oriented languages onto massively parallel computers," Journal of Parallel and Distributed Computing, 8, 119-34, February 1990. (Carnegie Mellon University, Pittsburgh; Thinking Machines Corp., Cambridge, Mass.) Before queries to a relational database are processed, it is important to determine a form that can be processed efficiently. Site selection is one consideration in handling distributed queries, and one that has received relatively little attention. Four algorithms are compared in the context of large distributed databases. T.P. Martin, K.H. Lam, J.I. Russell, "An evaluation of site selection algorithms for distributed query processing," The Computer Journal, 33, 61-70, February 1990. (Queen's University, Kingston, Ontario) Hypertext offers considerable flexibility in presenting large amounts of information, especially the kinds of loosely structured information that appear in documentation and instructional systems. Navigation - finding one's way through the text - remains a serious problem in hypertext systems. Some approaches to the problem are presented. J. Ni, "The art of negotiating through hypertext," Communications of the ACM, 33, 297-310, March 1990. (Technical University of Denmark, Copenhagen) Author: RON MAGOLDA Medical Products Department E.I. Du Pont de Nemours & Co. Wilmington, Del. The molecular level of understanding of the initiation of intracellular signals offers opportunities to regulate cell growth and differentiation. Conformational analysis of several X-ray structures of ras proteins with GTP and GDP offers a view of this molecular switch's "on" and "off" states. M.V. Milburn, L. Tong, A.M. deVos, A. Brunger, et al., "Molecular switch for signal transduction: structural differences between active and inactive forms of protooncogenic ras proteins," Science, 247, 939-45, 23 February 1990. (University of California, Berkeley; et al.) The latest modification of catalytic antibodies offers a significant advantage, which can improve turnover rates. K.D. Janda, M.J. Weinhouse, D.M. Schloeder, R.A. Lerner, S.J. Benkovic, "Bait and switch strategy for obtaining catalytic antibodies with acyl-transfer capabilities," Journal of the American Chemical Society, 112, 1274-5, 31 January 1990. (Scripps Clinic, La Jolla, Calif.; Pennsylvania State University, University Park) A new synthetic method describes the generation of simple starting materials into a polycyclic system, in a stereochemically controlled fashion. T.A. Brandvold, W.D. Wulff, A.L. Rheingold, "Efficient entry to bicyclic lactones via van Halban-White cyclizations," JACS, 112, 1645-7, 14 February 1990. (University of Chicago; University of Delaware, Newark) Atherosclerosis is a disease that afflicts a growing portion of the U.S. population. The early stages of the disease involve foam cells. The structure presented on the type I macrophage scavenger receptor should offer some insight into the generation of foam cells and the development of atherosclerosis. T. Kodama, M. Freeman, L. Rohrer, J. Zabrecky, et al., "Type I macrophage scavenger receptor contains à-helical and collagen-like coiled coils," Nature, 343, 531-5, 8 February 1990. (Massachusetts Institute of Technology, Cambridge; Massachusetts General Hospital, Boston; Integrated Genetics/Genzyme, Framingham, Mass.; et al.) A practical asymmetric synthesis of à-amino acids via an electrophilic animation method is presented. W. Oppolzer, O. Tamura, "Asymmetric synthesis of à-amino acids and à-N-hydroxyamino acids via electrophilic animation of bornanesultam-derived enolates with 1-chloro-1-nitrosocyclohexane," Tetrahedron Letters, 31 (7), 991-4, 1990. (Universit‚ de Genüve, Switzerland)

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