The present volume "Excitation Energy and Electron Transfer in Pho. tosynthesis" is dedicated to a colleague and dear friend Warren L. But ler. I first met Warren when he visited the University of Illinois at Ur bana during the early sixties; he left an indelible impression on me as a person with warmth and enthusiasm. Initially, he was someone I looked to for guidance, but later we also became friends. Whenever I passed through Los Angeles, I always telephoned Warren and often end ed up taking a plane to San Diego to stay with two wonderful people, Warren and his wife Lila. His invitations could never be refused. Below I reproduce the words of Herbert Stern on Warren L. Butler's life; these words express my sentiments as well as those of many of Warren's friends: ''A lifetime of acedemic creativity criss-crossed by streaks of highbrow and lowbrow fun. There is no summary to this adventure be cause we can neither make nor proclaim an end. Warren has bequeathed us his garden of academic treasures. It is ours to keep and tend. There is lots of joy in our many recollections of Warren's life and sorrow's foil can only brighten the brightness that the joy radiates."

Plant hormone research is the favorite topic of physiologists. Past three decades have witnessed that this subject has received much attention. The inquisitive nature of human mind has pumped much in literature on this subject and this volume is the product of such minds. In the following pages various hormonal-controlled physiological processes like, flowering, seed dormancy and germination, enzyme secretion, senes cence, ion transport, fruit ripening, root growth and development, thig momorphogenesis and tendril thigmonasty have been included. The volume also contains a review paper on 'Growth Regulating Activity of Penicillin in Higher Plants' and has been presented for the first time. The vast contents of each review paper have been written by erudite scholars who have admirably carried out their evangelic task to make the text up TO date. This volume, I am sure, would stimulate the appetite of researchers of peripheral disciplines of botany and agricultural sciences and they will continue to enjoy the fun and adventures of plant hormone research. Save one. my most outstanding debts are due to the rich array of the contributors and other plant physiologists specially to Prof. Thomas Gaspar (Belgium), Prof. E. E. Goldschmidt (Isreal), Prof. H. Greppin (Switzerland), Dr. K. Gurumurti (India), Prof. M. A. Hall (U. K. ), Prof. H. Harada (Japan), Dr. M. Kaminek (Czechoslovakia), Dr. J. L. Karm oker (BangIa Desh), Prof. Peter B. Kaufman (U. S. A. ), Dr. V. I. Kefeli . / (U. S. S. R. ), Dr. M. Kutaoek (Czechoslovakia), Prof. S."

These volumes contain an excellent, up-to-date review of the rapidly developing field of photosynthesis research. They comprise the proceedings of the Ninth International Congress on Photosynthesis, held in Nagoya, Japan, in 1992, which was the first in the series to be held in Asia. The Congress was attended by over 1000 active participants from 43 countries, who contributed plenary lectures, symposium talks, posters and discussions. The volumes contain most of these contributions, in the form of review papers and short communications, assembled in 26 chapters, which cover a wide variety of subjects, such as: fundamental aspects, excitation energy transfer, primary reactions, bioenergetic processes, carbon metabolism, and its metabolic and genetic regulations; applied aspects, herbicides and artificial photosynthesis, and environmental aspects, photosynthesis under stress conditions and global climate change. Research in Photosynthesis is therefore an important document, containing the latest high-level information about photosynthesis in its broadest sense. The four volumes are a valuable reference source. Furthermore, the volumes are important for background study for those enetering this exciting and promising field of investigation.

Forty years ago, when PLANT AND SOIL first appeared, Europe was still recovering from the devastating effects of World War II. During the war years, work in many centres of agricultural research had come to a virtual standstill. Buildings and equipment were destroyed, scientists were often forced to terminate their research and teaching activities and funds allocated to such work were diverted to other, at that time, more pressing needs. During the first post-war years reconstruction was undertaken with great zeal and in that light the founding of the new journal PLANT AND SOIL must be viewed. In the pre-war period most agricultural science journals were still primarily national ones and consequently many articles were published in languages mastered by only a limited number of potential readers. In small countries whose languages are not widely understood, the desire arose to publish research findings in one of the major languages. It is therefore understandable that in the early years of the journal's existence, large portions of PLANT AND SOIL were filled with articles from the Scandinavian countries and The Nether lands. Originally, rather frequent use was made of the opportunity to publish also in German and French, but with the advance of English as a major language of communication, a decline was noticeable in the number of German and French manuscripts submitted. As a consequence the Edi torial Board has recently decided to terminate the publishing of articles in these languages."

Thi s book is a general introduction into in vivo chlorophyll fluorescence and contains the contributions of the first International Ch 1 orophyll Fluorescence Sympos i urn he 1 din the Phys i kzentrum Bad Honnef, F. R. G. from June 6 to 8, 1988. This Symposium was made possible by a generous support from the Wilhelm and Else Heraeus Foundation, Hanau, which is gratefully acknmvledged. The book not only comprises all aspects of the applications of chlorophyll fluorescence in photosynthesis, stress physiol ogy, hydrobiology and remote sensing, but also gives access to measuring techni ques, data acqui si ti on and earl i er 1 iterature references. Thus it is far more than just a common proceedings book, it is a general introduction to all forms of application of the non-destructive in vivo chlorophyll fluorescence including the newest results. In a first chapter the inverse correlation between in vivo chlorophyll fl uorescence and photosynthet i c quantum convers i on and CO? -ass i mi 1 at ion is outlined, the origin and life-time of the chlorophyll fluOrescence at room and liquid nitrogen temperatures are given as well as the induction kinetics (Kautsky effect) and the methodo 1 ogi ca 1 approaches to regi ster different forms of chlorophyll-fluorescence signatures."

For centuries biologists have been extremely interested in the structure of desert plants as examples of natural selection to harsh environmental conditions. Indeed, desert plants are frequently used as examples in many biology classes and textbooks to illustrate natural selection, but this has led to an unfortunate litany of errors and misconceptions about desert plant adaptations.
This new synthesis focuses on plants of lowland tropical and subtropical arid deserts. Readers will be surprised to discover that many features commonly ascribed to desert plants are rareley observed in the most common species. Instead, the typical structural adaptations of nonsucculent warm desert plants are now viewed as ways to maximize photosynthetic rate.

The Second International Congress on Photosynthesis Research took place in Stresa, Italy during June 24-29, 1971; two centuries after the discovery of Photosynthesis by Joseph Priestley in 1971. This important anniversary was celebrated at the Congress by a learned account of Priestley's life and fundamental discoveries given by Professor Robin HILL, F. R. S. Professor HILL's lecture opens the first of the three volumes which contains the contributions presented at the Congress. The manuscripts have been distributed into three volumes. Volume I con tains contributions in the areas of primary reactions and electron transport; Volume II ion transport and photophosphorylation, and Volume III carbon assimilation, regulatory phenomena, developmental aspects, and from the two special sessions of the Congress devoted to evolution and photorespiration. It is realized that this division is necessarily somewhat arbitrary since many contributions relate to more than one of the above mentioned titles. However, the large number of contributions (over 3000 typed pages) made it impossible to publish the proceedings in less than three volumes. The printing of these volumes and the organization of the Congress were made possible by a contribution from the Consigio Nazionale delle Ricerche of Italy. The generous support of the Istituto Lombardo Acca demia di Scienze e Lettere to the publication of these proceedings is gratefully acknowledged. The editors wish to express their appreciation to all the scientists who contributed the results of the investigations, for their coopera tion; and to Drs."

Molecular biology, particularly molecular genetics, is among the newest and most powerful approach in modern photosynthesis research. Development of molecular biology techniques has provided new methods to solve old problems in many biological disciplines. Molecular biology has its greatest potential for contribution when applied in combination with other disciplines, to focus not just on genes and molecules, but on the complex interaction between them and the biochemical pathways in the whole organism. Photosynthesis is surely the best studied research area in plant biology, making this field the foremost candidate for successfully employing molecular genetic techniques. Already, the success of molecular biology in photosynthesis has been nothing short of spectacular. Work performed over the last few years, much of which is sum marized in this volume, stands in evidence. Techniques such as site-specific mutagenesis have helped us in examining the roles of individual protein domains in the function of multiunit complexes such as the enzyme ribulose-l,5-bisphos phate carboxylase/oxygenase (RUBISCO) and the oxygen evolving photo system (the photosystem II). The techniques of molecular biology have been very important in advancing the state of knowledge of the reaction center from the photosynthetic bacteria whose structure has been elegantly deduced by H. Michel and 1. Deisenhofer from the X-ray studies of its crystals."

Taking readers out of the laboratory and into the humid tropical forests, this comprehensive volume explores the most recent advances occurring in tropical plant ecophysiology. Drawing on the knowledge of leading practitioners in the field, this book synthesizes a broad range of information on the ways in which tropical plants adapt to their environment and demonstrate unique physiological processes. This book is arranged into four sections which cover resource acquisition, species interactions, ecophysiological patterns within and among tropical forest communities, and the ecophysiology of forest regeneration. These sections describe plant function in relation to ecology across a wide spectrum of tropical forest species and growth forms. How do different species harvest and utilize resources from heterogeneous tropical environments? How do patterns of functional diversity reflect the overwhelming taxonomic and morphological diversity of tropical forest plants? Such fundamental questions are examined in rich detail. To illuminate the discussions further, every chapter in this book features an agenda for future research, extensive cross referencing, timely references, and the integration of ecophysiology and the demography of tropical species where the data exist. Tropical Forest Plant Ecophysiology provides plant scientists, botanists, researchers, and graduate students with important insights into the behavior of tropical plants. Biologists and foresters interested in tropical ecology and plant physiological ecologists will also benefit from this authoritative and timely resource.

A recent volume of this series (Signals and Signal Transduction Pathways in Plants (K. Palme, ed.) Plant Molecular Biology 26, 1237-1679) described the relay races by which signals are transported in plants from the sites of stimuli to the gene expression machinery of the cell. Part of this machinery, the transcription apparatus, has been well studied in the last two decades, and many important mechanisms controlling gene expression at the transcriptional level have been elucidated. However, control of gene expression is by no means complete once the RNA has been produced. Important regulatory devices determine the maturation and usage of mRNA and the fate of its translation product. Post-transcriptional regulation is especially important for generating a fast response to environmental and intracellular signals. This book summarizes recent progress in the area of post-transcriptional regulation of gene expression in plants. 18 chapters of the book address problems of RNA processing and stability, regulation of translation, protein folding and degradation, as well as intracellular and cell-to-cell transport of proteins and nucleic acids. Several chapters are devoted to the processes taking place in plant organelles.

This is the third book chronicling the scientific activities of the European Society for Photobiology (ESP). It contains 56 chapters, written by authors from 16 countries, based on presentations at the 3rd Congress of the European Society for Photobiology held in Budapest, Hungary on the 27th August - 2nd September 1989. The science of photobiology, which can simply be defined as the study of the effects of light on living matter, covers so many subject areas that no single book can hope to do justice to them all. This multidisciplinary nature of photobiology is reflected by the material covered in this volume, which contains chapters on such diverse themes as motile photoresponses in bacteria, cancer therapy and photosynthesis. Interestingly, the emphasis placed on various subject areas differs quite markedly from the preceding volume ('Light in Biology & Medicine, volume 1, eds. R. Douglas, J. Moan & F. Dall'Acqua, Plenum Press, 1988). It is hoped that by highlighting different areas of photobiology these and future pUblications emanating from the ESP will, in time, produce a comprehensive record of photobiological research, . not only in Europe but throughout the world. Unlike many conference proceedings all the chapters con tained within this book have been subjected to rigorous peer review and several potential contributions were rejected during the editing process. Furthermore, most manuscripts underwent extensive editing to try and produce chapters of a uniform format and standard."

Electrical Manipulation of Cells provides an authoritative and up-to-date review of the field, covering all the major techniques in a single source. The book features broad coverage that ranges from the mechanisms of action of external electrical fields on biological material to the ways in which electrical stimuli are employed to manipulate cells. Bringing together the work of leading international authorities, the book covers membrane breakdown, gene delivery, electroporation, electrostimulation, cell movement, hybridoma production, plant protoplasts, electrorotation and stimulation, and electromagnetic stimulation. For each topic, the authors discuss the relevance of the approach to the current state of the art of biotechnology. Electrical Manipulation of Cells is an unmatched source of information for anyone involved in the manipulation of cells, particularly biotechnologists, cell biology, microbiologists, biophysicists and plant scientists. For researchers, the book provides technical material that ccan be employed in their own work. Students will gain thorough appreciation of the applications of this important technique.

This volume contains the papers, presented during a conference, organized jointly by the "Opzoekingsstation van Gorsem" and the "Limburgs Universitair Centrum", Belgium from 22 to 27 August 1982. For this third meeting, the chosen topic was the effect of different stresses on photosynthesis. Most of the research in this field is realized on water stress and temperature stress; this situation is refllected in the conference programme. However, the imp- tance of the other factors such as light, CO , salinity, anaerobiosis, was 2 also emphasized especially during the important discussion sessions. We express our gratitude to Drs. J. Gale, P. Jarvis, G.H. Krause, P.E. Kriedemann and P.S. Nobel for their excellent leadership during the discussion sessions. Particular thanks are also due to Dr. H.~i. Woolhouse who gave us an excellent inaugural address and whose erudition largely contributed to the interest of the discussions. For the first time in our experience of editors, we decided to use camera ready copies in order to publish more rapidly the proceedings and at a lower price. For a lot of reasons (among other things the bad choice of type of letter to be used and the choice of instructions to authors which were not perfectly followed by the authors), the technical presentation of this book will appear as non homogeneous; we accepted this lack of homogeneity with the hope tbat the publication time would be shorter in spite of the fact that, some authors delivered their manuscript with delay.

With the demonstration of the "triple response" in plants by Neljubow at the turn of the century, ethylene has been identified as a substance specifically affecting plant growth. Yet it took a few more decades to show that ethylene is a naturally occurring product of plants having all the characteristics of a phytohormone. Ever since much effort has been devoted to a wide variety of physiological and biochemical problems relevant to ethylene. A first meeting was organized in Israel in 1984 to bring together many people active in this rapidly expanding field of experimental research. It is the aim of the present symposium to provide once more a forum at which researchers might expose and comment progress in their work over the last few years. Speakers were invi ted and their contri buti ons ordered ina number of sessions, each of which was centered on a particular topiC. Much of the benefit came from ensuing discussion sessions which were conducted with much competence and expertise by Anderson, Ben-Arie, Goren, Morgan and Osborne. All of these colleagues are recognized leaders in ethylene research today and the organizers owe a very special gratitude to them for their substantial contribution to the programme. It is well to remember the friendly atmosphere, so essential to the success of the whole meeting and so much enjoyed by every partiCipant. Prompt publi ca tion of the papers was made possi ble by the camera-ready procedure offered by the publisher.

Fertilizer is a vital component of strategies for expanding foodproduction. The rapid growth in population and the widening food deficits inmany tropical countries of Asia, Africa, and Latin America call attention to those aspects of fertilization that have been neglected but are expected to yield large economic payoffs in the future. Fertilizer sulfur falls into this category. In the past fertilizer sulfur received little attention from researchers and policymakers since sulfur deficiency was not considered a serious problem. It was not a problem because of low crop yields, extensive cropping, and the incidental supply of sulfur through rain, irrigation water, manures, and sulfurcontaining fertilizers. However, the situation has changed in the last three decades. Moder nagriculture based on high crop yields, intensive cropping, improved crop varieties, and greater use of sulfur-free fertilizers and environmental regula tions restricting sulfur emissions are creating large gaps between sulfur sup ply and sulfur requirements. Sulfur deficiencies are widespread and grow ing. Consequently, the full potential of a modern agricultural system in tropical countries is not being realized. This research effort results from the recognition of the seriousness of the sulfur problem and its adverse impact on food production as well as IFDC's dedication to the development and transfer of economically ef ficient fertilizer technology to tropical countries. This study represents a comprehensive analysis of the technical and economic linkages between fer tilizer sulfur and food production, and it provides guidelines for future directions in fertilizer sulfur research and public policy."

The Twenty-Second Jerusalem Symposium reflected the high standards of these distinguished scientific meetings, which convene once a year at the Israel Academy of Sciences and Humanities in Jerusalem to discuss a specific topic in the broad area of quantum chemistry and 'biochemistry. The topic at this year's Jerusalem Symposium was Perspectives in Photosynthesis, which constitutes a truly interdisciplinary subject of central interest to biophysicists, and biologists. chemists The main theme of the Symposium was built around a conceptual framework for the acquisition, storage and useful disposal of energy in photosynthetic reaction centres. Emphasis was placed on the elucidation of primary charge separation processes in photosynthesis and their exploration within the framework of the electron transfer theory, on the interrelationship between structural data, inter actions and electron transfer kinetics, and on the role of protein dynamics in primary processes in photosynthesis. The interdisciplinary nature of these research areas was deliberated by intensive and extensive interactions between scientists from different disciplines and between theory and experiment. This volume provides a record of the invited lectures at the Symposium."

Large regions of the world are regularly burnt either deliberately or naturally. However, despite the widespread occurrence of such fire-prone ecosystems, and considerable body of research on plant population biology in relation to fire, until now there have only been limited attempts at a coherent conceptual synthesis of the field for use by students or researchers.

Studies in floral biology are largely concerned with how flowers function to promote pollination and mating. The role of pollination in governing mating patterns in plant populations inextricably links the evolution of pollination and mating systems. Despite the close functional link between pollination and mating, research conducted for most of this century on these two fundamental aspects of plant reproduction has taken quite separate courses. This has resulted in suprisingly little cross-fertilization between the fields of pollination biology on the one hand and plant mating-system studies on the other. The separation of the two areas has largely resulted from the different backgrounds and approaches adopted by workers in these fields. Most pollination studies have been ecological in nature with a strong emphasis on field research and until recently few workers considered how the mechanics of pollen dispersal might influence mating patterns and individual plant fitness. In contrast, work on plant mating patterns has often been conducted in an ecological vacuum largely devoid of information on the environmental and demographic context in which mating occurs. Mating-system research has been dominated by population genetic and theoretical perspectives with surprisingly little consideration given to the proximate ecological factors responsible for causing a particular pattern of mating to occur.

The Conference on Tropical Rainforest Research: Current Issues was organised by the University of Brunei Darussalam and The Royal Geographical Society, London, and held in Bandar Seri Begawan, Brunei Darussalam, over 8 days in April 1993. Over 160 participants from 22 countries attended the Conference, which was opened on the 9th April by the Brunei Darussalam Minister for Home Affairs, Yang Berhormat Pehin Orang Kaya Laila Setia Bakti Diraja Dato Laila Utama Haji Awang Isa bin Datu Perdana Menteri Dato Laila Utama Haji Awang Ibrahim. The conference was initially intended to provide a forum to present the results of the expedition into the lowland mixed dipterocarp forest of the Temburong District of Brunei Darussalam, which had been jointly organised by the University of Brunei Darussalam and The Royal Geographical Society, London. The 15-month expedition, lasting from January 1991 to March 1992, was based at the then newly-completed Kuala Belalong Field Studies Centre, a research and education facility set up by the University with funds provided by the Brunei Government and Brunei-Shell Petroleum Sdn Bhd. The expedition, with over 70 scientists taking part, received financial support from a wide range of sponsors and Corporate Patrons, including Royal Brunei Airlines, the Baring Foundation, Daiwa-Dicam, Greencard Trust, the Hongkong Bank, Morgan Grenfell and Nomura-Nimco. The conference itself was supported by donations from Brunei-Shell Sdn Bhd, Royal Brunei Airlines and Standard Chartered Bank.

China was the first country to use cytoplasmic male sterility to develop hybrid rice for commercial use in 1973. In 1986 more than 8 million hectares of hybrid rice were planted in China, which is one fourth of the total rice area and produces one third of the total rice in the country. Hybrids usually out yield the leading commercial varieties by -20-30%, giving an average yield advantage of 1 to 1. 5 t/ha, because of their better morphological traits, higher physiological efficiency, better resistance to major diseases and insects, and wide adaptability to various agro-ecological stresses. IMPROVEMENT OF HYBRID RICE A. Mutation techniques Almost all of the cultivated F1 rice hybrids in China are developed from cytoplasmic male sterile and restorer lines. According to surveys made in recent years, more than 30 sources of cytoplasmic male sterility in rice can be identified, among which only six are being commercially used (Table 1). Wild rice with aborted pollen (WA) cytosterility system is the most popular one in use to develop male sterile lines (MS line) in China. The main technique available for developing stable MS lines is sUbstitution backcrossing of the genome of one species into alien cytoplasm of another. Sufficient backcrosses are required to eliminate all nuclear genes derived from the cytoplasm donor species. A number of studies have shown that using interspecies crosses, such as the cross of wild rice (Q. perennis, Q. sativa, f.

The West African Sahel is the transition zone between the Saharadesert in the north of Africa and the more humid Sudanian zones in the south. Although diverse in many ways, the Sahelian countries have the common problem of a fragile agricultural sector. This predicament is mainly caused by low inherent soil fertility, limited and unpredictable rainfall, frequent droughts, and wind erosion that accelerates soil degradation and desertification, compounded by To assure food production in the future, means rapidly growing populations. of declining soil fertility and increasing must be found to offset the trends soil degradation through wind erosion. This is a challenge for agricultural research. Since 1985, the Special Research Program 308 'Adapted Farming in West Africa' at the UniversityofHohenheimin collaboration with the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) in Niger, has pursued the developmentof agricultural innovations for smallholder farmers in one of the most ecologically fragile regions of the world. The prevention of soil degradation, the restoration and maintenance of soil fertility, and the increase of land and labor productivity are key objectives of this multidisci plinary research program. From the beginning, a major focus of research has been wind erosion.

Nearly twenty years have passed since the publishing of a broadly-based texthook on hiolog ical control (Huffaker and Messinger 1976). In the interim, other works on biological control have been either briefer treatments (DeBach and Rosen 1991), or collections of essays on selected topics (Waage and Greathead 1986; Mackauer and Ehler 1990). Our text has been written to fill what we believe is a need for a well-integrated, broadly-based text of appropriate length and degree of technical detail for teaching a one semester upper level course in hiological control. We have attempted to focus on principles and concepts, rather than on biological control of particular taxa or hiological control by particular kinds of natural enemies. Therefore, for example, the reader will find the material on biological control of weeds integrated with biological control of insects and mites into chapters on principles, techniques, and applications rather than presented separately. Only biological control of plant pathogens is addressed independently, an appraoch made necessary by the many special features and concepts in plant pathogen biological control."

This book addresses the responses of plants to salinity. Although salinity is a common environmental factor for marine organisms, for the majority of land plants high soil salinity is an environmental constraint that limits growth, productivity, and normal plant functions. Salinity is particularly widespread in arid/semiarid climates where crop production depends on irrigation.

A comprehensive approach is taken in this book. After discussing salinity as an environmental soil factor and its global impact on ecosystems, plant responses are covered from the whole-plant level through metabolic changes to the underlying molecular and genetic mechanisms. In contrast to other books in this subject area, which focus on certain aspects of plant responses to salinity or are conference proceedings, this is the only comprehensive new book on this subject, written by experts in the field. The intended level of readership is graduate students and advanced researchers interested in environmental biology and specifically in the area of mechanisms of environment-plant interactions.

The last decade has seen tremendous progress in our knowledge of the pollen development and gene expression on one hand and the characterization of pollen specific proteins on the other. In compiling the chapters for this volume, we have pragmatically categorized these basic developments in pollen molecular biology and biotechnology into two sections based on their applications in agricul ture and implications in medicine. Pollen developmental biology and gene expression: applications in agricul ture. Pollen development is an extremely complex process encompassing a series of biochemical, physiological and genetic events. At the basic level, sporophyt ically expressed genes may expound our knowledge of unique processes of cellular differentiation which ultimately give rise to a full-fledged organism. At the applied level, the studies on the pollen and male sporophyte-specific gene expression, and of promoters and transcription factors of relevant genes have the potential to manipulate the fertility in certain cash crops leading to agricultural biotechnology."

Plants are composed of 17 essential and at least 5 beneficial elements, and these must be taken up as metal or nutrient ions to allow for growth and cell division. Much effort has been devoted to studying the physiology and biochemistry of metals and nutrients in plants. The aspect of cell biology, however, is an emerging new field and much needs to be learned about sensing, long-distance communication within plants, and cellular signal transduction chains in response to environmental stress. Cellular malfunction and consequently disease result when any of the key steps in metal and nutrient homeostasis are disrupted. Working together, leading experts in their respective fields provide a new concept that reaches beyond plant nutrition and plasmalemma transport into cellular physiology. Each chapter contains basic information on uptake, physiological function, deficiency and toxicity syndromes, long-distance and intracellular transport. The discussion is devoted to metals and nutrients where recent progress has been made and highlights the aspects of homeostasis and sensing, signaling and regulation, drawing parallels to other organisms including humans. Finally, the book identifies gaps in our current knowledge and lays out future research directions.