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0 Department of Botany, University of Toronto, Toronto, Ontario, M5S3B2 Canada
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INTRODUCTION |
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 TOP INTRODUCTION LITERATURE CITED |
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These six titles are fairly specialized, addressing germination biology (Seed germination in desert plants, Y. Guttermann, 1993, 253 pages), mineral nutrition (Mineral Nutrition of Desert Plants, A. D. Day and K.L. Ludecke, 1993, 117 pp.), psammophytes (Plants of Desert Dunes, A. Danin, 1996, 177 pages), structure (Structure-function relations of warm desert plants, A.C. Gibson, 1996, 215 pages), economic plants (Ecophysiology of economic plants in arid and semi-arid lands, G. E. Wickens, 1998, 343 pages), and dispersal (Dispersal Biology of Desert Plants, K van Rheede van Oudtshoorn and M. W. van Rooyen, 1999, 242 pages). Gibson's Structure-Function Relations of Warm Desert Plants initially appears to be a synthesis of structure and physiology, however, it is primarily an overview of the anatomy and morphology of desert species, with a relatively limited treatment of the associated physiology. With the exception of Plant Nutrients in Desert Environments (which is a very general summary of the mineral nutrition of economic plants and says little about the nutrition of wild plants of arid regions), all of these specialized books are valuable references. Although their price of $99 to $192 may preclude most folks from purchasing them, they provide valuable examples and case studies of the unique manner in which plants survive harsh, arid environments. Most of these titles are richly illustrated to show how specific features control the timing of germination and dispersal, or promote survival in shifting sands and during severe drought. Many wonderful examples are to be found for teaching or term papers, or simply enriching a field trip into an arid region. One notable contribution of these books is that they include desert species from around the world and thus allow for comparisons of many of the exotic forms to be found in diverse extremes of the planet. Wicken's Ecophysiology of Economic Plants of Arid and Semi-Arid Lands is particularly useful in this regard. For those familiar with only North American deserts, his comparisons of plants from the Asian, Australian, and African deserts will be a valuable, mind-expanding experience. The ethnobotanical value of Wicken's text is also substantial. The last third of the book describes the biology and use of some 170 species, most of which will be unfamiliar to readers in the developed world.
Because of the specialized focus of these six titles, however, the synthesis and integration of physiology and ecology are not emphasized, and in this light Smith et al.'s Physiological Ecology of North American Desert Plants stands out as a unique contribution.
Physiological Ecology of North American Desert Plants emphasizes the carbon, water, energy, and nutrient relations of desert plants of varying growth form, with comparatively little treatment of structural adaptations, herbivore defense, and reproductive strategies. The emphasis on carbon, water, energy, and nutrient relations—the bread and butter topics of plant physiological ecology—is a legacy of developments from the 1960s and 1970s that focused the discipline on biochemical controls and the dynamics of material and energy flux (Mooney, Pearcy and Ehleringer, 1987
). Desert studies were instrumental in the development of this new focus, as they were among the first to incorporate biophysical theory such as the water potential concept into experimental protocols. With the development of sophisticated gas exchange equipment, desert biologists were quick to exploit the new technology to assess trade-offs between water use and carbon acquisition. Later, when the theory linking carbon isotope discrimination to water use efficiency was developed, desert ecophysiologists were again pioneers in using this understanding to integrate seasonal and lifetime water use profiles of various functional types in heterogeneous environments (for example, see Ehleringer and Cooper, 1988
). This ability to integrate over multiple levels of complexity is key to current efforts to scale to the global level and explains in part the major contribution that physiological ecology has made to global change research.
In some ways, it is odd that desert research has had a major influence over the development of physiological ecology. From an anthropocentric viewpoint, deserts have been historically considered barren wastelands, with little intrinsic value. North American deserts were no different, and up until the 1950s, they were underpopulated backwaters that engendered little interest from society at large. So how is it that the deserts have had such an important role in plant biology, particularly physiological ecology? At one level, the focus on desert plants results from the extreme biology of so many resident species. An understanding of why these plants are unusual has led to a better understanding of adaptive mechanisms to environmental stress, and thus a more thorough view of evolution and physiological responsiveness. However, this has also contributed to a mistaken notion in the popular mind that deserts are freak shows populated by botanical oddballs. While an interest in extreme adaptations explains some of the work in desert systems, it does not explain how it was that so much research came about in biomes that just a few decades ago were outside the view of what most funding sources considered worthy of attention. Why the change?
In the North American case, the research emphasis on desert systems has resulted in considerable part from a combination of political and social factors associated with major historical events on the world scene. For one, the arid west of North America was the home of the cowboy, who became the romantic icon of the ranching industry. Ranching in the west was valuable as a means of initially populating the interior, both because it provided jobs and was the main source of protein for the many mining and railroad towns established throughout the region (Young and Sparks, 1985
). Because of the peculiarities of the American political system that gives disproportionately high political influence to the western states in general, and ranching in particular, substantial federal resources were allocated to the establishment of one or more land grant universities in each state of the arid region. Many of these land-grant universities were also associated with federal agriculture research stations, whose focus was the conservation of arid rangelands. Together, the land-grant universities and federal research stations brought to the region not just basic scientists, but also applied scientists, many of who were quick to adopt the new techniques of ecophysiology. Dwight Billings, for example, who is considered by many to be the father of modern physiological ecology, began his academic career at the University of Nevada in the early 1940s. Later, a number of his students conducted significant work at universities throughout the arid west. Both World War II and the Cold War played an important role. World War II led to substantial economic, population, and political growth in the western states, which later paid dividends in that the land grant institutions were better funded and staffed (Nash, 1985
). After the war, much of the arid west became a locale for military research, most notably at the nuclear test facility in the Nevada desert. Associated with this activity was the funding of research on desert ecosystems. For example, the atomic tests of the 1950s through early 1970s required assessments of radiation effects on local biota, but the funding for this work also supported background work on the physiology and ecology of desert vegetation (Wallace and Romney, 1972
). Importantly, the establishment of the Carnegie Institute of Washington Desert laboratory in Tucson, Arizona, and later the desert work of the Carnegie laboratory team at Stanford, California provided leadership and advanced training in desert ecophysiology (Billings, 1985
). One of the important contributions of the Carnegie group is that they networked extensively with leading international researchers, thereby bringing the latest advances from around the world into the local arena.
While Smith et al. do not address this background history, their book is a nice testament to the work of the many hundreds whose opportunities to study in the "arid wastes" resulted from much larger historical contingencies. As Smith et al. demonstrate, the consequence of the desert research in North America is a much-improved biological understanding that leaves us well poised to deal with some of the serious global challenges of the coming century.
Physiological Ecology of North American Desert Plants is loosely organized into three sections. The first section covered in Chapter 1 provides a background description of the geography, climate, soils and vegetation types within the deserts of North America. Herein, Smith et al. set forth many important distinctions. First, the deserts of North America are generally not true "deserts" as rigorously defined, but instead reflect a common perception of the general public. They are clearly arid systems, more properly called steppes or scrubland, with levels of precipitation and primary production that are above that of true deserts such as the Sahara. In recent decades, however, the North American deserts have been substantially desertified by overgrazing, biological invaders, and possibly climate change, so that the current systems increasingly resemble true deserts. The pattern and mechanism for the desertification of the arid lands of North American are common themes throughout the book and are treated here better than almost anywhere else. Second, the North American deserts are young by world standards. Aridification of the western region has occurred over the past 15–20 million years, driven in large part by long-term drying of the earth's climate and uplift of the Sierra/Cascade mountains, which created a rain shadow blocking moist air flow off the Pacific. The role of climate factors in distinguishing the deserts is well delineated and sets up the discussion of plant functional roles. The Great Basin, or "cold" desert, is distinguished from the others by harsh winters and dry summers, while the "warm" deserts (the Chihuahuan, Mojave, and Sonoran) are distinguished from each other by the timing of precipitation. The Mojave desert receives most of its precipitation in winter and thus has a rich, C3- dominated flora with relatively few perennial grasses. In contrast, the Chihuahuan desert has significant summer precipitation arising from monsoon air flow off the Gulf of Mexico. This supports a diverse flora of C4 grasses, many of them rhizomatous perennials that once formed extensive grasslands. The Sonoran desert has a bimodal pattern of winter and summer precipitation and is the warmest of the four. Its mild winters, coupled with high evapotranpiration, enable a subtropical flora of succulents and cacti to extend into the southwestern United States, thus providing the image of the lonesome saguaro and other "spiny, stunted and tenacious" xerophytes that dominate the popular literature of desert ecosystems (for example, see Abby, 1968
).
As Smith et al. point out, the plants actually found in the North American deserts represent a far greater collection of life forms than cacti and succulents, and Chapters 2 through 10 detail mechanistic explanations for the success of these various functional types. Chapter 2 represents the second section of the book in that it provides a general background of the major processes affecting performance of desert plants, particularly in relation to stress. Thus, C3, C4, and CAM (crassulacean acid metabolism) photosynthesis, drought adaptations, the role of osmotic adjustment, root/shoot allocation, and its influence on growth are all reviewed. Much of Chapter 2 could serve as a general ecophysiology chapter in a textbook and would be useful background reading for a graduate plant ecology or ecophysiology course. However, because it assumes substantial prior knowledge of photosynthesis, gas exchange and water relations, this chapter, and much of what follows, is too advanced for many nonspecialists who have not completed a preparatory course in plant physiology. For those who have the background, much is to be gained or refreshed; for those lacking it, this is not the best book to learn about desert systems or physiological ecology.
The third section of the book is represented by Chapters 3 through 10, which provide detailed case studies of the major plant life forms in the four deserts, typically comparing and contrasting species of similar form from different deserts, or species of different form within the same desert. It is in these chapters that one gains an appreciation for the variation in life form and physiology in the contrasting habits. Chapter 3 compares the varieties of the evergreen shrub Artemesia tridentata, the major codominant of the cold desert, with the varieties of Larrea tridentata, the dominant shrub of each of the warm deserts. While cacti and succulents form the common image of deserts in the popular mind, it is these two xerophytic shrubs that more than anything characterize the vegetation cover of the North American deserts. The dominance of these species is linked to their ability to withstand severe drought on the shallow soils of bajadas and basins that predominate throughout much of the region, while segregation of these species into distinct ranges of dominance is shown to result from different physiological thresholds. The spread of Artemesia southward into the warm desert is limited by aridity, while the northward spread of Larrea is limited by cold temperatures. In this chapter, the details of the physiological performance of desert species are best documented. Thus, for example, we learn that both shrubs have high levels of potential productivity on a leaf area basis, but because of frequent droughts that reduce leaf area, these species actually have low annual production. The population biology of desert species is also best developed in this chapter. Both shrubs are profligate seed producers, but the seeds are short lived (for example, only for a single growing season in Larrea). Recruitment is low, as few years favor establishment. Individuals in a stand of Larrea may reflect successful recruitment from only one to two good years per century, but because of the great longevity of the shrubs (some Larrea clones may be over 10 000 years old; Vasek, 1980
), their survival is ensured. As Smith et al. discuss, this could change if land managers embark on widespread attempts to enhance economic productivity of arid lands. Such attempts are a common legacy in the arid west, where landscapes have often been cleared of woody vegetation to enhance grass productivity.
Chapter 4 addresses drought-deciduous shrubs and trees. Encelia farinosa, a small shrub that is common to warm deserts, is the first species emphasized, in large part because it possesses leaves that develop a highly reflective pubescence with increasing drought. This phenomenon is an excellent example of the value of phenotypic plasticity. Pubescence production in response to drought and heat enhances the ability of E. farinosa to maintain a leaf canopy into the dry season as well as exploit drought-prone soils away from stream washes. By contrast, the related shrub E. frutescens lacks the ability to develop a dense pubsecence, and thus it is confined to washes. While this classic story is commonly repeated in numerous plant ecology and physiology texts, the treatment here is clear and concise, yet reasonably thorough—a fine summary for any course in environmental biology.
Many of the drought-deciduous trees and shrubs rely on photosynthesis in stems, and the remainder of Chapter 4 addresses the functional significance of stem photosynthesis in a range of species, although palo verde (Cercidium microphyllum) is emphasized. Surprisingly, relatively little is said about the photosynthetic role of the deciduous leaves. This is an oversight not so much because the discussion of stem carbon assimilation is weak or misguided, which it isn't, but because the lack of focus on deciduous leaf biology is a lost opportunity to cover one of the more fascinating desert adaptations. Many species are able to rapidly produce inexpensive, drought-intolerant leaves following rain events, and then shed these as drought progresses. To the many readers of the temperate world, this contrasts with the familiar pattern of deciduousness in response to winter cold. Also, desert plants often produce a crop of fine "deciduous roots" following rain, and it would have been interesting had the authors compared the biology of deciduous leaf and root production. Much of what is covered regarding deciduous behavior is in a three-page summary at the end of the chapter. Here, the relative costs of the deciduous vs. evergreen habit are clearly compared, but the treatment is a bit brief given the importance of the drought-deciduous habit in arid regions.
Chapter 5 discusses stem succulents, primarily by comparing two cacti (the saguaro, Carnegiea gigantea, and the barrel cactus Ferocactus acanthoides) with one of the major Agave species (Agave deserti). Succulents are perhaps the most fascinating of desert plants for the average person, because of their distinct form and habit. Interestingly, they are the least adapted to internalize severe drought; instead, they avoid tissue desiccation by relying on CAM photosynthesis and high capacity for water storage. This chapter presents a clear overview of succulent plant biology, and the comparison of the cacti and Agave highlights key similarities and differences between leading dicot (Cactaceae) and monocot (Lileaceae) families that have converged on the succulent life form in arid environments. In addition, the crucial role of nurse plants in promoting cacti establishment and thus ecological succession within desert communities is well summarized. Surprisingly, however, given the diversity of succulents and the popular image that succulents dominate deserts, this chapter is one of the least detailed in the book. It is brief (15 pages, including a three-page summary) and only superficially covers CAM photosynthesis, the major photosynthetic pathway of desert succulents. The brevity of the treatment may reflect the state of the art. The CAM succulents are not readily amenable to measurements using standard ecophysiology equipment, and their slow growth limits the ability of researchers to finish a study during a typical funding cycle. In addition, they have limited economic value in the desert, in contrast to many grass species that have been extensively studied by range scientists. Much laboratory-based physiology has been conducted on CAM species, but these mainly address responses of a few model species.
The economic value of grasses may explain why one of the more detailed and informative chapters is Chapter 6, which covers in depth the cold desert grasses, mainly through a case study of the C3 bunchgrass Leymus cinereus (Great Basin wild rye). This chapter's strength is its discussion of the adaptive significance of the bunch, or tussock, growth habit that is common in the cold desert, but not in the warm deserts, where rhizomatous C4 grasses prevail instead. Despite its strengths, Chapter 6 has some notable weaknesses. For one, the choice of Leymus cinereus as a typical desert grass is questionable, because it is a relatively mesic species generally found on deeper soils with greater water availability. Second, while the authors emphasize bunch grasses, their treatment of the perennial grasses of the warm desert is modest, so the strength of the comparison between the rhizomatous grasses of the different desert systems is much weaker than the comparison between evergreen shrubs. This is a lost opportunity to raise some globally important issues. Why is it that cold desert grasses are largely bunchgrasses, while warm desert grasses are more likely to be rhizomatous? Photosynthetic pathway is also associated with this dichotomy, with the perennial C4 grasses tending to be rhizomatous, while the bunchgrasses are largely C3. Could this association have widespread significance?
Chapters 7, 8, and 9 are relatively brief treatments of phreatophytes (deep-rooted woody plants that tap into groundwater), desert annuals, and poikilohydric plants, respectively. In the phreatophyte chapter, the case study focuses on Prosopis glandulosa (honey mesquite). This is another curious choice, because P. glandulosa is widespread in the warm deserts, living both where the roots can reach groundwater, and where they can't. While it makes for an interesting comparison between individuals within a species that can act either as phreatophyes exploiting groundwater, or drought-deciduous plants not using groundwater, the authors largely ignore the classic, obligate phreatophytes of the region, such as cottonwoods, willows and the Washington palm. The discussion of P. glandulosa does bring out a number of valuable points, however. Unlike the obligate phreatophytes that are restricted to riparian areas, facultative phreatophytes such as Prospopsis can reside on drought-prone upland soils, but to do so they require annual floods to recharge water in soil horizons above the water table. Otherwise, recruitment can be curtailed because there is not enough water to support seedlings during taproot extension deep into the soil. Human exploitation of ground water is currently lowering water tables in the region, and flood control has reduced the recharge of soil water. Thus, the establishment probability of Prosopsis and other facultative phreatophytes is being impaired, causing a shift to a more xeric type of vegetation.
The chapter on annual plants emphasizes two solar tracking species, Lupinus arizonicus and Malvastrum rotundifoium. Desert annuals are noted for their high growth and photosynthesis rates, which are viewed as adaptations to ensure rapid growth and seed set during the ephemeral period of resource richness following rain. One means of achieving high photosynthetic performance is to load the leaves with high levels of protein; for this reason, desert annuals have the highest photosynthetic rates recorded in terrestrial plants. A second strategy is to invest less protein into leaves and, instead, rotate the leaves so as to track the sun as it moves across the sky. Solar-tracking plants are able to maintain maximum rates of photosynthesis throughout the day, while nontrackers reach a maximum for only a few hours of the day when the solar angle is high. Smith et al. compare these two strategies to show that they can result in similar levels of carbon gain, but the authors do not elaborate on the relative costs and benefits of each strategy. One point they emphasize is that desert annuals have little capacity for photosynthetic acclimation, unlike evergreen species, and are unable to handle severe drought. These examples highlight some contrasts between perennial life forms that must deal with extreme stress, and others such as desert annuals that avoid stress by operating during a restricted period when resources are abundant.
The chapter on poikilohydric plants addresses the distribution and physiological performance of cryptogams and resurrection plants that are able to completely dehydrate during the dry season. Generally, these groups have been treated as botanical curiosities having limited ecological significance. As Smith et al. point out, this is a mistaken view, because nitrogen fixation by cyanobacteria in lichens and microbiotic crusts covering bare soil is a significant source of nitrogen in cold-desert communities. The lead case study in this chapter is Selaginella lepidophylla, the resurrection spikemoss of the Chihuahuan desert. This species has become a model for the study of poikilohydric plant adaptations in the desert environments, and their discussion focuses on the physiology of the desiccation process and its adaptive consequences. For example, the curling observed in resurrection plants appears to be a mechanism for avoiding photoinhibitory damage. From a broader evolutionary standpoint, the inclusion of a primitive vascular plant is useful, because it shows that the nonseed reproductive habit is not necessarily a barrier to life in arid regions, as is often indicated in introductory botany classes.
The final case study focuses on two Eurasian exotics, Bromus tectorum and Tamarix ramosissima. Bromus tectorum (cheatgrass) is an annual C3 grass that may be the single most destructive invader in western North America other than humans. It was first introduced over a century ago and has now reached dominance in much of the cold desert (Mack, 1986
). In addition to being an effective competitor for water, B. tectorum greatly accelerates fire cycles because it produces a dense canopy of tinder during the dry season. Although most of the cold desert is a fire-adapted system, the acceleration of fire cycles combined with intense competition during seedling establishment has allowed B. tectorum stands to exclude most, if not all of the native vegetation on a site. In is now common to enter vast tracts of the cold desert following a series of fires to find solid stands of B. tectorum with only a few isolated, often dead native plants (D'Antonio and Vitousek, 1992
). With each passing year, B. tectorum marches on, changing the landscape from a gray desert dominated by sagebrush to a tinder-dry grassland (Christensen, 2000). In the summer of 1999, 7% of the entire area of the state of Nevada (Seventh largest in the United States) burned, largely due to B. tectorum fires (Bremmer, 1999
). The consequences for the region are enormous. Land dominated by B. tectorum provides little for cattle or wildlife, and its propensity to burn will require restricting access to public lands during the fire season, altering the recreation styles of millions of people. The ghost towns of the old mining days could soon be joined by ghost towns from our day, as ranching and tourism dry up following ecosystem conversion. Similarly, Tamarix invaders from central Asia have converted diverse riparian zones into monospecific stands that support little in the way of wildlife and are prone to fire. Smith et al. effectively summarize the dispersal history of these two invaders, but their treatment excels in its discussion of the mechanisms promoting the success of these invaders, and the associated implications for management. As they describe, both species achieve dominance through similar mechanisms. Compared to native competitors, both invaders germinate over a wider range of conditions, both grow and establish more rapidly, and in the process, both produce relatively large root systems that pre-empt surface moisture from slower growing natives. They are effective colonizers of disturbed sites, and are able to alter fire cycles in ways that favor further dominance. Importantly, Smith et al. discuss situations where the invaders fail; for example, dry springtime promotes high mortality of B. tectorum. Understanding the success, as well as the failure, will be critical to any possible control developed for these invasions.
Each of the case study chapters has an extensive summary, but these are often less devoted to summarizing the previous material in the chapter than for presenting new, and often valuable information. The management implications of the physiological understanding are often highlighted in the summaries, and this placement of the basic biology into the context of desert management and ecosystem conversion produces summaries that are some of the more interesting portions of the book.
In summary, the Physiological Ecology of North American Desert Plants is a fine addition to the bookshelf of all plant physiological ecologists, if they can afford the rather steep price. A prior visit to the deserts would also help enhance appreciation of the book, as many of the species mentioned are not illustrated by photos or drawings and thus are just names that will often be unfamiliar to the uninitiated. (By contrast, a number of the other texts in the Adaptation of Desert Organisms series have beautiful illustrations that bring to life the features of exotic plants many readers will never see.) The book does have gaps in coverage, but for its limited length it is a rich synopsis of the main features of the four North American deserts and the major plant functional groups. Particularly valuable is the focus on the mechanisms by which the invaders have found success. It is clear from their treatment that the challenges in dealing with these obnoxious guests will be great; however, through the synthetic understanding that this book facilitates, there is reason to hope that humanity may some day be able to restore these ecosystems to their former magnificence.
Submitted by S. C. H. Barrett, Book Review Editor.
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FOOTNOTES |
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LITERATURE CITED |
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TOPINTRODUCTIONLITERATURE CITED |
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Billings, W. D. 1985 The historical development of physiological plant ecology. In B. F. Chabot and H. A. Mooney [eds.], Physiological ecology of North American plant communities, 1–15. Chapman and Hall, New York, New York, USA.
Bremmer, F. 1999 Simple weed threatens rangeland disaster. Reno-Gazette Journal. Nov. 11. Reno, Nevada, USA.
Christensen, J. 2000 A deadly dance on the steppes of Nevada. New York Times. February 1. page D3. New York, New York, USA.
D'Antonio C. M., and P. M. Vitousek. 1992 Biological invasions by exotic grasses, the grass/fire cycle, and global change. Annual Review of Ecology and Systematics 23: 63–87
Ehleringer, J. A., and T. A. Cooper. 1988 Correlation between carbon isotope ratio and microhabitat in desert plants. Oecologia 76: 562–566.[ISI]
Mack, R. N. 1986 Alien plant invasion into the intermountain west: a case history. In H. A. Mooney and J. A. Drake [eds.], Ecology of biological invasions in North America and Hawaii, 191–213. Springer-Verlag, New York, New York, USA.
Mooney, H. A., R. W. Pearcy, and J. Ehleringer. 1987 Plant physiological ecology today. BioScience 37: 18–20.[CrossRef][ISI]
Nash, G. D. 1985 The American West transformed: the impact of the Second World War. Indiana University Press. Bloomington, Indiana, USA.
Vasek, F. C. 1980 Creosote bush: long-lived clones in the Mojave desert. American Journal of Botany 67: 246–255.[CrossRef][ISI]
Wallace, A., and E. M. Romney. 1972 Radioecology and ecophysiology of desert plants at the Nevada test site. U.S. Atomic Energy Commission, TID-25954.
Young, J. A., and B. A. Sparks. 1985 Cattle in the cold desert. Utah State University Press. Logan, Utah, USA.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
