When I was asked by Timber Press to write a new book on resins, including amber—Howes's 1949 Vegetable Gums and Resins was the most recent such effort—the breadth of interdisciplinary coverage seemed too ambitious for an individual person. There have been so many advances in resin research in the past half century, including the development of new fields of research such as chemical ecology, and the exploration of other interesting facets about resins made possible by new chemical, molecular, and microscopic techniques. With a little thought, however, I realized that my years of resin research had prepared me to accept the challenge enthusiastically, a challenge that has been stimulating and rewarding.
My interest in resins began with ambers formed over geologic time and proceeded rapidly to the evolutionary significance of the ecological roles resins play in plants. These were natural interests, arising from my training as an ecologist and paleobotanist. Later, my queries turned to how humans have used resins throughout history, and my interest in that intensified when I taught an undergraduate course, Plants and Human Affairs, and coauthored a textbook on the subject. I became convinced that resins are remarkable materials indeed, especially in their diversity and the length of time they have been such versatile substances in the lives of plants and humans. A university colleague, a philosopher, suggested that resin had created a "cosmos" for me because of the variety of topics I had been led to investigate: paleobotany, chemistry, systematics, ecology, anthropology, ethnobotany, art history, etc. There is no doubt, however, that I could only have delved into such wideranging topics with the collaboration and expertise of many individuals, which increased the value and enjoyment of the experience. Although most of the people associated with the development of my research were not directly involved in my writing Plant Resins, I want to acknowledge their contribu tions to the learning experiences that enabled me to accept the challenge. It also is interesting how serendipity played a role in the people I met or the events that took place, helping me as my research interests ramified.
My research into plant resins began as a member of a paleoecological expedition to study amber in Chiapas, Mexico, led by entomologists from the University of California, Berkeley. My role in this expedition was to determine which trees produced the resin in which a diversity of insects had been beautifully preserved and the kind of forest in which the trees and insects had lived. Previously, amber had not been analyzed chemically as a resin but rather had been described inorganically as a gemstone. My first hint of the botanical source of the Mexican amber was a chemical one—its use by the Maya as incense. The burning incense did not smell like burning pine resin, which had long been thought to be the source of the well-known Baltic amber and was assumed to be the source of Mexican amber. Thus I collected resins from all the kinds of resin-producing trees in Chiapas for chemical comparison with the amber, ushering me into the world of tropical resins and the forests in which the trees grew.
Fortunately, at this time I became a research fellow at Harvard University in the laboratory of the geochemist and paleobotanist Elso Barghoorn, who enthusiastically encouraged my exploration of the chemical criteria for determining the botanical sources of amber through geologic time. It was necessary to use solid-state analytic techniques, such as infrared spectroscopy, because the polymerization of amber precluded dissolving it for standard organic chemical analysis. I subsequently collaborated with spectral chemist Curt Beck of Vassar College, who I had serendipitously discovered was using infrared spectroscopy to determine the archeological provenance of European amber. My approach at that time established a new direction in the study of plant origins of ambers, by including chemosystematic data. Additionally, my approach had an even larger perspective, of integrating paleo-ecological data into the understanding of amber-producing plants. These chemical and paleoecological studies, together with my background as a plant ecologist, prepared me to be intrigued by the correlation that the greatest diversity of trees producing copious amounts of resins are tropical angio-sperms (plants with true flowers). This interest coincided with the rapid advance of the field of biochemical ecology, and I was swept along with the tide of its development.
To understand tropical resin production, I decided to use the leguminous tree Hymenaea as a model, partly because I had determined it as the source of the amber in a number of large New World deposits. The genus has an amphi-Atlantic distribution, and the history of utilization of leguminous resins increased my growing interest in ethnobotany. Field investigation of Hymen-aea led me from Mexico through Central America to South America and Africa. The formation of the Organization for Tropical Studies (OTS) coincided with my early studies of Hymenaea in Central America, and assistance from numerous OTS colleagues from various colleges and universities (too many to name) helped promote the ramifications of my overall investigation of Hymenaea and my interest in other resin-producing plants.
The center of distribution of Hymenaea is Amazonia and I had the good fortune to be introduced to the region by the late Richard E. Schultes, longtime Amazonian ethnobotanical researcher at the Harvard University Botanical Museum. He helped initiate my Amazonian research, which continued for many years, and importantly, further enhanced my interest in ethnob-otany. Successful work on Brazilian Amazonian resin-producing plants also would not have been possible without the strong support and interest of Paulo Machado and Warwick Kerr, former directors of the Instituto Nacional de Pesquisas da Amazonia in Manaus; Paulo Cavalcante, Museu Goeldi in Belém; and others again too numerous to mention. Additionally, I had the unflagging interest and cooperation of Ghillean Prance, then director of research at the New York Botanical Garden, and later, director of the Royal Botanic Gardens, Kew, who was leading the research for Flora Amazónica.
Before I could investigate resin production throughout the geographic range of Hymenaea, revising the systematics of the genus was necessary since species had often been described from poor specimens collected during floris-tic surveys. This is a common situation for many of the plants belonging to tropical resin-producing families, a problem whose consequences are noted throughout Plant Resins. The Hymenaea revision, done in collaboration with a graduate student, Y. T. Lee, was approached as an interface between systematics and ecology, with amber providing the evolutionary context. During this revisionary work I interacted closely with tropical legume systematists such as the late Pat Brenan, Royal Botanic Gardens, Kew, and J. Léonard, Université de Bruxelles, a specialist on African copal producers. This opened my thinking on the important relationships of African and New World trees.
My interest in tropical resin-producing plants also expanded to discussion of taxonomic problems with specialists, including Douglas Daly, New York Botanical Garden (Burseraceae); T. C. Whitmore, Oxford University (Aga-this); and Peter Fritsch, California Academy of Sciences (Styracaceae), among others.
As my resin studies progressed, I had to learn more about the constituents of present-day rather than fossil resins. Thus I embarked on a determination of the components of Hymenaea resins with doctoral graduate students Susan Martin and Allan Cunningham, with assistance from chemists E. Zavarin, Forest Products Laboratory, University of California, Berkeley; George Hammond, University of California, Santa Cruz; A. C. Oehschlager, Simon Fraser University; and Duane Zinkel, Forest Products Laboratory, University of Wisconsin, Madison.
How resin is secreted into storage structures is significant to both plant defense and human use of resins. So another door to learning opened. In exploring the anatomy of secretory structures in Hymenaea, I was aided by the late Ralph Wetmore and I. W. Bailey as well as Margaret McCully, at Harvard University at the time, all of whom enthusiastically supplied the needed expertise. Lynn Hoefert, U.S. Department of Agriculture, Salinas, California, also assisted a graduate student, Gail Fail, with ultrastructural studies of resin secretion in Hymenaea. I increased my knowledge of resin secretory structures through contact with other researchers, too, including A. Fahn, Hebrew University of Jerusalem, and B. Dell and A. J. McComb, University of Western Australia, who studied secretory systems in a variety of resin-producing plants.
A major interest in the chemical ecology of Hymenaea was followed by comparison with the related legume, Copaifera. These investigations involved collaboration with another group of graduate students (Will Stubblebine, David Lincoln, José Carlos Nascimento, Matthew Ross, Craig Foster, Robert McGinley, Cynthia Macedo, Eric Feibert, and Susanne Arrhenius) on plant interactions with insects and fungi. Other avenues to understanding resin production were opened by graduate students (George Hall, Francisco Espinosa-Garda, and Wendy Peer) who worked on the chemical ecology of redwoods (Sequoia). I also enjoyed numerous stimulating discussions on defensive mechanisms of other resin-producing plants with colleagues, including Karen Sturgeon, then at the University of Colorado; Kenneth Raffa,
University of Wisconsin, Madison; Marc Snyder, Colorado College; John Bryant, University of Alaska; and numerous others.
Archeological and anthropological studies of resin and amber were carried out in Angola in collaboration with Desmond Clarke, University of California, Berkeley. By serving on doctoral dissertation committees at Yale University and the University of Texas, Austin, I learned about the use of resin by the Semelai in peninsular Malaysia (with Rosemary Gianno) and by the Maya in Mexico and Central America (with Kirsten Tripplett). Moreover, these kinds of studies provided opportunities to observe art objects made from amber, and contacts with museums around the world. And who would not avail themselves of the opportunities to collect and enjoy amber jewelry!
Thus, from my varied experiences in research on resin and amber, I saw the need for an up-to-date book because so much disparate information is scattered throughout the literature. I decided that the book should tell the whole story of these fascinating plant substances. Despite the importance of a multidisciplinary approach, and my hope of raising awareness of that, I divided the book into three parts to make it easier to use by readers with diverse backgrounds, interests, and goals, who I knew might turn to such a volume for information. These parts may be read in any order, depending on the reader's interest. A glossary is also provided. The three chapters in Part I, The Production of Resin by Plants, provide biochemical, developmental, and systematic information. However, this information is repeatedly projected toward discussion of the value of resins to plants and humans in Parts II and III. Central to understanding the remainder of the book is my operational definition of resin, presented in Chapter 1. This definition comes from my struggle with the confused and vague usage of the term resin that has persisted through the years. I hope that my definition provides rigor and clarity by distinguishing resins from other materials with which they are commonly confused (e.g., gums and mucilage) based on three criteria: chemistry, secretory structures, and ecological roles in the plant. Part I also includes a discussion of more recent major breakthroughs in the understanding of terpenoid biosynthesis and the ultrastructural evidence for its compartmentation, and how this new information solves mysteries encountered in ecological studies of resins. The secretory structures are characterized, and the importance of understanding their functions in ecological interactions and human use is discussed. Furthermore, I introduce the reader to the distribution of resin-producing plants throughout the plant kingdom and for the first time present evolutionary convergences in different aspects of resin production.
Part II, The Geologic History and Ecology of Resins, includes topics that have been at the heart of much of my own research. The two chapters have a phytocentric approach whereas other publications covering these subjects are more insect-oriented. Questions on when resins first evolved and on which groups of resin producers have a geologic record are addressed in Chapter 4. Chapter 5 addresses the question of whether resin production is primarily a defense against herbivores and pathogens, and presents ecological and evolutionary data that support this view.
Part III, The Ethnobotany of Resins, presents in six chapters the substantial roles that different kinds of resins have played in most cultures of the world throughout human history. In Chapter 11, I consider whether the importance of resin to humans will become a historical remnant as they are replaced by petrochemicals and other alternatives, or whether new technologies as well as policies that preserve plant resources, particularly in the tropics, will enable change in uses of resins and an important future for them. Plant Resins only provides a progress report on our current knowledge—I hope this synthesis of the many facets of resins will stimulate future research on these remarkable plant products.
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