Plant Resins: Chemistry, Evolution, Ecology, and Ethnobotany. Jean H. Langenheim. Timber Press, Portland, Oregon, 2003. 586 pp., illus. $49.95 (ISBN 0881925748 cloth).
This new work by Jean H. Langenheim is destined to become this generation's most widely read and cited book about plant resins. Dr. Langenheim is a professor emeritus of biology and a research professor at the University of California, Santa Cruz. She is eminently qualified to present this comprehensive examination of plant resins, since she has been one of the premier researchers in the field for over 40 years. Her work provides an overdue successor to Vegetable Gums and Resins, by F. N. Howes (Chronica Botanica, 1949), as the classic treatment of this subject.
The ambitious goal of the book is to present an interdisciplinary look at plant resins, encompassing their formation, their composition, their defensive functions for the plants that make them, and their utility to the many insects and mammals (including people) that also use them. The book is divided into three major parts. The first section covers the production of resin by plants, with chapters focusing on the definition and basic chemistry of different types of resin, an evolutionary overview of resinproducing plants, and a description of plant structures involved in resin secretion and storage. The second section covers the geological history and ecology of resins, with chapters that describe what is known and what remains a mystery about amber (fossil resin) and the interactions between plants, resins, and herbivores. The final section covers the ethnobotany of resins, with chapters focusing on the historical importance and future use of resins and on specific types of resins, including oleoresins, balsams, and varnish resins.
Because a variety of plant exudates have been loosely called resins, one of the book's simplest and most important contributions is presenting clear definitions of what plant resins and their finer categories are and, equally important, what they are not. These definitions give a solid foundation for understanding each chapter of the book and provide an invaluable reference to better comprehend and evaluate other works about plant exudates, written by less precise authors. For example, Langenheim helps clarify the misleading trade term “essential oil,” which refers to volatile monoterpenes and sesquiterpenes in some plants, by pointing out that these compounds are “neither essential to plant metabolism nor are they true oils; essential refers to their essence or fragrance, and oil to their feel.” As a nonchemist, I liked the early, clear descriptions of the basic classes of terpenoid and phenolic resins. The understanding I gained from these descriptions helped me appreciate discussions in later sections of the book on how differences in resin composition influence the extent to which a resin remains fluid or hardens after exposure to air, a key property that affects its ecological function. These differences also determine the probability that a resin may turn to amber over time.
Plant Resins is arranged in a logical progression of chapters, but each chapter is sufficiently self-contained that someone interested in a particular topic could readily digest that subject's main points. It would be incorrect to assume, however, that any one chapter provides total coverage of its main topic. The excellent chapter about the ecological roles of resins, for example, includes a summary of plants with floral structures that offer liquid resin to reward certain bee pollinators. I also found numerous other references to bee and resin interactions dispersed in other chapters, which expanded my comprehension of this topic even further. Although the index will aid such specific pursuits to some extent, I would encourage people to read the entire work, because it tells a fascinating multifaceted story.
I consider plant resins one of the most compelling topics in tropical forest ecology, because they connect so many plants, insect herbivores, stingless bee pollinators, and human collectors. My thorough reading of this book gave me an integrated understanding of what is known about the evolutionary processes that led to the formation of resins in independent lineages of plants; about the ways that the different chemical and physical properties and delivery systems of resins defend plants; about the ingenious strategies various insects have evolved to circumvent resin defenses; and about the profound effects (both mystical and practical) of plant resins on human cultures and economies over thousands of years. It helped me formulate some of the fundamental questions that I need to address in my own research with Copaifera oleoresin and Burseraceae balsam, investigating the roles that tree growth and senescence, microbes, and insects play in stimulating and terminating resin production and flow.
Any chemist, biologist, anthropologist, or entrepreneur who has studied or been interested in a plant resin and who reads this book will gain a deeper appreciation for resins by learning about them through the eyes and efforts of diverse investigators. The author writes about all of the book's major disciplines with consistent clarity, confidence, and precision; the text is accessible and valuable to scientists from any discipline. With the help of the well-thought-out glossary, a reader with no scientific background could also readily digest most of the information in the book with little or no effort. While the author draws heavily on her own extensive work, her scholarship, devoted to weaving together the salient points of the efforts of hundreds of other researchers, is rigorous and meticulous. Dr. Langenheim is fair-minded in her treatment of all investigators, but she does not shy away from expressing her skepticism about other researchers' conclusions if they are not supported by solid evidence.
Each paragraph contains numerous references, but not every claim is backed up by a citation. This saves some pages from the lengthy volume and makes it more readable; however, on occasion I was left hungry for a reference about a specific point. Extensive effort has gone into preparing illustrations and collecting photographs that do an excellent job of complementing the text. The clear photographs make it easier to understand the processes relating to resin formation and harvest. The figures are drawn with the right balance of detail and simplicity to portray chemical and botanical structures and processes.
Dr. Langenheim's prowess as a scholar has allowed her to synthesize in one book what has been learned so far about plant resins. Her depth of experience and insight into this topic have also allowed her to astutely assess this information and describe what questions can and should be explored in the future. Elaborating resin processes in plants, discerning the physiological and ecological aspects of resin's interactions with insects, and developing new plant resin contributions to human society could provide a host of researchers with countless compelling puzzles to probe for decades. Plant Resins will become a benchmark work, because it comprehensively summarizes the history of plant resin research and will inspire the current and next generation of detective researchers to tackle new plant resin mysteries.