Tree stems are black boxes. When mature, they can be very difficult to study in situ. They may be very tall (100 meters) and have large diameters (several meters). Their internal struc-ture is complex, ranging from fairly homogeneous wood com-prised largely of dead xylem elements in the conifers, to anastomosing primary vascular bundles embedded in a matrix of parenchyma in the monocotyledons, to heterogeneous wood with a variety of living and dead xylem elements in the dicoty-ledons. A stem changes structure and function with distance from its center (e.g., in woody stems: heartwood, sapwood, lateral meristems, phloem, and outer bark). The outer bark is viewed primarily as a protective sheath for the stem; it also acts as an efficient cloaking device to prevent easy access by researchers to the internal functions of the stem. All stem functions must integrate to support the rest of the tree. Stems are tied to leaves at stem tops and roots at stem bases. Water and nutrients in the stem destined for leaves enter through roots; photosynthate destined for roots is produced in the leaves. So, it is difficult to investigate excised stems and infer normal function from the results. Most tree stems are not disposable (as are most leaves and fine roots and the stems of many shrubs); destructive research on tree stems usually means sacrificing the entire tree. Nonetheless, understanding stem biology is critical to better ecological modeling, agricul-ture, wildlife management, lumber production, and other natu-ral resource management. As understanding and techniques and technology improve, the limitations that have faced the biologists and engineers are gradually being overcome.
The improvements in stem research are clearly seen in ecology. Ecology is a synthetic field that is based upon the integration of various other fields to understand how organ-isms relate to their environments. Historically, ecosystem modelers treated whole trees as black boxes, with the uptake and release of energy, water, nutrients, and carbon described with little consideration for the internal processes that influ-enced fluxes. At the same time, plant physiologists emphasized processes affecting photosynthesis, because the conversion of light and carbon to food is basic to life on earth. Physiologists also studied respiration and water relations which are integral to determining net carbon gain. The easily accessible leaves and somewhat less accessible fine roots were the site of most research. New knowledge from this research was incorporated into the ecological models, but then the tree stems, rather than the whole tree, were treated as black boxes. As understanding of the physiological ecology of trees improved over the last ten years and ecologists became aware of the critical role of tree stems, ecologists have begun to collaborate with taxonomists, morphologists, anatomists, physiologists, and engineers to in-vestigate tree stems in their own right and to incorporate the new data into ecosystem models (e.g., the dynamics of stem water storage). Interactions with pathologists and zoologists, including entomologists, are elaborating and expanding the
concepts of stem functions to include interactions with and internal responses to other taxa.
Plant Stems: Physiology and Functional Morphology is a collection of comprehensive review papers by leading re-searchers on what is known and what is not known about stems, with an emphasis on tree stems. The product of a workshop held recently in Oregon, it is also a call for the integration of research on stems from different fields. It is the first attempt to synthesize knowledge from diverse fields into a coherent concept of an integrated stem. Gartner, the editor, carefully organized the material into five sections: stem archi-tecture in plant performance, stems in the transport and storage of water, live stem cells in plant performance, stems in prevent-ing or reactprevent-ing to plant injury, and synthesis. The topics cov-ered in various chapters include an impressive array of research areas: structural support; short- and long-distance transport; water, nutrient, and metabolite storage; stem photo-synthesis; epiflora and epifauna; development; and defense against and reaction to disturbances such as fire, pollution, and pathogens. Authors frequently cross-reference other chapters. From necessity, some of the discussions are speculative and many examples are from herbaceous plants. Although much of the reported research is from the North American and Euro-pean temperate zones, African and Asian species and tropical and boreal species are mentioned (e.g., palms and tree ferns). In this context, ‘‘trees’’ are woody plants (including shrubs) or other arborescent vascular plants. Frequently, investigating organisms that operate at environmental extremes or that oc-cupy morphologically, anatomically, or functionally fringe po-sitions, gives insight into ‘‘typical’’ organisms. The authors cite such examples often, clearly indicating the features that distin-guish the example from typical plants. As a result, readers develop an appreciation for the breadth of solutions to func-tional problems. The approach challenges readers to think about the tradeoffs inherent in various solutions. Readers may disagree with some of the assertions, but ‘‘Plant Stems’’ will have challenged them to view stems in a more integrated way. The editor and authors have produced a good and timely synthesis on the function of tree stems. Most of my criticisms are minor. It is not possible to cover all aspects of stem physiology in such a volume. I especially missed more discus-sion on stem gas exchange and internal aeration. There is a chapter on stems and air pollution, although the book does not discuss the effects of groundwater pollution on stems. But given the wide structural and functional variation in stems, it is remarkable that this book has so comprehensively covered their functions and interactions.
Some chapters are very clear and easy to follow, others leave the reader to fill in gaps. Not all of the authors are clear about when they speculate. Terminology from several research fields is used in the text, but many terms are not defined. Defining more of the terminology would ease reading across disciplines.
Plant Stems: Physiology and Functional Morphology
BARBARA L. GARTNER, Editor
Academic Press, 525 B Street, Suite 1900, San Diego, CA 92101-4495; 1995,440 pages, hardcover, $89.95 US, ISBN 0-12-276460-9
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The chapters by Gartner, Givinish, Holbrook, Nilsen, Pate and Jeschke, and Weber and Grulke are especially well written and clear. Some authors clearly state where the most important areas for future research are, and some emphasize the impor-tance of more research in terms of real-world applications.
As I read the review chapters, I looked forward to a summary chapter that pulled all of stem biology together, but an entire second volume would be required to develop a synthesis of the preceding chapters with one voice. Hinckley and Schulte’s summary chapter reiterates some of the important concepts developed in the book, such as redundancy and modularity of construction, the incorporation of life history into the annual rings and structure of stems of individual trees, the energy costs of building disposable and nondisposable stems, re-sponding to environmental signals and communication among plant organs, difficulties in research on tree stems, interactions among stem functions and processes, and trade-offs among and optimization of functional strategies.
The book’s index limits its usefulness for reference. Not all occurrences of some terms used within the text are listed (e.g., integrated physiological unit), and I couldn’t find index entries for monocotyledons, palms, primary xylem, or vascular
bun-dles----all of which are discussed in various places throughout the volume. Also, inclusion of taxa in the index, or in a separate index, would have been very useful.
This book deserves a wide readership. Researchers, gradu-ate students, and advanced undergradugradu-ates in any field con-cerned with plant growth or function will find the book informative and thought provoking. College teachers and labo-ratory instructors will be able to glean ideas for student pro-jects from the many suggestions for proposed research. Referring to the ‘‘wide diversity of perspectives’’ in this vol-ume, Hinckley and Schulte write, ‘‘...we tend to study our small piece of the stem, but if we are to understand it as an entire structure we must take this broader view.’’ The next steps are for botanists, ecologists, and other scientists to read this book, do the research, and integrate the results among their fields. Clearly, the black box is being uncloaked----the time for inte-gration and synthesis has arrived!
Ann M. Lewis, Department of Forestry and Wildlife Manage-ment, Holdsworth Hall, University of Massachusetts, Amherst, MA 01003-4210, USA.
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