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Plant biomechanics: An engineering approach to plant form and function
TREE vol. 8, no. 3, March
7993
Plant Systematicsand Ecology Green Plants: Their Origin and Diversity by Peter R. Bell, Cambridge versity Press, 1992. f16.95 fv + 315 pages) /SBN 0 52143875 Diversity
and Evolution
Unipbk 6
of lend Plants
by Martin Ingrouille, Chapman Hall, 1992. f17.95 pbk (vi + pages) ISBN 0 4 12 44230 2
& 340
TWO books dealing with diversity and origin/evolution of plants have been published recently in Britain. Green P/ants by Bell is a revised edition of Bell and Woodcock’. Diversity and Evolution of Land Plants by lngrouille is new. Both books are aimed at high school or undergraduate students. The approaches adopted by the two authors are completely different. Bell follows the traditional path and describes diversity from the viewpoint of natural relationships of the plants. He states that the classification of plants facilitates the ordered treatment of diversity. Ingrouille, on the other hand, states that the systematic approach is dull to a modern audience not attuned to it, and he has chosen to relate diversity to evolutionary trends in plant structure rather than describing diversity in terms of taxonomic groups. This is a novel standpoint, and if thoroughly planned and worked up this approach could be an inspiring introduction to the amazing realm of green plants. However, to discuss the evolutionary trends of various organs in isolation from the whole organism demands a deep understanding of the systematics. How can the student relate to the unistratose sporangium wall, twolayered tapetum, sporogenous tissue and stalk development in lep-
tosporangiate ferns or the statement that leptosporangiate ferns evolved well after most other pteridophytes when the terms ‘ferns’ and ‘pteridophytes’ have not been explained? Furthermore, how can we know that heterospory evolved separately in several different groups and that the gametophyte of seed plants is reduced except through an understanding of phylogeny7 There is no explanation of modern phylogenetic methods and no evaluation of current phylogenetic schemes. The logic of the arrangement of the text is unclear to me and lngrouille is not completely consistent in his treatment. Some of the chapters are actually very systematic in approach, e.g. Chapter 5 on ‘Flowers: evolution and diversity’, but none of them tell the full story of the group or the organ or the processes involved. To understand pollination, for instance, the student must search in several different chapters and not always in obvious places. Bell’s systematic approach at least follows a strict order that is easier to read and facilitates information retreval. However, it suffers from the lack of a general phylogenetic framework and references to modern systematic studies. The various groups are treated separately. There are sometimes references to possible ancestors, but typically just as a short statement without thorough phylogenetic discussion. Many analyses have appeared over the past few years based on morphological or molecular data. The results do not always agree and the interpretation of these analyses is not straightforward. A sound policy would be to maintain a cautious and objective approach in the discussion of possible phylogenetic relationships, but to ignore these new
analyses is not in the spirit of science and does not serve students who will meet them almost on a daily basis if they choose to go further into the problems of the origin and evolution of plants. lngrouille has also mainly focused on quantitative diversity in his text and more or less neglected the evolutionary aspects of plant diversity. In his section on the evolution of the angiosperms he sets the scene by claiming that ‘primitive angiosperms are either early fossils or Iiving taxa with a high proportion of primitive characters. Primitive characters are those which appeared early in the fossil record’. As a palaeobotanist, I have every reason to be happy that the results of our work are considered useful, but this is taking it a bit too far. Although both authors adopt very different approaches to the same subject, the two books are surprisingly similar in layout and spirit. The illustrations are rather dull, sometimes looking like rough sketches and sometimes very sloppily mounted. There are also abundant spelling errors and too many factual errors, particularly in Ingrouille. These problems may reflect a wish to keep to a low budget and can be improved in future editions without too much effort. The lack of enthusiasm that marks both books, however, will be more difficult to deal with. Else Marie
Friis
Dept of Palaeobotany, Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm. Sweden Aoforenca 1 Bell, P.R. and Woodcock, C.L.F. (1983) Diversity of Green Plants (3rd edn), Edward Arnold
VegetableMechanics Plant Biomechanics: An Engineering Approach to Plant Form and Function by Karl J. Niklas, University of Chicago Press, 1992. f23.95/$29.95 pbk, f59.95/$75.00 hbk (622 pages) /SBN 0 226 586316/O 226 58630 8 I suspect that I am not alone in the pleasure I find in contemplating the shapes and forms of plants. My favorite time to go ‘plant watching’ is early evening. I go in search of the moment when my eye is held by the latticework of darkening branches and then all of a sudden I 116
find that I am looking at the lighter spaces of sky between them. Figure and ground exchange, what was foreground becomes backdrop, and there exists for a fleeting moment a perfect balance between what we focus on as real and of substance and what is, for the most part, deemed formless and invisible. Reading Karl Niklas’ book, P/ant Biomechanics, resulted in a similar experience. Over and over again the distinction between figure and ground gave way to a new understanding of the interdependence between those components that we
commonly think of as contributing to a plant’s structure and the material that fills in between. For example, the structural integrity of cell walls is typically attributed to its network of cellulose microfibrils. Indeed. the tensile strength of cellulose equals or exceeds that of many steels. Yet the ‘amorphous’ matrix in which the microfibrils are embedded must be sheared as the wall extends or deforms, and thus the physical properties of the matrix and how the microfibrils are attached influences the extent to which stresses are transmitted from
TREE vol. 8, no. 3, March
1993
cell to cell. At the level of an intact cell we might again make the distinction between the cell wall as the primary structural element and the relatively ‘formless’ protoplast as that which needs support. Cell walls, however, are much weaker in compression than in tension. The accumulation of osmotically active solutes by the cell sap places the wall in tension such that externally applied compressive forces are experienced as tensile stresses. Even the shapes, sizes and distribution of air spaces contributes to the mechanical properties of plant tissues - although in this case primarily by influencing the modes of failure and deformation that may occur. By bringing into focus the contribution of ‘non-structural’ elements, Niklas emphasizes the integrated nature of plant construction and the importance of always perceiving the plant as a living organism. Despite the aesthetic sensibilities inherent in the contemplation of plant farm and function, Plant Biomechanics is not for the fainthearted. The book is fairly mathematical in places and draws heavily from both physics and engineering. The strength of this book, however, is not as a compendium of formulae, but in demonstrating how biological solutions to mechanical constraints are in accord with the basic design principles that make up the field of engineering. Yet plants are not machines and nowhere in Plant Biomechanics are we allowed to take the easier road of considering them as such. As living organisms, plants
~--- ----- BooksReceived
are self-assembling, they must function while they are constructed, and they consist of materials whose properties may change over time and in response to internal conditions. If plants cannot be reduced to lifeless structures, then neither can they be understood within the confines of the relatively better developed field of animal biomechanics. The existence of a cell wall creates a mechanical demarcation between the two kingdoms and places strong constraints on the development of plant structures. The would-be botanical engineer is thus confronted with the necessity of coming to terms with how plants grow, including details of irreversible cell wall expansion, secondary modifications of cell wall composition, and the organization and deployment of meristems throughout the plant as a whole. As Plant Biomechanics is the first comprehensive treatment of plant structure from a mechanical point of view, it is perhaps idle praise to describe any of its features as unique. Nevertheless, two themes that run throughout the book should set it apart for a long time to come. The first of these is the insistence that at all levels plants are made up of structures, not materials. Niklas describes cell walls as ‘periodic microstructured composites’, while parenchyma forms a ‘pressurized cellular solid’. Although this perspective necessarily complicates any engineering analysis, it does explain why the mechanical behavior of plant structures cannot be
independent of either their anatomy or geometry. A second attribute of Plant Biomechanics that contributes to its particular flavor is its attempt to place the mechanical properties of the vegetable kingdom within an evolutionary context. I liked this book because its uncompromisingly botanical orientation forced me to think hard about the relationship between form and function in plants. I particularly applaud the inclusion of plant water relations as part of the foundation necessary for understanding plant structure. Nevertheless, I hesitate to recommend that anyone sit down and read this volume cover-to-cover as I did. Niklas has provided us with a very rich feast and readers might be better advised to savor its contents. In a lyrical moment Niklas describes secondary growth as creating an internal sarcophagus upon which the living tissues are draped. Yet the dominant paradigm of P/ant Nomechanics is that at all levels plants are constructed and function as a living, integrated whole. By demonstrating the interdependence and developmental integration of all aspects of plant structure, Niklas provides a strong argument that neither the traditional disciplines of plant anatomy and morphology nor any portion of the plant itself can justly be treated as dead wood.
N. Michele
Halbrook
Dept of Biologrcal Scrences. Stanford University. Stanford, CA 94305-5020, USA
7993
Plant Systematicsand Ecology Green Plants: Their Origin and Diversity by Peter R. Bell, Cambridge versity Press, 1992. f16.95 fv + 315 pages) /SBN 0 52143875 Diversity
and Evolution
Unipbk 6
of lend Plants
by Martin Ingrouille, Chapman Hall, 1992. f17.95 pbk (vi + pages) ISBN 0 4 12 44230 2
& 340
TWO books dealing with diversity and origin/evolution of plants have been published recently in Britain. Green P/ants by Bell is a revised edition of Bell and Woodcock’. Diversity and Evolution of Land Plants by lngrouille is new. Both books are aimed at high school or undergraduate students. The approaches adopted by the two authors are completely different. Bell follows the traditional path and describes diversity from the viewpoint of natural relationships of the plants. He states that the classification of plants facilitates the ordered treatment of diversity. Ingrouille, on the other hand, states that the systematic approach is dull to a modern audience not attuned to it, and he has chosen to relate diversity to evolutionary trends in plant structure rather than describing diversity in terms of taxonomic groups. This is a novel standpoint, and if thoroughly planned and worked up this approach could be an inspiring introduction to the amazing realm of green plants. However, to discuss the evolutionary trends of various organs in isolation from the whole organism demands a deep understanding of the systematics. How can the student relate to the unistratose sporangium wall, twolayered tapetum, sporogenous tissue and stalk development in lep-
tosporangiate ferns or the statement that leptosporangiate ferns evolved well after most other pteridophytes when the terms ‘ferns’ and ‘pteridophytes’ have not been explained? Furthermore, how can we know that heterospory evolved separately in several different groups and that the gametophyte of seed plants is reduced except through an understanding of phylogeny7 There is no explanation of modern phylogenetic methods and no evaluation of current phylogenetic schemes. The logic of the arrangement of the text is unclear to me and lngrouille is not completely consistent in his treatment. Some of the chapters are actually very systematic in approach, e.g. Chapter 5 on ‘Flowers: evolution and diversity’, but none of them tell the full story of the group or the organ or the processes involved. To understand pollination, for instance, the student must search in several different chapters and not always in obvious places. Bell’s systematic approach at least follows a strict order that is easier to read and facilitates information retreval. However, it suffers from the lack of a general phylogenetic framework and references to modern systematic studies. The various groups are treated separately. There are sometimes references to possible ancestors, but typically just as a short statement without thorough phylogenetic discussion. Many analyses have appeared over the past few years based on morphological or molecular data. The results do not always agree and the interpretation of these analyses is not straightforward. A sound policy would be to maintain a cautious and objective approach in the discussion of possible phylogenetic relationships, but to ignore these new
analyses is not in the spirit of science and does not serve students who will meet them almost on a daily basis if they choose to go further into the problems of the origin and evolution of plants. lngrouille has also mainly focused on quantitative diversity in his text and more or less neglected the evolutionary aspects of plant diversity. In his section on the evolution of the angiosperms he sets the scene by claiming that ‘primitive angiosperms are either early fossils or Iiving taxa with a high proportion of primitive characters. Primitive characters are those which appeared early in the fossil record’. As a palaeobotanist, I have every reason to be happy that the results of our work are considered useful, but this is taking it a bit too far. Although both authors adopt very different approaches to the same subject, the two books are surprisingly similar in layout and spirit. The illustrations are rather dull, sometimes looking like rough sketches and sometimes very sloppily mounted. There are also abundant spelling errors and too many factual errors, particularly in Ingrouille. These problems may reflect a wish to keep to a low budget and can be improved in future editions without too much effort. The lack of enthusiasm that marks both books, however, will be more difficult to deal with. Else Marie
Friis
Dept of Palaeobotany, Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm. Sweden Aoforenca 1 Bell, P.R. and Woodcock, C.L.F. (1983) Diversity of Green Plants (3rd edn), Edward Arnold
VegetableMechanics Plant Biomechanics: An Engineering Approach to Plant Form and Function by Karl J. Niklas, University of Chicago Press, 1992. f23.95/$29.95 pbk, f59.95/$75.00 hbk (622 pages) /SBN 0 226 586316/O 226 58630 8 I suspect that I am not alone in the pleasure I find in contemplating the shapes and forms of plants. My favorite time to go ‘plant watching’ is early evening. I go in search of the moment when my eye is held by the latticework of darkening branches and then all of a sudden I 116
find that I am looking at the lighter spaces of sky between them. Figure and ground exchange, what was foreground becomes backdrop, and there exists for a fleeting moment a perfect balance between what we focus on as real and of substance and what is, for the most part, deemed formless and invisible. Reading Karl Niklas’ book, P/ant Biomechanics, resulted in a similar experience. Over and over again the distinction between figure and ground gave way to a new understanding of the interdependence between those components that we
commonly think of as contributing to a plant’s structure and the material that fills in between. For example, the structural integrity of cell walls is typically attributed to its network of cellulose microfibrils. Indeed. the tensile strength of cellulose equals or exceeds that of many steels. Yet the ‘amorphous’ matrix in which the microfibrils are embedded must be sheared as the wall extends or deforms, and thus the physical properties of the matrix and how the microfibrils are attached influences the extent to which stresses are transmitted from
TREE vol. 8, no. 3, March
1993
cell to cell. At the level of an intact cell we might again make the distinction between the cell wall as the primary structural element and the relatively ‘formless’ protoplast as that which needs support. Cell walls, however, are much weaker in compression than in tension. The accumulation of osmotically active solutes by the cell sap places the wall in tension such that externally applied compressive forces are experienced as tensile stresses. Even the shapes, sizes and distribution of air spaces contributes to the mechanical properties of plant tissues - although in this case primarily by influencing the modes of failure and deformation that may occur. By bringing into focus the contribution of ‘non-structural’ elements, Niklas emphasizes the integrated nature of plant construction and the importance of always perceiving the plant as a living organism. Despite the aesthetic sensibilities inherent in the contemplation of plant farm and function, Plant Biomechanics is not for the fainthearted. The book is fairly mathematical in places and draws heavily from both physics and engineering. The strength of this book, however, is not as a compendium of formulae, but in demonstrating how biological solutions to mechanical constraints are in accord with the basic design principles that make up the field of engineering. Yet plants are not machines and nowhere in Plant Biomechanics are we allowed to take the easier road of considering them as such. As living organisms, plants
~--- ----- BooksReceived
are self-assembling, they must function while they are constructed, and they consist of materials whose properties may change over time and in response to internal conditions. If plants cannot be reduced to lifeless structures, then neither can they be understood within the confines of the relatively better developed field of animal biomechanics. The existence of a cell wall creates a mechanical demarcation between the two kingdoms and places strong constraints on the development of plant structures. The would-be botanical engineer is thus confronted with the necessity of coming to terms with how plants grow, including details of irreversible cell wall expansion, secondary modifications of cell wall composition, and the organization and deployment of meristems throughout the plant as a whole. As Plant Biomechanics is the first comprehensive treatment of plant structure from a mechanical point of view, it is perhaps idle praise to describe any of its features as unique. Nevertheless, two themes that run throughout the book should set it apart for a long time to come. The first of these is the insistence that at all levels plants are made up of structures, not materials. Niklas describes cell walls as ‘periodic microstructured composites’, while parenchyma forms a ‘pressurized cellular solid’. Although this perspective necessarily complicates any engineering analysis, it does explain why the mechanical behavior of plant structures cannot be
independent of either their anatomy or geometry. A second attribute of Plant Biomechanics that contributes to its particular flavor is its attempt to place the mechanical properties of the vegetable kingdom within an evolutionary context. I liked this book because its uncompromisingly botanical orientation forced me to think hard about the relationship between form and function in plants. I particularly applaud the inclusion of plant water relations as part of the foundation necessary for understanding plant structure. Nevertheless, I hesitate to recommend that anyone sit down and read this volume cover-to-cover as I did. Niklas has provided us with a very rich feast and readers might be better advised to savor its contents. In a lyrical moment Niklas describes secondary growth as creating an internal sarcophagus upon which the living tissues are draped. Yet the dominant paradigm of P/ant Nomechanics is that at all levels plants are constructed and function as a living, integrated whole. By demonstrating the interdependence and developmental integration of all aspects of plant structure, Niklas provides a strong argument that neither the traditional disciplines of plant anatomy and morphology nor any portion of the plant itself can justly be treated as dead wood.
N. Michele
Halbrook
Dept of Biologrcal Scrences. Stanford University. Stanford, CA 94305-5020, USA