sign in sign up
<<
Home , Plant physiology

Encyclopedia of

New Series Volume 1

Editors A. Pirson, Gottingen M. H. Zimmermann, Harvard Transport in I Phloem Transport

Edited by M.H.Zimmermann and J.A.Milburn

Contributors M.J.P. Canny J.Dainty A.F. G.Dixon WEschrich D. S. Fensom D. R. Geiger W Heyser W Holl J.A.Milburn T. R.F. Nonweiler M. V. Parthasarathy J.S.Pate AJ.Peel S.A.Sovonick D.C.Spanner P. M. L.Tammes M. T. Tyree J. Van Die H. Ziegler M. H. Zimmermann

With 93 Figures

Springer-Verlag Berlin Heidelberg New York 1975 ISBN-13: 978-3-642-66163-1 e-ISBN-13: 978-3-642-66161-7 DOl: 10.1007/978-3-642-66161-7

Library of Congress Cataloging in Publication Data. Main entry under title: Transport in plants I: phloem transport. (Encyclopedia of plant physiology; v. I) Bibliography: p. Includes index.!. Plant translocation. 2. Phloem. I. Zimmer• mann. Martin Huldrych, 1926~ II. Milburn, John A., 1936~ QK871.T73 582'.041 75-20178 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, by photocopying machine or similar means, and storage in data banks. Under §54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Verlag Berlin· Heidelberg 1975 Softcover reprint of the hardcover 1st edition 1975 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Preface

When WILHELM RUHLAND developed his plan for an Encyclopedia of Plant Physiol• ogy more than three decades ago, could still be conveniently subdivided into classical areas. Even within plant physiology, subdivisions were not too difficult to make, and general principles could be covered sufficiently in the two introductory volumes of the Encyclopedia on the physical and chemical basis of biology. But the situation changed rapidly even during the 12-year publication period of the Encyclopedia (1955-1967). The new molecular direction of genetics and structural research on biopolymers had an integrating effect on all other biological fields, including plant physiology, and it became increasingly difficult to keep previously distinct areas separated. RUHLAND'S overall plan included 18 volumes and about 22,000 pages. It covered the entire field of plant physiology, in most cases from the very beginning. But, as each volume appeared, it was clear that its content would soon be outdated. Discussions between the publisher and plant physiologists were therefore initiated to determine if and how the series could be continued. A difficult question concerned the degree of independence of individual fields. Modern biologists, particularly cell biologists, have a tendency to generalize, particularly if their background is primarily in physics or chemistry. Indeed, many basic principles of biology can be considered today well proven for all organisms. On the other hand, nature is extremely diverse; any generalized information that has been obtained in the laboratory with "standard" organisms and in vitro systems has to be compared with results obtained with the wide variety of organisms in nature. In plant physiol• ogy, as in other fields, this apparent antagonism between general principles and diversity has a stimulating effect on research. processes of plants are being studied with all available methods of modern biology. Electron microscopy, penetrating more and more into the dimen• sions of , shows increasing concern with the functional aspects of structure. One of the most fascinating ways of studying life processes is the study of their regulation. In this respect, plant physiology is particularly dependent upon the progress made in and genetics. It will probably not be long before methods and principles of bacterial genetics can be applied to the much more complex eucaryotic organisms, including green plants with their specific genetic material. Thus, on the one hand, new methods of studying regulation and adaptation break down old barriers between formerly separate fields such as genetics, physiology and . On the other hand, certain areas have remained well defined, particularly those concerning functions of the organism as a whole, such as phloem transport. Today it is impractical or impossible to merely issue supplementary volumes or a revised edition of the Encyclopedia of Plant Physiology. Even though there are still clearly-defined fields, too many of the boundaries have crumbled and new VI Preface combinations of interests are developing. For this reason it was decided not to make subdivisions into predetermined areas in a grand overall publication plan, as it was done in the first edition, but to publish a "New Series" in a much more flexible way. New and expanding fields will be treated separately as the need arises. There will be no introductory volumes to discuss basic principles. Historical concepts, already treated in the old Encyclopedia, will be discussed only if they need to be reconsidered in the of newer findings. As each volume is to be more or less self-contained, overlapping will become unavoidable in the long run. Such overlap is not too disadvantageous if similar chapters are written by different authors and if the publication dates of the respective volumes are reasonably far apart from each other. Furthermore, it is always desirable that in the case of contro• versial issues individual representatives can defend their own point of view. The subject matter of larger individual fields will be covered in several volumes, each one however self-contained and complete in itself. Thus the first volumes of the New Series consist ofa set of three, covering transport and exchange phenomena at three levels of organization: the whole plant, and cells, and structures within cells. The New Series differs not only in material from the old Handbook, but also in appearance. The individual volumes will be less extensive, and consequently probably also less costly. They will be written exclusively in English, the language now established as the most suitable for communication in the natural sciences. Moreover, once a manuscript is complete, it will be published within a shorter time than before, probably two or three volumes in the course of each year. These measures should greatly contribute towards distributing the New Series in greater number than the old Handbook, and to making it for years to come one of the most important literary references in plant physiological research. We hope that within the next few years, the New Series will once again cover the whole field of plant physiology, although in a quite different way from the old Handbook.

A.PIRSON M.H. ZIMMERMANN Introduction

Research on long-distance transport in plants probably began with the work of MALPIGHI in the later 17th century, following the discovery of blood circulation in by HARVEY. But the fact that there are two separate long-distance transport channels, the xylem and the phloem, was not recognized until more than 100 years later. The significance of phloem transport remained unclear until the assimilation of dioxide had been discovered by DE SAUSSURE in 1804. Outstanding experi• mental work early during the 19th century by COTTA, DE CANDOLLE and KNIGHT established the movement of carbohydrates from into stems and an~ their storage in the form of . THEODOR HARTIG discovered the sieve tube in 1837 and described exudation from both xylem and phloem in 1860. Thus, a reasonably clear understanding of long-distance transport pathways had been reached by the mid-19th century, though the transport mechanisms were unknown. Toward the end of the last century, translocation research suffered from the some• what dogmatic statement by the great plant physiologist SACHS that diffusion is the mechanism by which assimilates are distributed in plants. SACHS was undoubtedly an outstanding scientist, but his influence was decidedly negative in the field of translocation. Interest in translocation research developed rather slowly at the beginning of the 20th century, possibly because SACHS' influence still lingered on and because interest in plant physiology moved away from whole-plant physiology towards prob• lems at the tissue and cellular level. During the 1920s, phloem-transport research was re-established in a number of laboratories and for the first time it became quantitative. Mass-transfer studies by DIXON and his students (e.g. MASON) estab• lished beyond doubt that diffusion was inadequate, by several orders of magnitude, as a mechanism for assimilate distribution over long distances. The search began for a mechanism to explain the phenomenally efficient way in which plants transport solutes rapidly over long distances. The chapters of this volume show that this search has not yet ended to the satisfaction of everyone. In 1926 MiiNCH proposed his pressure-flow hypothesis which was published in greater detail in his book in 1930 by GUSTAV FISCHER in Jena. It had a very profound and lasting effect on virtually all subsequent work. It was considered carefully by MASON and MASKELL and eventually supported by DIXON. Nevertheless, though MiiNCH'S book is unquestionably the most-cited single literature report of phloem transport, it may equally well merit the reputation of the least-read book in the field, at least so far as the English-speaking scientific community is concerned. Interest in the phenomenon of phloem transport has increased steadily during the past 50 years. The number of publications has multiplied annually and several books on the subject have appeared recently. These books, taken individually and all cited in various chapters of this volume, represent strongly the points of view VIII Introduction of the particular authors, and thus each presents an individual view of the state of knowledge. During the month of August, 1974, a conference was held at Banff, Canada, on phloem transport, the proceedings of which may be published before the end of the year. In this, considerable emphasis was placed on recent research in structural aspects and on theoretical models of phloem transport. The present volume does not duplicate this effort. We have striven to produce a more text-book• like presentation with a good deal more emphasis on experimental work. The subject matter for this volume and the chapter authors have been carefully selected to represent as balanced an overall coverage as possible. Naturally, this resulted in the presentation of widely varied opinions, a fact which some readers may find confusing. At the present stage of knowledge, this is unavoidable. It has been said that the mechanism of phloem transport is still a matter of faith. What appears perfectly clear and logical to one person, may seem incredible to another. This is largely the result of the authors' interpretation of certain experimental results. For example, a whole chapter is devoted to the proposal of electroosmotically driven transport. Yet, in another chapter (Chapter 16, p.367) the authors state that they consider it so unlikely an explanation of transport that they refuse "to belabor the point any further". Some authors dismiss the concept of a flowing solution on the basis of experimental results which appear highly questionable to others. For proponents of the mass-flow concept it is difficult to believe in conclusions drawn from electronmicrographs of fixed plant material which appear to preclude mass-flow when in the same plant they can observe, for hours or days, exudation from a single tube through a severed aphid stylet at a rate which requires refilling of the sieve element three to ten times per second. What else but a flowing solution could do this? On the other hand it is often very healthy if established "facts" and dogmatic views are challenged. Scientific endeavor is perhaps more often than we like in direct conflict with human nature. Once we have made a statement we have a natural reluctance to withdraw it. Too often our attitude is to do experiments and use arguments to prove our point, rather than to find out honestly how the plant works, at the risk of finding an answer contrary to our wishes. Phloem research has developed a mystique which seems in tum to produce a search for certainties. These can easily become dogmatic assertions, for example: that glutaraldehyde is the best fixative for phloem; that radio-active tracer profiles correspond with the contents of sieve tubes; that is always "immobile" in the phloem; that all sieve tubes behave in unison at a given time; that sieve-tube is always alkaline; that sieve tubes are short-lived; that osmotic adjustments are always through changes in concentration; that sieve tubes occupy 20% of the (non-fibrous?) phloem. Although the above statements are often correct, there are good reasons to challenge each one of them. We have cited them in the hope that they will produce a note of caution in future research. In spite of the present disagreement about transport mechanisms, this volume shows that considerable progress has been made during recent years. Much of the research that accumulated this knowledge was stimulated by MUNCH'S controver• sial book and the apparent simplicity of his proposed mechanism. If the present volume has a similarly stimulating effect, it will have served its purpose well. We do not doubt that in spite of the present-day controversy, a more thorough under• standing of how are transported over long distances throughout the plant Introduction IX is not too far distant. We are inclined to doubt that appreciable new advances will be made using conventional electronmicroscopy to elucidate structure alone. The need here is for experimental investigations into the structures seen in sieve tubes. Similarly we are convinced that the present trend towards quantification of the parameters must be the main avenue of future advances and this in turn depends on the development of better experimental techniques. We would like to thank all our friends and colleagues who contributed to this book. Their collaboration is greatly appreciated. Those who delivered their chapter promptly had to bring it up to date one or more times before the volume was complete. We thank specifically the many authors who reviewed chapters other than their own. Many persons whose names do not appear elsewhere in the book also gave invaluable help during the course of production of individual chapters. This help ranged from discussing ideas, and in some cases assisting with experiments, to reviewing the text of the manuscripts. They are for Chapters 2, 4 and 10, Prof. RAY F. EVERT; Chapter 3, Dr. c.B. OSMOND; Chapter 6, Dr. J.M.S. FORREST; Chapters 9 and 15, Drs. R.G. THOMPSON and R.D. LEE; Chapters 11 and 17, Dr. ROBERT J. FELLOWS; Chapter 13, Dr. ENID A.C. MACRoBBIE; Chapter 14, Prof. P.E. WEATHERLEY, F.R.S.; Chapter 19, Profs. BRIAN E.S. GUNNING and OWEN A.M. LEWIS, and Dr. PATRICK J. SHARKEY. We also thank Springer-Verlag for prompt publication and high quality production. Last but not least, we thank the many individuals in the publishing-house whose names never appear in books, but whose conscientious work makes it "their" book as well as "ours".

Petersham, Massachusetts and MARTIN H. ZIMMERMANN Glasgow, Scotland JOHN A. MILBURN October 1975 Contents

I. Structural Considerations in Phloem Transport

1. Sieve-Element Structure M. V. PARTHASARATHY 3 A. Introduction 3 B. Terminology, Sieve-Element Size and Shape 3 C. Structure of Sieve Elements ...... 5 Angiosperms 5 - 21 - Vascular Cryptogams 24 D. Longevity of Sieve Elements ...... 26 E. Structure of Sieve Elements and Translocation ...... 27 Undifferentiated S.E. 27 - S.E. Structure and Possible Mechanisms of Trans• port 28 F. Conclusion 32 References . . 33

2. Sealing Systems in Phloem W. ESCHRICH 39 A. Introduction 39 B. Callose .. 39 Identification and Distribution 39 - Structure and Properties 41 - Changes of Callose Content 42 - Sieve-Tube Callose in Relation to Transport 45 - Conclusions 47 C. Sieve-Tube Slime 48 General Considerations 48 - Distribution and Origin 48 - Position of Fila• ments 50 - Conclusions 52 References ...... 52

II. Nature of Substances in Phloem

3. Nature of Transported Substances H. ZIEGLER 59 A. Introduction 59 B. Methods for the Analysis of the Transported Substances 59 C. Different Groups of Transported Substances .... 61 61 - Carbohydrates 63 - Nitrogenous Substances 67 - Lipids 73 - Organic Acids 73 - Constituents and ATP 74 - Growth Substances and Inhibitors 78 - Vitamins 83 - Other Organic Substances 84 - Inorganic Substances 85 - and Flagellates 91 - Physical Properties of S.T. Exudate 92 XII Contents

D. Concluding Remarks 93 References ...... 94

4. Biochemistry of Phloem Constituents w. ESCHRICH and W. HEYSER 101 A. Introduction ...... 101 B. Enzymic Activities in Sieve Tubes 102 Sources ofS.T. Exudates 102 - in S.T. 104 C. Maintenance of the Living State of Sieve Tubes ...... 104 Respiration 104 - Enzymes of Glycolysis and Gluconeogenesis 114 D. Origin of Enzymes in Sieve Tubes ...... 116 E. Biochemical Reactions in Sieve Tubes in Relation to Function 118 Enzymes Normally Present 118 - Metabolism of Sucrose and Derivatives 119 - Nucleic Acids 122 - Synthesis and Breakdown of Callose 124 - P- and 127 F. Conclusions 130 References . . 132

III. Phloem Transport: Assessment of Evidence

5. Mass Transfer M.J.P. CANNY 139 A. Introduction 139 B. Methods of Measuring Mass Transfer 139 Gain by Sinks 140 - Loss from Sources 143 - Transfer through Trunks 144 --,--- Special Systems: Palm 145 - Aphid Sty- let 145 - Results of the Measurements 146 C. Factors Related to Mass Transfer ...... 146 Concentration Gradient 146 - Path Distance 148 - Morphology and Pat- terns of Movement 149 - Flow Models 150 - Speed Components 151 D. Summary 152 References ...... 152

6. Aphids and Translocation A.F.G. DIXON 154 A. Introduction 154 B. Distribution of Aphids and Its Bearing on Translocation 158 C. Aphid Honeydew asa Sourceoflnformation on Substances Translocated in Phloem Elements ...... 159 Sterols 159 - Phenols 160 - Minerals 161 - Plant 162 D. Growth Efficiency of Aphids ...... 162 E. Damage Done to Plants by Aphids ...... 163 Galling 163 - Premature Senescence and Death of Plant Tissue 164 - Speed of Response of Plants to Aphid Infestation 165 - Hormonal Changes In• duced 165 Contents XIII

F. Aphids as "Sinks" ...... 165 G. Aphid Sty1ets and Phloem Transport 166 H. Summary 167 References ...... 167

7. Investigations with Aphid Sty1ets into the Physiology of the Sieve Tube A.J. PEEL ...... 171

A. Introduction ...... 171 B. The Aphid Technique as a Method of Obtaining Samples of Sieve-Tube Sap ...... 172 Severed Stylets 172 - Whole Aphids 172 - Comparison of Stylet Technique with Incisions 173 C. Evidence That Exudation from Stylets Involves Longitudinal Transport through the Sieve Tubes ...... 174 Experiments with Incisions and Radioactive Tracers 174, 175 D. The Pattern of Fluxes into and along the Sieve Tube in Relation to Stylet Puncture ...... 176 E. Severed Sty1ets and Aphids as Sinks ...... 177 Polarity Initiated by Stylets 178 - Velocity and Specific Mass Transfer of Sucrose in Pierced S.T. 178 F. The Control of Solute Loading into Sieve Elements ...... 179 Potential 179 - Specificity 179 - Metabolism and Loading 181 G. The Nature of the Longitudinal Movement Produced in Response to Stylet Puncture ...... 181 Water Movement 181 - Hydrostatic Pressures in Sieve Tubes 183 - Effect of Hydrostatic Pressure Gradients in Xylem upon Stylet Exudation 184 - Simultaneous Bidirectional Movement 185 - Metabolism and Movement of Solutes 187 H. Conclusions 192 References . . 192

8. Phloem Exudation from Monocotyledonous Axes J. VAN DIE and P.M.L. TAMMES ...... 196

A. Introduction ...... 196 B. Exudation from Wounded Palms and Agaves ...... 197 Sugar-Rich Saps 197-Use of Yuccaflaccida 199 - Composition and Nature of Exudates 200 C. Experimental Analyses of the Exudation Process ...... 206 Translocation to the Site of Bleeding 206 - Feeding Detached Inflorescence Axes 211 - Cooling Experiments with Yuccaflaccida 212 D. The Exudation Process and Its Relation to Phloem Transport 213 Flow between Source and Artificial Sink 213 - Structure ofthe Sieve Plate 214 - Conduding Remarks 216 References ...... 218 XIV Contents

9. Work with Isolated Phloem Strands D.S. FENSOM ...... 223 A. Introduction ...... 223 B. Studies of Heat-Pulse Movement 224 C. Electrical Studies ...... 225 D. Light Microscopy of Isolated Phloem Strands 228 E. Micro-Injection of Tracers ...... 234 F. Feeding Sucrose Solution to Isolated Phloem Strands 237 G. Electronmicroscopy of Isolated Strands 240 H. Summary .... 243 References . . . . . 243 10. Bidirectional Transport W. ESCHRICH 245 A. Introduction 245 Historical Survey 245 - Aims 245 - Anatomical Aspect 246 B. Experiments ...... 246 Analysis of S.T. between Two Sources 246 - Bidirectional Spreading from a Single Source 249 C. Bidirectional Transport in Relation to Vascular Differentiation 251 Anatomy 251 - Differentiation and Transport 251, 253 D. Conclusions 254 References ...... 255

11. Effects of , Anoxia and Other Metabolic Inhibitors on Translocation D.R. GEIGER and S.A. SOVONICK ...... 256 A. Introduction 256 The Importance of Translocation Modification by Metabolic Inhibitors 256 - Possible Difficulties Encountered in Studying the Effect of Metabolic Inhibi• tors on Translocation 256 B. Effects of Temperature, Anoxia and Metabolic Inhibitors on Transloca- tion ...... 260 Whole-Plant Experiments 260 - Results of Path, Sink and Source Treat• ments 261, 272, 276 C. Significance of Effects of Various Inhibitors ...... 278 Relation to Mechanism 278 - Effects on Growth and Productivity 278 - Future Work 281 D. Summary 281 References . . . . . 283

IV. Possible Mechanisms of Phloem Transport

12. Protoplasmic Streaming M.l.P. CANNY . . . 289 A. Introduction . . . . 289 B. The Phenomenon of Streaming 290 Contents XV

C. The Fine Structure of Streaming Protoplasm ...... 291 D. Chemical Basis of Protoplasmic Streaming ...... 292 E. Translocation Models Using the Streaming Properties of Protoplasm 293 Rotation Streaming 293 - Transcellular Strands 293 - Peristalitic Pump- ing 294 F. The Occurrence of Streaming in Sieve-Element Protoplasm .... 295 G. Structural Components of Sieve Elements That Might be Related to Streaming 296 H. Conclusion 297 References . . 298

13. Electroosmotic Flow D.C. SPANNER . 301 A. Presuppositions of the Theory 301 B. Electroosmosis as a Phenomenon 302 Anomalous Osmosis 304 - Design for an Electroosmotic Pump 304 - A Modified Design 306 C. Statement of the Theory ...... 307 Further Comments 311 - Initiation of Transport 312 - Insertion of P-Pro• tein into Pores 312 - Active Uptake of Potassium 313 - Membrane Expan• sion 314 - Limitation of Potassium Circulation 314 - Sieve-Element Length 315 - Function of Companion Cells 316 D. Objections to the Potassium Theory 316 E. Further Evidence for the Potassium Theory 319 F. Comparison with Other Theories .... 320 Miinch Hypothesis 320 - Contractile Protein Theories 321 G. Conclusion ...... 323 Addendum 1 : Hydrodynamic Polarization of Sieve Plate 323 Addendum 2: Donnan Equilibrium between Lumen and Pores 325 Addendum 3 . 325 References . . . 326 14. Pressure Flow J.A. MILBURN 328 A. Introduction .... 328 B. Pressure-Flow Systems 329 Miinch Osmotic Pressure-Flow Models 329 - Intact Plants: Elaboration on Miinch Model 329 - Effect of Gravity 332 - Reverse-Osmosis Experi• ments 333 - Role of Extra-Floral Nectaries 334 C. Objections to Osmotic Pressure Flow ...... 335 Structural Considerations 335 - Bidirectional Transport 337 - Differential Transport 337 - Polarized Flow 338 D. Observations Supporting Osmotic Pressure Flow ...... 338 E. Quantitative Studies on Phloem Transport ...... 340 Influence of Water Potential 341 - Sieve-Tube and Gra• dients 342 - S.T. Hydraulic Conductivity from Specific Mass Transfer, Sap Velocity or by Volume Transfer 346, 347, 349 F. Summary 350 References ...... 351 XVI Contents

15. Other Possible Mechanisms D.S. FENSOM 354 A. Introduction 354 B. Peristalsis of Cell Walls 354 C. Micro-Electro-Kinesis 355 D. Surface Active Movement Mechanism 355 E. Reciprocating Flow Hypothesis 357 F. Contractile 357 G. Concluding Considerations 363 References ...... 365

16. Theoretical Considerations M.T. TYREE and J. DAINTY 367 A. The Basic Transport Equations 367 Fick's Equations 368 - Localized Membrane Permeation 369 - Laminar Flow Equations 370 - Conservation and Phenomenological Equations 371 B. Mathematical Models for Mass Flow: Energetics and Forces 372 Miinch Model Treated Quantitatively 372 - Peristaltic Pumping Model 378 - Electroosmosis and Transcellular Cyclosis 380 C. Mathematical Models Treating the Kinetics of Spatial and Temporal Distribution of Labeled Translocate ...... 381 Kinetics of Loading and the Through-Put Component 381, 385 D. Summary 390 References ...... 391

V. Phloem Loading: Storage and Circulation

17. Phloem Loading D.R. GEIGER 395 A. Introduction 395 Concept of Phloem Loading 395 - Miinch Hypothesis and Phloem Load- ing 395 - Experimental Verification of Phloem Loading 396 B. Structural Aspects of Phloem Loading ...... 398 Distribution of Minor Veins 398 - Structural Features of Intermediary and Transfer Cells 399 - Close Functional and Structural Association of Sieve Elements and Companion Cells 400 C. Physiological Aspects of Phloem Loading ...... 400 Involvement of Free Space in Path from Mesophyll to Minor Vein Phloem 400 - Selectivity of Phloem Loading 407 - Mechanism of Phloem Loading 413 D. Control Aspects of Phloem Loading ...... 419 Production of Osmotic Pressure in Sieve Elements and Companion Cells Gener- ates Motive Force for Translocation 419 - Onset of Export Capacity 421 - Relationship between and Translocation 422 E. Summary 424 References ...... 425 Contents XVII

18. Radial Transport in Rays W. HOLL ..... 432 A. Introduction . . . . . 432 B. Structural Organization of Rays 432 General 432 - Physiological Differentiation of Rays in Gymnosperms and 433, 436 C. Storage in Rays ...... 438 Stored Material 438 - Seasonal Variations 440 D. Movement of Assimilates in Rays ...... 441 General 441 - Loading and Unloading of Rays via Strasburger Cells 442 - Possible Mechanisms of Radial Transport in Rays 444 - Exchange between Rays and Vessels 445 E. Summary 447 References ...... 448

19. Exchange of Solutes between Phloem and Xylem and Circulation in the Whole Plant 1.S. PATE 451 A. Introduction ...... 451 B. Structural Considerations Relating to Phloem-Xylem Exchange of Solutes ...... 451 C. Solutes Present in the Xylem and Their Origin in the . . . . . 455 D. Exchange of Xylem Solutes with the Axis and Interchange with Shoot Phloem ...... 458 E. Solutes Present in the Phloem and Their Origin in Leaves 459 F. Transport to Centers of Growth or Storage in the Shoot 462 G. Solute Transport and Circulation in the Whole Plant 464 H. Summary 467 References ...... 468

Appendix I: Flow of Biological Fluids through Non-Ideal Capillaries T.R.F. NONWEILER ...... 474

Appendix II: On the Simultaneous Movement of THO, 14C-Sucrose, and 32p04 in the Sieve-Tube of Willow M.T. TYREE and M.H. ZIMMERMANN 478 References ...... 479

Appendix III: List of and Sugar Alcohols in Sieve-Tube Exudates M.H. ZIMMERMANN and H. ZIEGLER ...... 480

Author Index . 505 Subject Index . 525 List of Contributors

MJ.P. CANNY M.V. PARTHASARATHY Department, Monash University, Division of Biological Sciences, Clayton, Victoria 3168/Australia Section of Genetics, Development and Physiology, Cornell University, J. DAINTY Ithaca, N.Y. 14853/USA Department of Botany, University of Toronto, Toronto M5S IAI/Canada J.S. PATE Department of Botany, University A.F.G. DIXON of Western Australia, Nedlands, School of Biological Sciences, W.A. 6009/Australia University of East Anglia, Norwich NR4 7TJ/Great Britain A.J. PEEL Department of Plant Biology, W. ESCHRICH University of Hull, Hull HU6 7RX/ F orstbotanisches Institut der Great Britain Universitiit, 34 G6ttingen-Weende, Biisgenweg 2/Federal Republic of S.A. SOVONICK Germany Harvard University, Harvard Forest, Petersham, MA 01366/USA D.S. FENSOM Department of Biology, Mount Allison D.C. SPANNER University, Sackville EOA 3CO, Department of Botany, Bedford Col• New Brunswick/Canada lege, University of London, London NW 14NS/Great Britain D.R. GEIGER Department of Biology, University P.M.L. TAMMES of Dayton, Dayton, Ohio 45469/USA Centrum voor Plantenfysiologisch Onderzoek, Bornsesteeg 47, W. HEYSER Wageningen/The Netherlands Forstbotanisches Institut der Universitiit, 34 G6ttingen-Weende, M.T. TYREE Biisgenweg 2/Federal Republic of Department of Botany, University of Germany Toronto, Toronto M5S IAI/Canada W. HOLL Institut fUr Botanik und Mikro• J. VAN DIE biologie, Technische Universitiit, Botanical Laboratory of the Uni• 8 Miinchen 2, Arcisstr. 21/Federal versity, Lange Nieuwstraat 106, Republic of Germany Utrecht/The Netherlands

J.A. MILBURN H. ZIEGLER Department of Botany, University of Institut fUr Botanik und Mikro• Glasgow, Glasgow G 12 8QQ/Great biologie, Technische Universitiit, Britain 8 Miinchen 2, Arcisstr. 21/Federal Republic of Germany T.R.F. NONWEILER Department of Mathematics, M. H. ZIMMERMANN Victoria University of Wellington, Harvard University, Harvard Forest, Wellington, C. l/New Zealand Petersham, MA 01366/USA