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Plant Cell Biology PDF

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Preface This book is in essence the lectures I give in my plant Recognizing the basic similarities between all living cell biology course at Cornell University. Heretofore, the eukaryotic cells (Quekett, 1852, 1854; Huxley, 1893), I lecture notes have gone by various titles, including “Cell discuss both animal and plant cells in my course. Although La Vie,” “The Book Formerly Known as Cell La Vie,” the examples are biased toward plants (as they should be in “Molecular Theology of the Cell,” “Know Thy Cell” (with a plant cell biology course), I try to present the best exam- apologies to Socrates), “Cell This Book” (with apolo- ple to illustrate a process and sometimes the best examples gies to Abbie Hoffman), and “Impressionistic Plant Cell are from animal cells. I take the approach used by August Biology.” I would like to take this opportunity to describe Krogh (1929); that is, there are many organisms in the this course. It is a semester-long course for undergradu- treasure house of nature and if one respects this treasure, ate and graduate students. Since the undergraduate biol- one can find an organism created to best illuminate each ogy majors are required to take genetics, biochemistry, and principle! I try to present my course in a balanced manner, evolution as well as 1 year each of mathematics and phys- covering all aspects of plant cell biology without empha- ics, and 2 years of chemistry, I have done my best to inte- sizing any one plant, organelle, molecule, or technique. grate these disciplines into my teaching. Moreover, many I realize, however, that the majority of papers in plant of the students also take plant anatomy, plant physiology, cell biology today are using a few model organisms and plant growth and development, plant taxonomy, plant bio- “-omic” techniques. My students can learn about the suc- chemistry, plant molecular biology, and a variety of courses cesses gained though this approach in a multitude of other that end with the suffix “-omics”; I have tried to show the courses. I teach them that there are other approaches. connections between these courses and plant cell biology. Pythagoras believed in the power of numbers, and I Nonbotanists can find a good introduction to plant biology believe that the power of numbers is useful for under- in Mauseth (2009) and Taiz and Zeiger (2006). standing the nature of the cell. In my class, I apply the Much of the content has grown over the past 20 years power of numbers to help relate quantities that one wishes from the questions and insights of the students and teaching to know to things that can be easily measured (Hobson, assistants who have participated in the class. The students’ 1923; Whitehead, 1925; Hardy, 1940; Synge, 1951, 1970; interest has been sparked by the imaginative and insight- Feynman, 1965; Schrödinger, 1996). For example, the area of ful studies done by the worldwide community of cell biolo- a rectangle is difficult to measure. However, if one knows its gists, which I had the honor of presenting. length and width, and the relation that area is the product of I have taken the approach that real divisions do not length and width, the area can be calculated from the easily exist between subject areas taught in a university, but only measurable quantities. Likewise, the circumference or area in the state of mind of the teachers and researchers. With of a circle is relatively difficult to measure. However, if one this approach, I hope that my students do not see plant cell measures the diameter and multiplies it by π, or the square of biology as an isolated subject area, but as an entrée into the diameter by π/4, one can easily obtain the circumference every aspect of human endeavor. One of the goals of my and area, respectively. In the same way, one can easily esti- course is to try to reestablish the connections that once mate the height of a tree from easily measurable quantities if existed between mathematics, astronomy, physics, chemis- one understands trigonometry and the definition of tangent. try, geology, philosophy, and biology. It is my own personal My teaching was greatly influenced by a story that attempt, and it is an ongoing process. Consequently, it is Hans Bethe told at a meeting at Cornell University com- far from complete. Even so, I try to provide the motivation memorating the 50th anniversary of the chain reaction pro- and resources for my students to weave together the threads duced by Enrico Fermi. Bethe spoke about the difference of these disciplines to create their own personal tapestry of between his graduate adviser, Arnold Sommerfeld, and the cell from the various lines of research. his postdoctoral adviser, Enrico Fermi. He said that, in the Plant Cell Biology Copyright © 22000099, Elsevier, Inc. All rights of reproduction in any form reserved. xiii xiv Preface field of atomic physics, Sommerfeld was a genius at cre- each term. Some specialized terms are essential for pre- ating a mathematical theory to describe the available data. cise communication in science just as it is in describ- Sommerfeld’s skill, however, depended on the presence of ing love and beauty. However, some terms are created data. Fermi, on the other hand, could come up with theories to hide our ignorance, and consequently prevent further even if the relevant data were not apparent. He would make inquiry, because something with an official-sounding name estimates of the data from first principles. For example, he seems well understood (Locke, 1824; Hayakawa, 1941; estimated the force of the first atomic bomb by measuring Rapoport, 1975). In Goethe’s (1808) “Faust Part One,” the distance small pieces of paper flew as they fell to the Mephistopheles says: “For at the point where concepts fail. ground during the blast in Alamogordo. Knowing that the At the right time a word is thrust in there. With words we force of the blast diminished with the square of the distance fitly can our foes assail.” Francis Bacon (1620) referred to from the bomb, Fermi estimated the force of the bomb rela- this problem as the “Idols of the Marketplace.” Often we tive to the force of gravity. Within seconds of the blast, he think we are great thinkers when we answer a question calculated the force of the bomb to be approximately 20 with a Greek or Latin word. For example, if I am asked, kilotons, similar to which the expensive machines recorded “Why are leaves green?” I quickly retort, “Because they (Fermi, 1954; Lamont, 1965). have chlorophyll.” The questioner is satisfied, and says In order to train his students to estimate things that they “Oh.” The conversation ends. However, chlorophyll is just did not know, Fermi would ask them, “How many piano the Greek word for green leaf. Thus, I really answered the tuners are there in Los Angeles?” After they looked befud- question with a tautology. I really said “Leaves are green dled, he would say, “You can estimate the number of piano because leaves are green” and did not answer the question tuners from first principles! For example, how many peo- at all. It was as if I was reciting a sentence from scripture, ple are there in Los Angeles? One million? What percent- which I had committed to memory without giving it much age has pianos? Five percent? Then there are 50,000 pianos thought. However, I gave the answer in Greek, and with in Los Angeles. How often does a piano need to be tuned? authority … so it was a scientific answer. About once a year? Then 50,000 pianos need to be tuned in In “An Essay Concerning Human Understanding,” John a year. How many pianos can a piano tuner tune in a day? Locke (1824) admonished that words are often used in a Three? Then one tuner must spend 16,667 days a year tun- nonintellectual manner. He wrote, ing pianos. But since there are not that many days in a year, and he or she probably only works 250 days a year, then … he would not be much better than the Indian before- there must be around 67 piano tuners in Los Angeles.” mentioned, who, saying that the world was supported by My students apply the power of numbers to the study a great elephant, was asked what the elephant rested on; of cellular processes, including membrane transport, pho- to which his answer was, a great tortoise. But being again tosynthesis, and respiration, in order to get a feel for these pressed to know what gave support to the broad-backed tor- toise, replied, something he knew not what. And thus here, as processes and the interconversions that occur during these in all other cases where we use words without having clear processes between different forms of energy. My students and distinct ideas, we talk like children; who being questioned apply the power of numbers to the study of cell growth, what such a thing is, which they know not, readily give the chromosome motion, and membrane trafficking in order to satisfactory answer, that it is something; which in truth signi- be able to postulate and evaluate the potential mechanisms fies no more, when so used either by children or men, but that involved in these processes, and the relationships between they know not what; and that the thing that they pretend to these processes and the bioenergetic events that power know and talk of is what they have no distinct idea of at all, them. Becoming facile with numbers allows the students to and so are perfectly ignorant of it, and in the dark. understand, develop, and critique theories. “As the Greek origin of the word [theory] implies, the Theory is the true Sometimes terms are created to become the shibbo- seeing of things—the insight that should come with healthy leths of a field, and sometimes they are created for political sight” (Adams and Whicher, 1949). reasons, financial reasons, or to transfer credit from some- Using the power of numbers to relate seemingly unre- one who discovers something to someone who renames it lated processes, my students are able to try to analyze all (Agre et al., 1995). Joseph Fruton (1992) recounted (and their conclusions in terms of first principles. They also learn translated) a story of a conversation with a famous chemist to make predictions based on first principles. The students in Honoré de Balzac’s La Peau de Chagrin: must be explicit in terms of what they are considering to be facts, what they are considering to be the relationship “Well, my old friend,” said Planchette upon seeing Japhet between facts, and where they are making assumptions. This seated in an armchair and examining a precipitate, “How provides a good entrée into research, because the facts must goes it in chemistry?” be refined and the assumptions must be tested (East, 1923). “It is asleep. Nothing new. The Académie has in the mean- I do not try to introduce any more terminology in my time recognized the existence of salicine. But salicine, aspar- class than is necessary, and I try to explain the origin of agine, vauqueline, digitaline are not new discoveries.” Preface xv “If one is unable to produce new things,” said Raphael, “it present concepts are only the last approximation in a long seems that you are reduced to inventing new names.” series of similar attempts which, of course, is not ended.” “That is indeed true, young man.” I teach my students that it is important to be skeptical when considering old as well as new ideas. According to I teach plant cell biology with a historical approach and Thomas Gold (1989), teach “not only of the fruits but also of the trees which have borne them, and of those who planted these trees” (Lenard, New ideas in science are not always right just because they 1906). This approach also allows them to understand the are new. Nor are the old ideas always wrong just because origins and meanings of terms; to capture the excitement they are old. A critical attitude is clearly required of every of the moment of discovery; to elucidate how we, as a sci- scientist. But what is required is to be equally critical to the entific community, know what we know; and it empha- old ideas as to the new. Whenever the established ideas are accepted uncritically, but conflicting new evidence is brushed sizes the unity and continuity of human thought (Haldane, aside and not reported because it does not fit, then that par- 1985). I want my students to become familiar with the great ticular science is in deep trouble—and it has happened quite innovators in science and to learn their way of doing sci- often in the historical past. ence (Wayne and Staves, 1998, 2008). I want my students to learn how the scientists we learn about choose and pose To emphasize the problem of scientists unquestioningly questions, and how they go about solving them. I do not accepting the conventional wisdom, Conrad H. Waddington want my students to know just the results and regurgitate (1977) proposed the acronym COWDUNG to signify the those results on a test (Szent-Györgyi, 1964; Farber, 1969). Conventional Wisdom of the Dominant Group. I do not want my students to become scientists who merely In teaching in a historical manner, I recognize the impor- repeat on another organism the work of others. I want my tance of Thomas H. Huxley’s (1853) warnings that “Truth students to become like the citizens of Athens, who accord- often has more than one Avatar, and whatever the forgetful- ing to Pericles “do not imitate—but are a model to others.” ness of men, history should be just, and not allow those who Whether or not my students become professional cell biolo- had the misfortune to be before their time to pass for that gists, I hope they forever remain amateurs and dilettantes in reason into oblivion” and “The world, always too happy to terms of cell biology. That is, I hope that I have helped them join in toadying the rich, and taking away the ‘one ewe lamb’ become “one who loves cell biology” and “one who delights from the poor.” Indeed, it is often difficult to determine who in cell biology” (Chargaff, 1986)—not someone who can- makes a discovery (Djerassi and Hoffmann, 2001). I try to not recognize the difference between a pile of bricks and an the best of my ability to give a fair and accurate account of edifice (Forscher, 1963), not someone who sells “buyology” the historical aspects of cell biology. (Wayne and Staves, 2008), and not someone who sells his or My course includes a laboratory section and my stu- her academic freedom (Rabounski, 2006; Apostol, 2007). dents perform experiments to acquire personal experience Often people think that a science course should teach in understanding the living cell and how it works (Hume, what is new, but I answer this with an amusing anec- 1748; Wilson, 1952; Ramón y Cajal, 1999). Justus von dote told by Erwin Chargaff (1986): “Kaiser Wilhelm I Liebig (1840) described the importance of the experimen- of Germany, Bismark’s old emperor, visited the Bonn tal approach this way: Observatory and asked the director: ‘Well, dear Argelander, what’s new in the starry sky?’ The director answered Nature speaks to us in a peculiar language, in the language of promptly: ‘Does your Majesty already know the old?’ phenomena; she answers at all times the questions which are The emperor reportedly shook with laughter every time he put to her; and such questions are experiments. An experiment is the expression of a thought: we are near the truth when the retold the story.” phenomenon, elicited by the experiment, corresponds to the According to R. John Ellis (1996), thought; while the opposite result shows that the question was falsely stated, and that the conception was erroneous. It is useful to consider the origins of a new subject for two reasons. First, it can be instructive; the history of science pro- My students cannot wait to get into the laboratory. In vides sobering take-home messages about the importance of fact, they often come in on nights and weekends to use the not ignoring observations that do not fit the prevailing con- microscopes to take photomicrographs. At the end of the ceptual paradigm, and about the value of thinking laterally, in semester, the students come over to my house for dinner case apparently unrelated phenomena conceal common prin- (I worked my way through college as a cook) and bring ciples. Second, once a new idea has become accepted there is often a tendency to believe that it was obvious all along— their best photomicrographs. After dinner, they vote on the hindsight is a wonderful thing, but the problem is that it is twelve best, and those are incorporated into a class cal- never around when you need it! endar. The calendars are beautiful and the students often make extra to give as gifts. The historical approach is necessary, in the words of In 1952, Edgar Bright Wilson Jr. wrote in An George Palade (1963), “to indicate that recent findings and Introduction to Scientific Research, “There is no excuse for xvi Preface doing a given job in an expensive way when it can be car- the people who they consider to be the best scientists. This ried through equally effectively with less expenditure.” is a shame. They read the work of others … but not the best. Today, with an emphasis on research that can garner sig- Interestingly, they usually are well read when it comes to nificant money for a college or university through indirect reading the best writers (e.g., Shakespeare, Faulkner, etc.). costs, there is an emphasis on the first use of expensive Typically, the people on my students’ lists of best scien- techniques to answer cell biological questions and often tists have written books for the layperson or an autobiogra- questions that have already been answered. However, the phy (Wayne and Staves, 1998). Even Isaac Newton wrote very expense of the techniques often prevents one from a book for the layperson! I give my class these references performing the preliminary experiments necessary to learn and encourage them to become familiar with their favorite how to do the experiment so that meaningful and valuable scientists first hand. The goal of my lectures and this book data and not just lists are generated. Unfortunately, the lists is to facilitate my students’ personal and continual journey generated with expensive techniques often require statisti- in the study of life. cians and computer programmers, who are far removed My goal in teaching plant cell biology is not only to from experiencing the living cells through observation and help my students understand the mechanisms of the cell measurement, to tell the scientist which entries on the list and its organelles in converting energy and material mat- are meaningful. Thus, there is a potential for the distinction ter into a living organism that performs all the functions between meaningful science and meaningless science to we ascribe to life. I also hope to deepen my students’ ideas become a blur. I use John Synge’s (1951) essay on vicious of the meaning, beauty, and value of life and the value in circles to help my students realize that there is a need to searching for meaning and understanding in all processes distinguish for themselves what is fundamental and what is involved in living. derived. I thank Mark Staves and my family, Michelle, Katherine, By contrast, this book emphasizes the importance of the Zack, Beth, Scott, my mother and father, and aunts and scientists who have made the great discoveries in cell biol- uncles, for their support over the years. I also thank my col- ogy using relatively low-tech quantitative and observational leagues at Cornell University and teachers at the Universities methods. But—and this is a big but—these scientists also of Massachusetts, Georgia, and California at Los Angeles, treated their brains, eyes, and hands as highly developed sci- and especially Peter Hepler and Masashi Tazawa, who taught entific instruments. I want my students to have the ability to me how to see the universe in a living cell. get to know these great scientists. I ask them to name who they think are the 10 best scientists who ever lived. Then I Randy Wayne, ask if they have ever read any of their original work. In the Department of Plant Biology, Cornell University majority of the cases, they have never read a single work by Academic Press is an imprint of Elsevier 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 525 B Street, Suite 1900, San Diego, California 92101-4495, USA 84 Theobald’s Road, London WC1X 8RR, UK © 2009 Elsevier, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or   mechanical, including photocopying, recording, or any information storage and retrieval system, without   permission in writing from the publisher. Details on how to seek permission, further information about the  Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance   Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.  This book and the individual contributions contained in it are protected under copyright by the Publisher   (other than as may be noted herein).  Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our   understanding, changes in research methods, professional practices, or medical treatment may become necessary.  Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using   any information, methods, compounds, or experiments described herein. In using such information or methods   they should be mindful of their own safety and the safety of others, including parties for whom they have a   professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors,   or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability,  negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained   in the material herein. Library of Congress Cataloging-in-Publication Data Wayne, Randy.   Plant cell biology / Randy Wayne.     p. cm.   Includes bibliographical references and index.   ISBN 978-0-12-374233-9 (hardback : alk. paper)  1.  Plants—Cytology.  I. Title.    QK725.W39 2009   571.6’2—dc22   2009018976 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN: 978-0-12-374233-9 For information on all Academic Press publications   visit our website at www.elsevierdirect.com Companion Website Available http://www.elsevierdirect.com/companions/9780123742339 09 10 11 12 13  5 4 3 2 1 Printed in the United States of America Dedicated to President John F. Kennedy   for inspiring my generation to be courageous in the pursuit of science Chapter 1 On the Nature of Cells The world globes itself in a drop of dew. The microscope cannot find the animalcule which is less perfect for being little. Eyes, ears, taste, smell, motion, resistance, appetite, and organs of reproduction that take hold on eternity—all find room to consist in the small creature. So do we put our life into every act. The true doctrine of omnipresence is that God reappears with all His parts in every moss and cobweb. — Ralph Waldo Emerson, “Compensation” . IntroductIon: what Is a cell? In the introduction to his book, Grundzüge der Botanik, Matthias Schleiden (1842), often considered the cofounder FIgure .  Cells of cork. (Source: From Hooke, 1665.) of the cell theory, admonished, “Anyone who has an idea of learning botany from the present book, may just as well put it at once aside unread; for from books botany is not learnt” (quoted in Goebel, 1926). Likewise, I would like to stress that an understanding of plant cell biology, and what a plant cell is, comes from direct experience. I hope that this book helps facilitate your own personal journey into the world of the cell. Exploring the world made accessible by the invention of the microscope, Robert Hooke (1665) discovered a regular, repeating structure in cork that he called a cell. The word cell comes from the Latin celle, which in Hooke’s time meant “a small apartment, esp. one of several such in the same building, used e.g. for a store-closet, slave’s room, prison cell; also cell of a honeycomb; … also a monk’s or hermit’s cell” (Oxford English Dictionary, 1933). Hooke used the word cell to denote the stark appearance of the air-filled pores he saw in the honeycomb-like pattern in the cork that he viewed with his microscope (Figure 1.1). FIgure .2  The cortical cells of a small root of asparagus. (Source: From Grew, 1682.) Hooke’s perspective of the emptiness of cells was propa- gated by Nehemiah Grew (1682), who compared the cells of the pith of asparagus to the froth of beer (Figure 1.2), and is of wood or piths, as the valves in the heart, veins and other still implied in words with the prefix cytos, which in Greek passages of animals, that open and give passage to the con- tained fluid juices one way, and shut themselves, and impede means “hollow place”. Hooke, however, did realize that the passage of such liquors back again, yet have I not hitherto there might be more to a cell than he could see. He wrote, been able to say anything positive in it; … but … some dili- Now, though I have with great diligence endeavoured to find gent observer, if helped with better microscopes, may in time, whether there be any such thing in those microscopical pores detect [them]. Plant Cell Biology Copyright © 22000099, Elsevier, Inc. All rights of reproduction in any form reserved.  2 Plant Cell Biology FIgure .3  Stellate cells from the petiole of a banana. (Source: From von Mohl, 1852.) FIgure .5  Photomicrograph of a swimming Dunaliella cell taken with Nomarski differential interference contrast optics. To further emphasize the vitality of cells, von Mohl also stressed that cells were endowed with the ability to perform all kinds of movements. In the world of the living cell, the only thing that is certain is change—movement occurs at all levels, from the molecular to the whole cell. While I was taught that plants, unlike ani- mals, do not move, some plants can constantly change their position. Get a drop of pond water and look at it under the microscope. Watch a single-celled alga like Dunaliella under the microscope (Figure 1.5). See it swim? These plant cells are Olympic-class swimmers: they swim about 50 m/s— FIgure .4  Spiral vessels, sap tubes, and cells of Marantha lutea. equivalent to five body lengths per second. Not only can the (Source: From deCandolle and Sprengel, 1821.) cells swim, but they can also change their motile behavior in response to external stimuli. When a bright flash of light Hugo von Mohl (1852) pointed out in Principles of the (from the sun or a photographic flash) strikes swimming Anatomy and Physiology of the Vegetable Cell, the first Dunaliella cells, like synchronous swimmers, they all swim textbook devoted to plant cell biology, that indeed plant backward for about a half second. From this observation, cells are not vacuous when viewed with optically corrected even a casual observer will conclude that individual cells have microscopes, but contain a nucleus and “an opake, viscid well-developed sensory systems that can sense and respond to fluid of a white colour, having granules intermingled in external stimuli (Wayne et al., 1991). it, which fluid I call protoplasm.” Von Mohl, echoing the In contrast to Dunaliella, some cells, particularly those conclusions of Henri Dutrochet (1824) and John Queckett of higher plants, remain static within an immobile cell wall. (1852), further revealed through his developmental stud- Yet, if you look inside the cell, you are again faced with ies that cells have a variety of shapes (Figure 1.3) and give movement. You see that the protoplasm dramatically flows rise to all structures in the plant including the phloem and throughout a plant cell, a phenomenon known as cytoplas- xylem. This was contrary to the earlier opinions of deCan- mic streaming (Kamiya, 1959). Look at the giant internodal dolle and Sprengel (1821), who believed that there were cell of Chara (Figure 1.6). The cytoplasm rotates around three elementary forms in plants—dodecahedral-shaped the cell at about 100 m/s. If you electrically stimulate the cells, noncellular tubes, and noncellular spirals (Figure 1.4). cell, the cytoplasmic streaming ceases instantly. As the By focusing on mature plants, deCandolle and Sprengel neurobiologists say, the cell is excitable and responds to had not realized that the tubelike vessels and the spiral-like external stimuli. In fact, action potentials were observed in protoxylem developed from dodecahedral-shaped cells. characean internodal cells before they were observed in the chapter |   On the Nature of Cells 3 FIgure .8  Bright-field photomicrograph of the streaming cytoplasm of the slime mold Physarum polycephalum. FIgure .6  Photomicrograph of a portion of a giant internodal cell of Chara showing several nuclei being carried by cytoplasmic streaming. The velocity in one direction is slightly greater than the velocity in the opposite direction. This causes the cell to migrate in the direction of the more rapid streaming. Since the plasmodium migrates toward food, the velocity of cyto- plasmic streaming in each direction is probably affected by the gradient of nutrients. Nobody knows how this cell per- ceives the direction of food and how this signal is converted into directions for migration. Will you find out? While looking at Physarum, notice that the protoplasm is not homogeneous, but is full of relatively large round bodies rushing through the cell (Figure 1.8). Is what you see the true nature of protoplasm, or are there smaller enti- ties, which are invisible in a light microscope, that are also important in the understanding of cells? Edmund B. Wilson (1923) describes the power and the limitations of the light microscope in studying protoplasm: When viewed under a relatively low magnification … only the larger bodies are seen; but as ... we increase the magnification FIgure .7  Dark-field photomicrograph of the slime mold Physarum … we see smaller and smaller bodies coming into view, at every polycephalum. stage graduating down to the limit of vision … which in round numbers is not less than 200 submicrons. … Such an order of magnitude seems to be far greater than that of the molecules nerve cells of animals (Cole and Curtis, 1938, 1939). The of proteins and other inorganic substances. … Therefore an events that occur between electrical stimulation and the immense gap remains between the smallest bodies visible with cessation of streaming are relatively well understood, and the microscope and the molecules of even the most complex I discuss these throughout the book. organic substances. For these reasons alone ... we should be Lastly, take a look at the large single-celled plasmodium certain that below the horizon of our present high-power micro- of the slime mold Physarum (Figure 1.7; Coman, 1940; scopes there exists an invisible realm peopled by a multitude of Kamiya, 1959; Carlisle, 1970; Konijn and Koevenig, 1971; suspended or dispersed particles, and one that is perhaps quite as complex as the visible region of the system with which the Ueda et al., 1975; Durham and Ridgway, 1976; Chet et al., cytologist is directly occupied. 1977; Kincaid and Mansour, 1978a,b; Hato, 1979; Dove and We have now arrived at a borderland, where the cytologist Rusch, 1980; Sauer, 1982; Dove et al., 1986; Bailey, 1997; and the colloidal chemist are almost within hailing distance of Bozzone and Martin, 1998). Its cytoplasm streams at about each other—a region, it must be added, where both are tread- 2000 m/s. The force exerted by the streaming causes the ing on dangerous ground. Some of our friends seem disposed plasmodium to migrate about 0.1 m/s. Why does it move to think that the cytologist should halt at the artificial bound- so slowly when streaming is so rapid? Notice that the cyto- ary set by the existing limits of microscopical vision and hand plasmic streaming changes direction in a rhythmic manner. over his inquiry to the biochemist and biophysicist with a

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