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Meristems, Growth, and Development in Woody Plants

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Meristems, Growth, and Development in Woody Plants MERISTEMS, GROWTH, AND DEVELOPMENT IN WOODY PLANTS An Analytical Review of Anatomical, Physiological, and Morphogenic Aspects By J. A. Romberger U.S. Department of Agriculture, Forest Service (The author is Plant Physiologist at the Forest Physiology Laboratory, Plant Industry Station, Beltsville, Md.) Technical Bulletin No. 1293 October 1963 12 88725 PREFACE Apical meristems are small. Lateral meristems are thin. Together they constitute a physically insignificant fraction of the total mass of a tree or shrub. Yet the whole future of the plant depends upon the activity of its meristems. Growth and morphogenesis, and the control of these processes, are largely localized in the meristems proper and in their ancillary regions of occasional cell division, con- tinuing cell enlargement, and cell differentiation. The subject area encompassing meristems, growth, and development is basic to a wide range of research problems in forestry and horticulture. This bulletin is intended for students and research workers, in plant physiology, horticulture, and the forest sciences, who are inter- ested m the control of growth and development in woody plants. It is not a textbook. nTustrations duplicating those readily avail- able in texts have not been provided. Headers are assumed to have knowledge of the basic principles of the anatomy, physiology, and biochemistry of plants, and to have access to textbooks on these sub- jects. I have attempted to go beyond the textbook level in analyzing complex problems, in searching for interrelations between the various islands of research information, and in providing a guide to the early as well as the more contemporary literatuijB. The approach is nonauthoritarian. Many questions are asked. Few are answered. Eeaders are encouraged to speculate and to doubt and question my interpretations as they see nt. I wish to be regarded not as an expert, or a teacher, but as a fellow student. Although growth control in woody plants has many special aspects, it cannot be considered as a subject completely separate from that of growth control in herbaceous species. Much, or most, of the ex- perimental work on growth regulators, photoperiodism, and photo- morphogenesis was done with herbaceous species. Some of the evi- dence discussed and literature cited in this review is not directlj concerned with trees or shrubs, but such citation and discussion is nonetheless prerequisite to intelligent consideration of the specific problems of growth control in woody plants. Throughout the review, emphasis is put upon lines of work spe- cifically aimed at increasing our basic knowledge of meristems and the control of their activities. The voluminous literature concerning purely empirical experimentation aimed at early application in the field IS not stressed. As a knowledge of political and social history is indispensable to social scientists, a knowledge of the history of biology is likewise indispensable to the biological theoretician and experimenter. With- out the past, without an appreciation of past successes and failures, and their significance to us, our future advance would be wavering in direction and lacking in momentum. Such considerations, and the belief that discussions of sincere attempts to arrive at truth are IV PREFACE never obsolete, prompted use of the historical method of exposition in most sections of this review. Plant names are generally the Latin names given in the works cited. Many original sources give no authorities for the names em- ployed. None are given here. Some of the names used herein are not current or are in dispute. Readers who need current names and authorities must seek information in the papers cited, and elsewhere. No review of this type can cover all related areas in addition to the central subject. The very important and closely related subjects of the control of flowering in woody plants, and the physiology of seed dormancy and the germination process, are treated only mci- dentally. Also outside the area of immediate concern are breaking of dormancy by deliberate wounding of plants or by applications of any of a great variety of chemicals having no known relation to any endogenous regulators. This review is not exhaustive even within the subjects covered. The goal was to provide access to important lines of work rather than to cite all significant papers. Some references were intention- ally omitted because they are included in bibliographies of other works cited. Some important papers were undoubtedly overlooked, and numerous recent ones came to my attention too late to be included. Covera^ of some subject areas was modified because of the existence of relatively recent and readily available reviews by other authors. With these limitations imderstood, I hope that these discussions will encourage and facilitate further work on the fascinating subject of meristems and their activity or dormancy in woody plants. A written discussion is Imear. Only one aspect of a subject can be presented at a time. Words, sentences, and paragraphs follow one another. Each separate fact or idea in turn briefly commands the reader's attention. But the realm of ideas is not one dimen- sional. The numerous facts and ideas embodied in this review are related to each other more like various points within the volimie of a sphere than like points on a straight line through space. To pro- mote escape from linearity, numerous cross references have been provided in the text. These are indicated in italics within parenthe- ses, either alone or separated from citations to other works by a semicolon. I sincerely appreciate the assistance and advice received from many people during the preparation of this bulletin. Particularly helpful were Edward R. Moser, Librarian, Division of Biology, California Institute of Technology, and the staff members of the National Agricultural Library in Washington, D.C., and Beltsville, Md. Drs. Bruce M. Pollock, Harry A. Borthwick, Thomas O. Perry, and Robert M. Allen made many constructive suggestions after reading all or parts of the manuscript. CONTENTS Pace Part I. Anatomy and physiological morphology 1 Organization of meristems 1 Development of the meristem concept 1 The origin of cells 1 The apical cell theory _ 3 The histogen theory of apical organization 4 Transition to modern concepts 5 Organization of gymnosperm shoot apices 6 Cytohistological zonation 6 Zone apicale, anneau initial, and méristèm médullaire 12 Organization of angiosperm shoot apices 13 Tunica-corpus theory 13 Cytohistological zonation 16 Méristèm d'attente, anneau initial, and méristèm médullaire 18 Metrameristem 20 Synopsis on shoot apices 20 Organization of root apices 21 Root apex versus shoot apex 21 Ajpical cells and histogens 22 Körper-Kappe theory 23 Many-celled promeristems versus central cells 24 The quiescent center 27 Physiological morphology of shoot meristems and buds 30 Reactivity of shoot meristems 30 Metabolic differentiation within the meristem 30 Culture of isolated apices 31 Morphogenic regions of the apex 32 Special significance of the subapical region 34 Origin of leaves, cataphylls, and vascular tissue 35 Initiation of primordia 35 Procambium ^ 38 Development of leaves 41 Development of cataphylls 44 Vegetative buds and the morphogenic cycle 46 The bud concept 46 A peculiar anatomical feature—the crown 47 Terminal buds 49 Axillary buds 54 Adventitious buds 58 Physiological processes in buds 59 Shoot tip abortion 62 Inability to form terminal buds 62 Physiology of apical abortion 63 Physiological anatomy and development in the cambium 65 Developmental anatomy 65 Morphogenic cycles in the vascular cambium 68 Part II. Episodic growth and dormancy of shoots 71 Concepts, nomenclature, and definitions 71 The dormancy concept and its development 71 Kinds of dormancy—definitions 73 . Alternate growth and dormancy 76 Implications of episodic growth 76 Associated anatomical and cytological changes 78 Analysis of the control problem 80 Internal physiological factors 80 Why summer growth inhibition? 80 Possible root influences 81 Correlated inhibition and apical. dominance 81 v VI CONTENTS Part II. Episodic growth and dormancy of shoots—Continued Analysis of the control problem—Continued Page Experimental control of growth and dormancy in various species 84 An introduction to photoperiodism in woody plants g4 Pinits sylvestris 86 Faaus aylvatica 89 Robinia paeudoacacia. _ 92 Catalpa bignonioides 93 Weigela florida .- — 94 Comtí8 florida ._ 95 ÄÄtis typhina 96 Acer p8ev4oplatantL8 96 The significance of photoperiodism 97 Are photoperiodic receptor and response mechanisms general? 97 Mechanistic implications of photoperiodic responses 99 Possible mechanisms of growth and dormancy control 100 Photomorphogenesis 100 Early work on light intensity and spectral quality 100 Duration of light — _-.- 102 Etiolation - 104 Phytochrome—a photomorphogenic receptor 106 Responses to light of limited spectral regions 111 A second photomorphogenic receptor? 115 Some kinetic aspects of photomorphogenesis and photoperiodism 117 Circadian rhythms in relation to photo- and thermoperiodism 119 Endogenous circadian rhythms 119 Circadian rhythms and photoperiodism 121 Circadian rhythms and thermoperiodism. 124 Endogenous growth regulators 125 Introduction __> 125 The auxin concept 126 Auxins in buds and shoots.. 128 Auxins in developing long shoots versus short shoots 130 Auxins and cambial activity 133 The significance of auxins in dormancy control 138 Gibberellins 140 Kinins 146 Other possible regulators 148
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