The 50th volume of Progress in Botany appears in new guise. In cooperation with Springer-Verlag we have changed from the less attractive typewriter composition to the direct reproduction of a manuscript which was writ­ ten by means of a text editing system and produced by a laser printer. We, the editors, should like to take the appearance of Volume 50 as the occasion for a few short remarks. Our younger readers are perhaps not aware that our Book Series was founded in 1931 by Fritz von Wettstein, based on the following thoughts and considerations, aptly formulated by him in the Preface to the first volume. "One of the greatest dangers threatening progress in the science of botany is the absolutely unbelievable growth in volume of the literature. The quality of journals, books and individual works that are daily sent to us makes it impossible for anyone person to maintain a general view of the progress made in botany in all the specialized fields, let alone to find time for results from associated su bjects. For varying reasons, every botanist must find this state of matters insupportable. Let us endeavor, in the general interest, to retain a wide background of knowledge, and not become limited specialists. The vitally necessary connections between the specialized fields can only flourish, or even exist, if the general view of botany as a whole can be maintained.

A. Structural Botany.- I. General and Molecular Cytology: The Plasma Membrane and the Tonoplast.- 1. Introduction.- 2. The Plasma Membrane (Plasmalemma).- a) PM Isolation and Purification.- b) Lipids.- c) Proteins.- d) Exocytosis and PM Recycling.- e) In- and Evaginations of the PM.- 3. The Tonoplast (Vacuolar Membrane).- a) Tonoplast Isolation and Purification.- b) Lipids.- c) Proteins.- d) Vacuole and Tonoplast Origin.- References.- II. Cytology and Morphogenesis of the Prokaryotic Cell.- 1. Cytology and Macromolecular Structure: Technical Aspects.- a) Artifacts.- b) Elemental Microanalysis.- c) Structure Analysis of Isolated Cytoplasmic Enzymes.- d) Enzyme Localization by Electron Microscope Immunocytochemistry.- 2. New Isolates and Taxonomical Aspects.- a) New Isolates.- b) A Bacterium Forming up to Five Endospores per Cell.- 3. Structure-Function Aspects at the Cellular Level.- a) Vesicle Formation; Cellulose Degradation.- b) Polysaccharide Storage Material in Methanogenic Bacteria.- c) The Inclusion Bodies in Bacillus thuringiensis subsp. israelensis.- 4. Structure-Function Aspects at the Macromolecular Level.- a) The Cellulolytic Enzyme Complex in Clostridium thermocellum.- b) An Enzyme Complex Involved in Methane Formation.- c) Organization of the Bacterial Nucleoid.- 5. Location of Bacterial Enzymes.- 6. Morphogenesis and Differentiation.- a) Septation and Filament Splitting in Methanospirillum hungatei.- b) Dynamics and Differentiation in Cell Envelope Structure.- c) Size Variation and Differentiation in Phototrophic Bacteria.- d) Spore Formation and Germination.- 7. Recognition and Attachment.- References.- B. Physiology.- I. Plant Water Relations.- 1. Water Relations of Cells and Tissues.- 2. Root Water Uptake and Water Movement Through the Plant.- 3. Stomatal Control of Transpirational Water Loss.- 4. Effects of Water Relations on Germination and Growth.- 5. Implications of Water Stress.- a) Drought Effects on Metabolism.- b) Drought Resistance.- 6. Implication of Waterlogged Conditions.- 7. Habitat Water Relations and Plant Performance.- 8. Water Relations of Poikilohydric Plants.- References.- II. Mineral Nutrition: Potassium.- 1. Introduction.- 2. The Molecular Level: Membrane-Transport Proteins.- 3. The Membrane Level.- a) Interactions and Exchange of Ions and Energy Coupling.- b) Potassium Channels.- 4. The Cellular Level.- a) Transport Kinetics.- b) Compartmentation and the Regulation of Transport.- ?) Feedback Systems.- ?) Cytosolic Potassium Homeostasis.- ?) Potassium and Cytosolic pH Homeostasis.- 5. The Physiological Level.- a) Physiological Processes in General.- b) Stomatal Guard Cell Movements.- c) Rhythms.- d) Growth and Phytohormone Interactions.- e) Na+/K+-Selectivity and Salinity Responses.- f) Sugar Translocation in the Phloem.- 6. The Whole-Plant and Nutritional Level.- 7. Conclusions.- References.- III. Photosynthesis. Carbon Metabolism: New Regulators of CO2 Fixation, the New Importance of Pyrophosphate, and the old Problem of Oxygen Involvement Revisited.- 1. Introduction.- 2. Inorganic Carbon Uptake.- 3. Investigations with Isolated Plastids.- 4. The Return of RuBP Carboxylase.- 5. RuBP Carboxylase and the Limits to Photosynthesis.- 6. The Other Calvin Cycle Enzymes.- 7. Light-Mediated Enzyme Regulation.- 8. Starch Metabolism.- 9. Sucrose Metabolism: Fructose-2,6-P2, and a Role for PPi?.- 10. Mitochondrial Respiration in Illuminated Photosynthetic Cells?.- 11. Photorespiration.- 12. The Many Roles of O2 in Photosynthetic Carbon Metabolism.- 13. C4 Plants.- a) Transport of Pyruvate.- b) Transport of PEP.- c) Transport of Malate.- d) Transport of Inorganic Carbon.- e) Enzymes.- f) Control of Carbon Partitioning.- 14. C3-C4 Intermediates.- 15. CAM Plants.- a) Carbon Partitioning.- b) Malic Acid Accumulation.- References.- IV. Secondary Plant Substances. Nonsteroidal Triterpenoids.- 1. Introduction.- 2. Oleanane and Ursante-Type Triterpenoids.- 3. Different Triterpenoids.- 4. Limonoids.- 5. Cucurbitacins.- References.- V. Growth.- 1. General Reviews.- 2. Abscicic Acid.- a) Reviews.- b) Methods of Determination.- c) Occurrence and Biosynthesis.- d) Metabolism.- e) Physiological and Molecular Mode of Action.- 3. Ethylene.- a) Reviews.- b) Methods of Determination.- c) Biosynthesis and Its Regulation.- d) Physiological and Molecular Mode of Action.- References.- VI. Developmental Physiology.- 1. Gene Expression.- a) Organ- and Tissue-Specific Gene Expression.- b) Phytochrome-Regulated Gene Expression.- c) Heat Shock Responses.- ?) Heat Shock Proteins.- ?) Gene Expression.- ?) Functions.- d) Wounding.- 2. Phytochrome.- a) Serological Properties.- b) Intracellular Localization.- c) Ubiquitin.- ?) Electron Microscopic Studies.- ?) Biochemical Studies.- 3. Elongation Growth.- a) Extensibility of the Outer Epidermal Cell Wall as Growth-Limiting Factor.- b) Microfibril Orientation as Regulatory Element.- c) Matrix Components.- d) Cell Wall Proteins.- e) Molecular Mechanisms of Growth Induction.- 4. Circadian Rhythm.- a) Clock Mutants.- b) The Role of Protein Synthesis.- c) Temperature Compensation.- References.- VII. Physiology of Movements in Space Experiments.- 1. Introduction.- 2. Growth and Orientation of Gravitropic Organs.- 3. Structure of Root Statocytes.- 4. Circumnutation and Epinasty.- References.- C. Genetics.- I. Replication.- 1. Introduction.- 2. Structural Analysis of the Origin of Replication (OriC).- 3. Proteins and Enzymatic Functions Required for Replication Initiation.- a) DnaA-Protein.- b) DnaB/DnaC Proteins.- c) RNA-Priming.- d) DNA Synthesis by DNA Polymerase III.- e) Elongation.- 4. Control of the Initiation of Replication.- 5. Termination of Replication.- 6. Outlook.- References.- II. Recombination: Recombination of Mobile Genetic Elements from Plants and Cyanobacteria.- 1. Introduction.- 2. Mobile Elements in Higher Plants.- a) Transposons.- ?) Transposons Have Been Identified in a Number of Higher Plants.- ?) Transposons Generate DNA Rearrangements.- ?) Transposons May Influence Plant Gene Expression.- ?) Plant Transposons Have a Characteristic Sturcture and May Code for Polypeptides.- b) Plasmids.- ?) Linear Plant Plasmids Contain Terminal Inverted Repeats.- ?) Plasmids S1 and S2 Generate Genome Instabilities.- ?) Plasmids Have Been Identified in a Number of Different Plants.- c) Promiscuous DNA.- ?) Genetic Information May Be Transferred Between Organelles.- ?) Mechanisms for Transfer of Genetic Information Between Organelles.- 3. Mobile Elements in Cyanobacteria.- ?) In Anabaena a Mobile Genetic Elem…