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.

Inhalt
A. Structural Botany.- I. General and Molecular Cytology: The Plasma Membrane and the Tonoplast.- II. Cytology and Morphogenesis of the Prokaryotic Cell.- B. Physiology.- I. Plant Water Relations.- II. Mineral Nutrition: Potassium.- III. Photosynthesis. Carbon Metabolism: New Regulators of CO2 Fixation, the New Importance of Pyrophosphate, and the old Problem of Oxygen Involvement Revisited.- IV. Secondary Plant Substances. Nonsteroidal Triterpenoids.- V. Growth.- VI. Developmental Physiology.- VII. Physiology of Movements in Space Experiments.- C. Genetics.- I. Replication.- II. Recombination: Recombination of Mobile Genetic Elements from Plants and Cyanobacteria.- III. Extranuclear Inheritance: Mitochondrial Genetics.- IV. Recognition and Pathogenicity in Bacterial Plant Pathogens.- D. Taxonomy.- I. Systematics and Evolution of the Algae.- II. Taxonomy and Phylogeny of Fungi.- III. Systematics, Morphology and Ecology of Lichenized Fungi.- IV. Systematics of the Bryophytes.- V. Systematics of the Pteridophytes.- VI. Palaeobotany.- E. Geobotany.- I. The History of Flora and Vegetation During the Quaternary.- II. Vegetation Science (Sociobiological Geobotany).- F. Special Topics.- I. Floral Ecology.