It is instructive to compare the response of biologists to the two themes that comprise the title of this volume. The concept of the cell cycle-in contra distinction to cell division-is a relatively recent one. Nevertheless biologists of all persuasions appreciate and readily agree on the central problems in this area. Issues ranging from mechanisms that initiate and integrate the synthesis of chro mosomal proteins and DNA during S-phase of mitosis to the manner in which assembly of microtubules and their interactions lead to the segregation of metaphase chromosomes are readily followed by botanists and zoologists, as well as by cell and molecular biologists. These problems are crisp and well-defined. The current state of "cell differentiation" stands in sharp contrast. This, one of the oldest problems in experimental biology, almost defies definition today. The difficulties arise not only from a lack of pertinent information on the regulatory mechanisms, but also from conflicting basic concepts in this field. One of the ways in which this situation might be improved would be to find a broader experimental basis, including a better understanding of the relationship between the cell cycle and cell differentiation."

Plant cells contain various types of plastid, the best known among which is the chloroplast. Apart from their predominant interest for the work on photo synthesis, however, chloroplasts have attracted considerable attention for other reasons. This pertains to extranuclear inheritance of cell organelles and, particularly important for this series, to the participation of chloroplasts as discrete and partly autonomous cell constituents in the developmental biochemistry of plants. This volume is composed of articles by investigators who are actively involved in work on various aspects of research on chloroplasts. Each author has independently covered and analyzed as comprehensively as possible the particular aspects assigned to him. This has the advantage of bringing out many different facets of the situation, though some overlapping has-to be taken into account. We are sure that this volume will enable the reader to gain a broad theoretical and experimental basis for the understanding of the development of chloroplasts and the relationship between plant cells and these organelles."

The first volume of the series, on "The Stability of the Differentiated State" received many favorable reviews from the scientific community. Many readers seem to agree with us that publication of topical volumes is a worthwhile alternative to periodic compilations of rather unrelated, though up-to-date reviews. Production of topical volumes is however, plagued with one great difficulty, that of "author synchronization." This difficulty explains the lag between volumes 1 and 2 of the series. Nevertheless we hope that the present volume will be appreciated as a valuable source of information on its central topic: How do cell organelles originate, and what mechanisms assure their continuity? Tubingen, Berlin, Zurich, \V. BEERMANN, J. REINERT, H. URSPRUNG, Heidelberg H. -W. HA GENS Contents Assembly, Continuity, and Exchanges in Certain Cytoplasmic Membrane Systems by W. GORDON WHALEY, MARIANNE DAUWALDER, aud ]OYCE E. KEPHART 1 I. The Nature of the Membrane. . . . . . H. The Assembly of Membranes . . . . . . 5 III. The Growth and Transfer of Membranes. 6 A. The Nuclear Envelopc . . . 6 B. The Endoplasmic Reticulum 13 C. The Golgi Apparatus . 17 D. The Plasma Membrane 28 E. Vacuoles and Vesicles 31 IV. Concluding Remarks 37 References . . . . . 38 Origin and Continuity of Mitochondria by ROBERT BAXTER 1. Introduction . . . . . . . . . . . . . 46 H. Mitochondrial Biogenesis: thc Machincry 46 III. Limitations of Mitochondrial Autonomy 50 IV. The Replication of Mitochondria 53 V. Discussion and Conclusion 58 Referenccs . . . . . . . . . 59 Origin and Continuity of Plastids by \VILFRIED STUBBE 1. Introduction . . . . . . . . . . . . . 65 II. Arguments for the Continuity of Plastids ."

H. BINDING and J. REINERT In collaboration with the first authors of this volume This volume is devoted to the development of cell clones and plants from ma nipulated cells: isolated protoplasts, cell fusion bodies, and transformed cells. Isolated protoplasts represent cells which are liberated from their walls and separated from the differentiation pattern of the organism. Investigations on re generation from protoplasts provide a better understanding of the process and control of developmental pathways. Whereas protoplast isolation results in alteration of the state of differentiation of a cell, protoplast fusion is a means for the creation of cells with novel genetic constitution. Fascinating features are (1) to hybridize cells which-unlike gametes -did not derive from meiosis products, (2) to bring together foreign plastids and mitochondria and to investigate their parasexual reactions, and (3) to match ge netic traits which had been separated for long periods of evolution. Highly sophisticated techniques have already been elaborated for the transfer of genes by the use of isolated DNA and gene transfer systems. Highly promising results have already been obtained by the use of Ti plasmids of Agrobacterium, but direct DNA transformation is also proving to be useful. Most of the results in these areas are preliminary and/or limited to a few sys tem.lt is the aim of this volume to present the main features, but at the same time to draw attention to problems and perspectives of protoplast regeneration and so matic cell genetics in order to stimulate further investigations."

Letters to John is about my relationship with my Spirit Companion John, who took quite seriously his assignment to protect, guard and guide me in crises and in beautiful and subtle moments. Each event strengthened my faith in John's presence during my entire eighty-six years. A Guardian Angel truly is the generous expression of God's love for His people.