When WILHELM RUHLAND developed his plan for an Encyclopedia of Plant Physiol ogy more than three decades ago, biology could still be conveniently subdivided into classical areas. Even within plant physiology, subdivisions were not too difficult to make, and general principles could be covered sufficiently in the two introductory volumes of the Encyclopedia on the physical and chemical basis of cell biology. But the situation changed rapidly even during the 12-year publication period of the Encyclopedia (1955-1967). The new molecular direction of genetics and structural research on biopolymers had an integrating effect on all other biological fields, including plant physiology, and it became increasingly difficult to keep previously distinct areas separated. RUHLAND'S overall plan included 18 volumes and about 22,000 pages. It covered the entire field of plant physiology, in most cases from the very beginning. But, as each volume appeared, it was clear that its content would soon be outdated."

Trees have the distinction of being the largest and oldest living organisms on earth. Although the herbaceous habit has made unprecedented evolutionary gains since the middle and late Cenozoic, trees still are the most conspicuous plants covering the habitable land surface of the earth. Man has long sought their shelter and protection, utilized their food and fiber, and often exploited them to his own detriment. Trees have always been of much interest to botanists, and many of the early investigations concerning the structure and function of plants were conducted with trees. At the beginning of the present century the use of trees for basic investigations began to decline. The reasons for this are obvious. Investigations of structure and function began to shift from whole organisms to tissues, then to individual cells, and finally to cellular organelles and subcellular particles. Physiological research became increasingly more detailed and complex, requiring more and more precisely controlled laboratory conditions. Hence, a relatively small number of herbaceous plants, various unicellular algae, fungi, and bacteria have become standard research material in most laboratories.