First published in 1977, this volume is a short and integrated account of the dynamic mechanisms involved in the defence of plant cells against attack by parasitic bacteria and fungi. The central interest of the volume is with the processes by which plant cells perceive the approach of an intruder and occasionally permit, but usually discourage, its further progress. How do the genes of host and parasite communicate to determine the outcome of attempted parasitism? Is there a universal defence mechanism in all plants and, if so, what is it? What contribution does the much studied process of phytoalexin formation make to the defence of plants? These are the main questions considered by Professor Deverall, and they are approached from a basis of our understanding of the genetical, cytological and biochemical interactions between plants and parasites. Plant pathologists, mycologists, botanists, microbiologists, plant physiologists and plant biochemists who are professionally concerned with plant disease will find that this monograph reviews past advances in an area that was once the subject of much attention, and provides suggestions for solving some of the problems.