This book delves into the many facets of the study of plant development and demonstrates the importance of ontogenetic studies in analyzing the structure, function, and evolution of glands, structures, and organs. The nine chapters that make up this book represent the state of the art of the scientific knowledge on the subject and bring unpublished contributions and important reviews of the most diverse developmental issues of plants compiled for the first time in a single book. Chapter 1 describes an unprecedented way secretory ducts form as the result of cavity coalescence in Malvaceae. Chapter 2 describes a case of homeoheterotopic alteration between nectaries and colleters in Passifloraceae and analyzes the evolution of leaf glands in the family. Chapter 3 emphasizes the importance of anatomical, histochemical and ontogenetic studies in distinguishing colleters and nectaries in leaves of Sapium (Euphorbiaceae) and highlights the importance of ontogenetic studies for the observation of new colleters since many of them are deciduous. Chapter 4 reviews leaf development in vascular plants using traditional morphological and anatomical knowledge combined with the most recent data obtained in molecular studies. Chapter 5 reviews the morphogenesis and evolution of haustoria in mistletoes and evaluates the likely reasons that led to the change from root parasitism to aerial parasitism in Santalales. Chapter 6 demonstrates the importance of the meristematic activity and bud formation for the structure of inflorescences in Acanthaceae, revealing the causes of their architectural diversity in genera such as Lepidagathis. Chapter 7 reveals the ontogenetic causes that resulted in varying degrees of flower reduction, separation of sexes, and their relationship with pollination in urticalean rosids. Chapter 8 uses the ontogenetic study of flowers to analyze the diversity of polyads in Leguminosae and their importance in the taxonomy of the family and the dispersal mode of pollen. Chapter 9 investigates the mode of formation of the pseudomonomerous gynoecia using Anacardiaceae as a model and discusses the evolution of this morphological reduction of gynoecium in other lineages of angiosperms.