A similar plan is followed in the other great groups. The processes of fertilization and reproduction can be most easily observed in the lower plants like the algæ and fungi, and this is an additional argument in favor of giving emphasis to these forms of plant life as well as the advantage of proceeding logically from simpler to more complex forms. Having also learned some of these plants in our study of physiology, we are following another recognized rule of pedagogy, i.e., proceeding from known objects to unknown structures and processes. Through the study of the organs of reproduction of the lower plants and by general comparative morphology we have come to an understanding of the morphology of the parts of the flower, and of the true sexual organs of the seed plants, and no student can hope to properly interpret the significance of the flower, or the sexual organs of the seed plants who neglects a careful study of the general morphology of the lower plants. Part III. _Plant members in relation to environment._ This part deals with the organization of the plant body as a whole in its relation to environment, the organization of plant tissues with a discussion of the principal tissues and a descriptive synopsis of the same. This is followed by a complete study from a biological standpoint of the different members of the plant, their special function and their special relations to environment. The stem, root, leaf, flower, etc., are carefully examined and their ecological relations pointed out. This together with the study of physiology and representatives in the groups of plants forms a thorough basis for pure plant ecology, or the special study of vegetation in its relation to environment. There is a study of the factors of environment or ecological factors, which in general are grouped under the physical, climatic, and biotic factors. This is followed by an analysis of vegetation forms and structures, plant formations and societies. Then in order are treated briefly forest societies, prairie societies, desert societies, arctic and alpine societies, aquatic societies, and the special societies of sandy, rocky, and marshy places. _Acknowledgments._ The author wishes to express his gratefulness to all those who have given aid in the preparation of this work, or of the earlier editions of Elementary Botany; to his associates, Dr. E. J. Durand, Dr. K. M. Wiegand, and Professor W. W. Rowlee, of the botanical department, and to Professor B. M. Duggar of the University of Missouri, Professor J. C. Arthur of Purdue University, and Professor W. F. Ganong of Smith College, for reading one or more portions of the text; as well as to all those who have contributed illustrations.