=26. The place of function.= In the foregoing it is understood that the immediate effect of the physical factors of the habitat is to be found in the functions of the plant, and that these determine the plant structure. Function has so long been the especial theme of plant physiology that methods of investigation are numerous and well known, and it is unnecessary here to consider it further than to indicate its general bearing. The essential sequence in ecological research, then, is the one already indicated, viz., habitat, plant, and formation, and this will constitute the order of treatment in the following pages. That portion of floristic which is not mere descriptive botany belongs to the consideration of the formation, and in consequence there will be no special treatment of floristic as a subdivision of ecology. CHAPTER II. THE HABITAT CONCEPT AND ANALYSIS =27. Definition of the habitat.= The habitat is the sum of all the forces or factors present in a given area. It is the exact equivalent of the term environment, though the latter is commonly used in a more general sense. As an ecological concept, the habitat refers to an area much more definite in character, and more sharply limited in extent than the habitat of species as indicated in the manuals. Since the careful study of habitats has scarcely begun, it is impossible to recognize and delimit them in an absolute sense. Visible topographic boundaries often exist, but in many cases, the limit, though actual, is not readily perceived. Contiguous habitats may be sharply limited, or they may pass into each other so gradually that no real line of demarcation can be drawn. Whatever variations they may show, however, all habitats agree in the possession of certain essential factors, which are universally present. On the other hand, a few factors are merely incidental and may be present or absent. The relative value and amount of these is probably similar for no two habitats, though the latter readily fall into groups with reference to the amount of some particular factor.