There may be two possible explanations of, or reasons for, the presence of any given type of compound in the tissues of any particular species of plant. First, it may be supposed that this particular type of compounds is elaborated by the plant to satisfy its own physiological needs, or for the purpose of storing it up in the seeds as synergic food for the growth of the embryo, in order to reproduce the species. For this rôle of the various organic food materials, etc., we will employ the term "physiological use." On the other hand, it is often conceivable that certain types of compounds, which have properties that make them markedly attractive (or repellent) as a food for animals and men, or which are strongly antiseptic in character, or which have some other definite relationship to other living organisms, have had much to do with the survival of the particular species which elaborates them, in the competitive struggle for existence; or have been developed in the plant by the evolutionary process of "natural selection." For this relation of the compound to the plant's vital needs, we will use the term "biological significance." Such a segregation of the rôles which the different compounds play in the plant's economy may be more or less arbitrary in many cases; but it will be clear that when _physiological uses_ are discussed, reference is being made to the plant's own internal needs; while the phrase _biological significance_ will be understood to refer to the relation of the plant to other living organisms. PHYSIOLOGICAL USES OF THE ORGANIC COMPONENT GROUPS From the standpoint of the rôle which each plays in the plant economy, the several groups of organic compounds may be roughly divided into three classes. These are: (_a_) the framework materials, including gums, pectins, and celluloses; (_b_) synergic foods, including carbohydrates, fats, and proteins; and (_c_) the secretions, including the glucosides, volatile oils, alkaloids, pigments, and enzymes. The _framework material_, as the name indicates, constitutes the cell-wall and other skeleton substances of the plant. It is made up of carbohydrate complexes, produced by the cell protoplasm from the simpler carbohydrates. The _synergic foods_, or "reserve foods" as they are sometimes called, produced by the excess of synthetized material over that needed for the immediate use of the plant, are accumulated either in the various storage organs, to be available for future use by the plant itself or by its vegetative offspring, or in the seed, to be available to the young seedling of the next generation. Proteins not only serve as reserve food materials but also make up the body of the living organism itself. Carbohydrates and fats serve as synergic and reserve foods.