But apart from these possible protective influences which may have had an important effect upon the preservation and perpetuation of the species of plants which secrete them, there is no known biological necessity for the presence of these aromatic substances in plants. REFERENCES ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze, Kautschuk," 822 pages, Berlin, 1912. ALLEN'S Commercial Organic Analysis, Vol. IV, "Resins, Rubber, Guttapercha, and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th ed.). HEUSLER, F., trans. by Pond, F. J.--"The Chemistry of the Terpenes," 457 pages, Philadelphia, 1902. PARRY, E. J.--"The Chemistry of Essential Oils and Perfumes," 401 pages, 20 figs., London, 1899. CHAPTER XII THE VEGETABLE BASES We come, now, to the consideration of the characteristically nitrogenous compounds of plants. None of the groups of compounds which have been considered thus far have, as a group, contained the element nitrogen. This element is present in the chlorophylls and in certain other pigments, but not as the characteristic constituent of the molecular structure of the group of compounds, nor do these compounds serve as the source of supply of nitrogen for the plant's needs. The characteristic nitrogen-containing compounds may all be regarded as derived from ammonia, or ammonium hydroxide, by the replacement of one or more hydrogen atoms with organic radicals of varying type and complexity. If the group, or groups, which be considered as having replaced a hydrogen atom in ammonia, in such compounds, is an alkyl group, the compound is strongly basic in character and is known as an _amine_; whereas if the replacing group is an acid radical, the resulting compound may be neutral (known as _acid amides_), or weakly acid (known as _amino-acids_) in type. Compounds of the first type constitute the _vegetable bases_; while those of the second type are the _proteins_. The vegetable bases may be divided into three groups. These are (_a_) the _plant amines_, which are simple open-chain amines; (_b_) the _alkaloids_, which are comparatively simple closed-ring amines, containing only one nitrogen atom in any single ring; and (_c_) the _purine bases_, which are complex compounds containing a nucleus with four carbon atoms and four nitrogen atoms arranged alternately to form a double-ring group. THE PLANT AMINES The simple amines bear the relation to ammonia, or ammonium hydroxide, represented by the following formulas, in which the R indicates any simple alkyl radical: