These compounds may be either secretions or excretions of plants, sometimes normally present in the healthy tissue, and sometimes produced as the result of injury or disease. The essential oils and the resins often occur associated together in the plant; or, the resins may develop from the oily juice of the plant after exposure to the air. THE ESSENTIAL OILS These may be divided, according to their chemical composition, into two major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated and sulfuretted oils. The =terpenes= are of three different types, namely: (_a_) the hemiterpenes, C_{5}H_{8}, unsaturated compounds of the valerylene series, of which _isoprene_ (found in crude rubber) is the best-known example; (_b_) the terpenes proper, C_{10}H_{16}, which constitute the major proportion of the whole group; and (_c_) the polyterpenes (C_{5}H_{8})_{_n_}, of which _colophene_ and _caoutchouc_ are the most common examples. Eleven different terpenes having the formula C_{10}H_{16} have been isolated from various plant juices, and their molecular arrangement carefully worked out. The following three examples will serve as typical of the general structural arrangement of these hydrocarbons: Limonene Camphene Pinene H | C /|\ / | \ / | \ / | \ / | \ CH_{3} H / | \ | | / | \ C C /CH_{3} | CH_{3}\ / \\ CH_{3} / | \ H_{3}C \|/ CH_{2} / \\ \ / | \ | C | H_{2}C CH C | CH_{2} | / | | | /| | | | / | | | CH_{3} | HCH | | / | | | | | | | / | H_{2}C CH_{2} CH_{2}=C | CH_{2} | / | \ / \ | / | / | \ / \ | / | / | CH C |/ | | | HC CH C H \ // // \ \ //