The hard resins are amorphous substances of vitreous character, which consist of very complex aromatic acids, alcohols, or esters, combined with other complicated structures, known as resenes, whose definite chemical nature is not yet known. Among the hard resins are many substances which are extensively used in the manufacture of varnishes, such as copal, amber, dammar, sandarach, etc.
There are also resinous substances, such as asafœtida, myrrh, gamboge, etc., which are mixtures of gums (see [Chapter VI]) and true resins. Some of these have considerable commercial value for medicinal or technical uses.
PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF ESSENTIAL OILS
No theory has yet been advanced concerning the possibility of the use of essential oils and resins by plants in their normal metabolic processes. The very great diversity in their chemical nature makes it impossible that they should all be considered as having the same physiological function, if indeed any of them actually have any such function.
It is evident that those aromatic compounds which occur as normal secretions of plants and which give to the plants their characteristic odors may act either as an attraction to animals which might utilize the plants as food and so serve to distribute the seed forms, or as a repellent to prevent the too rapid destruction of the leaves, stems, or seeds of certain species of plants whose slow-growing habits require the long-continued growth of these portions of the plant for the perpetuation of the species. The presence of these compounds in larger proportions in those species of conifers, etc., which grow in tropical regions, in competition with other rapid-growing vegetation, suggests the latter possibility. It must be admitted, however, that their presence in such cases may be the result of climatic conditions, as indicated by the fact that most spice plants are tropical in habit, rather than the result of their protective influence in the struggle for survival during past ages.
Many of the oils and resins which are secreted as the result of injury by disease or wounds have marked antiseptic properties and undoubtedly serve to prevent the entrance into the injured tissue of destructive organisms.
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.).