Many resins, and at least till producpd by the metamorphosis of the cell-tissues, contain also tissue-remains (o. g. dragon'e-bluod, xanthorrhcea). These are the more valuable according as the metamorphosie has been more complete. Red xanthorrhcea often shows pieces of tissue between the resinifled masses. Organic remains not derived from the reein-forming orgaue of the plant also occur ; e. g. fungi. Thus the fine green crust which coats old specimens of gamboge is due to a delicate mycelium fungus.
The microscopic examination of resins at once reveals some facts concerning their origin. While the structure of red xanthorrhcea, exhibiting all stages from unresinified to completely reeinified tissue (according to Wiesner), indicates that this resin is solely due to chemical metamorphosis of the whole tissue, the microscope showe that gamboge must exist in solution in tho tissue of the plant-stem, and consequently proceeds from a cellular secretion. Gamboge really consiets of a gummy groundwork permeated by microscopic grains of resin. On cutting the gamboge tree, the resin-holding sap eecapee, and mingles with the watery sap of other tissues, and thus occurs the secretion of the hard resin.
Dragon's-blood originatee much in the sante manner as xanthorrlicen ; and according to the researches of Karsten and Wigand, it is probable that the ordinary resins of the Comferce (rosins) are produced in the sante way. But it must not he supposed that the metamorphosis from the cellulose of the cell-walls to the resin is sitnplo aud direct. It is indirectly uvident that the cell-walls of the resiuified tissue, before their complete conversion into resin, yield a quantity of tannin. This tannin seems to be a medium between the hydrocarbon and the resin. In the resinification uf whole tissue, it is dnubtlese not only the cell-membrame, but also more or less of the cell-contents, that is utilized. When a rosin, like mastic, occurs in reudy-formed veins, and is not produced by the resorption of cells or whole tissue, it must be coneidered as a secretion-product.
The molting-point is a characteristic test for different kinde of resin, and even for different grades of the eame kind. The lowest met with is that of Siam henzoin-75° (167° F.); the higheet, that of the hardest copale--360° (698° F.) The solubilitiee of the vai ions resins in such eolvents
as alcohol, ether, carbon bisulphide, turpentine-oil, cajuput-oil, chloroform, &c., are well-known guides to their identification (see pp. 1624-7).
The resins exhibit no such chemical relationship as do the fats, for instance. Some few show a general reeemblance, g. mastic, eandsraeh, dammar, olibanum ; hut the grouter number are not only dissimilar, but do not even admit ef being grouped under sections. Resins are as a rule very complicated bodiee, and their origin, as proviouely explained, makes it difficult to expe,ct other wise. The classification of reeins according to the effects of simple reagente is impossible, and recourse has been had to oxidation with caustic alkali, with or without dry distillation. Hlasiwetz has proved in Borne resins a family relationehip with the aromatic series, which clearly indicates that many resins are derived from essential oils ; indeed from the terpenes (e. g. turpentine-oil, lavender-oil), coluphony-like resios can be produced by oxidation, aud it ie quite probable that the sn-called terpeue-reeine exist in the plants as essential oils. The terpene-resins are weak, some times crystalline, acide. Nature affords them mixed with unaltered terpene, which may be distilled off with water. The terpene-free resin is odourless, hard, and but little changed by caustic alkali. Many aldehydes are converted iuto resins by oxidation ; acryl-resin and the resin of acetic aldehyde are familiar examples.
That resins are among the most widely-distributed plaut-producte, is well known. They are found in almost all forms of vegetable life, even in fungi. They chiefly occur in the bark, and either flow out over its surface, or aggregate within it. The chief resin-yielding plants are as follows :— Leguittittesce.—Myrospermum spp. (Peru, Tolu); Vuuapa spp. ; Cop2ifira spp. (Copaiba). emsalpinicc.—Trachylobium spp., Hymenaa spp. (Copal, Jutahy-seca).
Anacardiacecs.—Pistachia spp. (Mastic, Chian turpentine); Rhus spp.
Amyridacew.—Bostoellia spp. (Frankincense, Olibanwn) ; Lica spp. (Copal, Elerni); Burscra spp. (Elemi); Canariunt spp. (Dammar); Amyris spp. ; Hedwigtia balsamifera ; Bulsamodendron spp, (Bdellium, Elemi, Myrrh, Balm of Gilead); Elaphrium spp, spp.