236 A TEXT-BOOK OF BOTANY. 



and stems of many plants are supposed to be useful in preventing 

 depredations by animals. The odorous principles which occur in 

 many flowers are supposed to exert a directive influence upon 

 insects and thus assist in the work of cross-pollination. While 

 biologists usually consider the volatile oils as serving ecological 

 uses yet those investigators, who study the perfume-yielding con- 

 stituents very closely, are inclined to consider them as being in the 

 nature of food materials that are used after the fertilization of the 

 flower and during the development of fruit and seeds. 



RESINS, GUM-RESINS AND BALSAMS. A large number of this 

 class of plant products are found in commerce and used in medi- 

 cine and in the arts. A few of these occur as normal products 



FIG. 126. Menthol: A, individual crystals obtained by sublimation; B, the commonly 

 occurring aggregates of very fine needles. 



in living plants, as the gum-resins of the Umbelliferse, the gum- 

 resin euphorbium, and the resins of mastiche and sandarac. Most 

 of the others arise as a result of wounds in plants and are in the 

 nature of pathological products, as benzoin, styrax, Tolu balsam, 

 Peru balsam, etc. Until recently not much was known except in a 

 general way regarding the composition of resins. Largely through 

 the researches of Tschirch and his students the nature and the 

 constitution of the important constituents in a number of the 

 resins have been worked out. As a result of these studies seven 

 principal groups of resins are recognized. 



i. Tannol Resins, These are esters of aromatic phenols and 



