SECRETION. 103 



and fruits ; sometimes in cellules, in particular structures 

 for themselves, and again in vesicles on the surface of 

 the vegetable. In the Labiatae, almost the whole of the 

 plant is covered with these vesicles, whilst in Umbelli- 

 ferae they are found on the fruit in particular receptacles, 

 called Vittse. 



124. The odours of plants are chiefly owing to these 

 products and sometimes arise from their spontaneous 

 exudation, but in other cases require the plant to be 

 rubbed or broken. In fine sunny weather the oil is 

 secreted in greater quantity, and of a stronger odour 

 than in wet seasons. 



125. These oils, (of which oil of Cloves, Cajeput, and 

 Cinnamon may be taken as examples), contain two 

 principles, Igreusine and Sereusine. 



126. Resin is a term applied to many compound sub- 

 stances ; when it is combined with oil it forms an oleo 

 resin, when withBenzoicAcida^afoam, and when volatile 

 oil and resin exist together in a viscid and strong- smelling 

 state, turpentine; and there are intermediate states 

 between resin and gum, called gum-resins. Resins com- 

 bined with extractive matter and fecula, occur in the 

 roots of many plants, as in the Ipomsea jalapa, which 

 furnishes the Jalap of commerce. 



127. Caoutchouc or Indian Rubber, is obtained by 

 exudation, according to Meyen, for commercial purposes, 

 chiefly from the Siphonia cahuca and Siphonia elas- 

 tica, though many other plants yield it as well ; when 

 newly exuded it is of a whitish yellow colour, but 

 becomes black on exposure to the air ; it is insoluble in 

 water and alcohol, but soluble in naptlia and in some of 

 the volatile oils, and likewise in a fluid termed caoutchou- 

 cine, obtained by distillation from itself. Scarcely any 

 oxygen enters into its composition. 



128. Camphor, as obtained by distillation from the 

 Laurus camphora and Dryabalanops camphora, is of 

 a white colour and crystalline texture, insoluble in water 

 but soluble in alcohol, acetic acid, and the fixed and 

 volatile oils. According to Proust, 100 per cent of 



