ASSIMILATION. 



117 



Mentha viridis ; oil of lemons and oranges from the rinds 

 of those fruits; oil of cloves from the fruit of Caryophyllus 

 aromaticus ; and oil of turpentine from various species of 

 Abies and Pinus. Volatile oils are generally produced and 

 stored up in certain special receptacles, expressly organized 

 for this purpose. They are represented in 



Fig. 24. 



Vertical section of the rind of an orange, the reservoirs" of volatile oil being 

 marked r, r, r. The cellular tissue of the rjnd is seen surrounding the oil 

 cavities, and the cells are elongated and condensed so as to form a compact 

 tissue in the walls. 



The transparent dots which are seen in the leaves of the 

 lemon, the orange, and the myrtle are produced by these 

 receptacles, which, being filled with volatile oil, are neces- 

 sarily more translucent than the other parts of the leaves. 

 When held up to the light, these leaves appear as if punc- 

 tured with numerous fine holes. Several species of Hy- 

 pericum present the same appearance. 



The fixed oils are found chiefly in the seeds where they 

 supply the place of starch, as in the cotyledons of some 

 of the Cruciferse, Composite, and many other plants. 

 Fixed oils are generally obtained by pressure. Their 

 economical applications are very numerous. Olive, almond, 

 linseed, rape, cocoa-nut, and castor oils are familiar ex- 

 amples. 



Such are the facts which have been ascertained respect- 

 ing the nature of these vegetable products. Chlorophyl, 

 starch, gum, and sugar, are almost universally diffused; 



