138 PART II. — VEGETABLE HISTOLOGY. 



they replace starch as a reserve food material, and hence are of 

 great importance to the life of the plant. Chemically consid- 

 ered, they are combinations of glycerine with various fatty acids, 

 as oleic, stearic, palmitic and myristic acids. 



The waxes, which, in their chemical nature, are allied bodies, 

 being compounds of fatty acids with complex alcohols other 

 than glycerine, sustain very different relations to the life of the 

 plant. Instead of serving as reserve food-materials, they appear 

 to possess no nutritive value whatever, but are purely protective 

 in their function. They occur as excretions on the cuticularized 

 epidermis of many plants. The "bloom " of certain fruits is of 

 this nature, and the glaucous appearance of the leaves and stems 

 of many plants is due to the same cause. The parts covered by 

 it are protected from wet, and so from the spores of destructive 

 fungi ; it doubtless serves also to check excessive evaporation 

 from the plant. Occasionally, as in the case of the Wax Palm 

 of New Granada and the Wax Myrtle of our New England 

 coast, the secretion is sufficiently abundant to be of commercial 

 importance. 



Volatile Oils. Ethereal or volatile oils bear some resem- 

 blance in their appearance and physical properties to the fixed 

 oils or fats, but they differ from them both in chemical nature 

 and in origin. Moreover, they are incapable of serving as reserve 

 food materials, and are of no nutritive value to the plant. They 

 are more or less volatile at ordinary temperatures, are slightly 

 soluble in water (to which they communicate their taste and 

 odor), are not converted into soaps by the action of alkalies, and 

 they may be completely volatilized by heat without undergoing 

 chemical change. Some of them are of service to the plant in 

 protecting it against injurious insects or other animals, and per- 

 haps also against destructive fungi, while others, as the floral 

 perfumes, are useful, as we have already seen, in attracting 

 insects to flowers, and so in effecting cross-fertilization by their 

 agency. They occur in solution in the cell-sap of most flowering 

 plants, and are often excreted along with resins into secretion 

 reservoirs. 



They may be classified according to their composition into 

 hydrocarbons, represented by common oil of turpentine (C 10 H 16 ), 

 oil of cubebs, oil of copaiva, etc.; oxygenated essences, as the oils 



