Ch. Ill, 13] USES OF THE PLANT'S FOOD 



107 



protoplasm is wholly insignificant in comparison with the 

 magnitude and importance of the phenomena it displays. 



4. THE SECRETIONS. These are numerous and di- 

 verse substances having each a special meaning in the plant's 

 economy. Chemically they are as different as well can be. 

 Some are carbohydrates; others are hydrocarbons (con- 

 taining carbon and hydrogen only) ; some contain nitrogen 

 like the amides; while still others are obvious transfor- 

 mations from proteins. Some secretions have a perfectly 

 obvious function ; others clearly have some function though 

 it is not known; but in many cases the substances seem 

 to represent simply by-products of functional changes, or, 

 like autumn colors, the incidental result of conditions which 

 happen to occur in certain parts. Some of them serve well 

 certain needs of man, who takes them for his purposes, often 

 extracting and refining them to this end. The principal 

 classes of secretions are the following. 



The Essential Oils, or aromatic oils, best known in 

 Clove oil, Cedar oil, oil of Lavender, and of " Lemon Ge- 

 ranium," and the oil of Orange rind, differ greatly from the 

 fatty oils in being volatile, 

 and hence giving odors. They 

 occur in plants in special cells, 

 or in special collections of cells 

 called glands (Fig. 67). They 

 are the basis of practically all 

 the odors of plants, including 

 the fragrance of flowers, to 

 which they serve to guide 

 insects in connection with 

 cross-pollination, later to be 

 more fully considered.* In 

 leaves they have been sup- 

 posed to give protection, by their acrid taste, against insect 

 enemies, or to have other uses, for all of which the evidence 

 is still insufficient. Chemically they are in part hydrocar- 



Fig 67. — A gland, in section, 

 oontaining ethereal oil, in Die- 



tamnufi Frasiiiilla; much magni- 

 fied. (From Sachs.) 



