The Various Substances Made by Plants 117 



tion by man. They are formed in living cells from starch, and 

 therefore ultimately from grape sugar, to which they can be 

 changed back in germination and digestion by the action of suit- 

 able enzymes. 



Related to the fats in some respects, though to the later- 

 described proteins in others, are the lecithins, widely distributed 

 in plants, and possessing a considerable interest as the probable 

 basis for the formation of the vastly-important and complicated 

 substance chlorophyll, the composition of which, aside from the 

 presence of carbon, hydrogen, oxygen and phosphorus, is still 

 rather uncertain. 



The other kind of oils, the ethereal, essential, or volatile 

 oils, are very different in composition and meaning. They are 

 familiar to us chiefly in the fragrant oil of lemon and oil of cloves, 

 and are the causes also of the odors, sometimes fragrant and 

 sometimes acrid, of many kinds of leaves (e. g., Lemon Geranium) 

 when cut or crushed; and they cause likewise the fragrance of 

 flowers and fruits. Camphor, and some other aromatic materials 

 are related substances. They are not food products, as the fats 

 are, but serve mostly ecological uses, either in connection with 

 the protection of plants against insects or Fungi, or for the at- 

 traction of animals in connection with dissemination of seeds 

 and cross pollination of flowers, as we shall later consider in detail 

 along with those respective subjects. They are stored as a rule 

 in special receptacles or glands, often of considerable size (figure 

 37). Chemically they are most diverse, some of them consisting 

 only of carbon and hydrogen, approaching near to the formula 

 C 10 H 16 . Little is known as to their exact mode of formation. 



It is a non- volatile oil (called toxicodendrol) , which is the 

 poisonous susbtance in the Poison Ivy; and the fact that it is a 

 non-evaporating oil explains why it is so very difficult to remove 

 from the skin, and why it persists in plants which are long dead 

 and dried. 



Plant Acids. These are agreeably familiar to us as the sub- 



