ALLIED AND MISCELLANEOUS PRODUCTS 187 



The last two form the basis of the tannins and will be discussed 

 later. Gallic acid occurs in tea and wine and the barks of many- 

 trees, including the oak and hemlock. 



Essential Oils and Resins. — Associated with these are the es- 

 sential oils and resins, which are also aromatic compounds. To 

 the essential oils are due most of the characteristic odors of plants. 

 They may have different structures but are similar in their pos- 

 session of distinctive odors and tastes. The group is thus a loose 

 one which comprises many substances that are not very closely 

 related chemically. Here are the terpenes of which isoprene from 

 rubber is a well-known derivative. Others are menthol (alcohol) 

 and camphor (ketone). AUyl sulphide or garlic oil, found as a 

 glucoside in onions, garlic, radishes, etc., is one of the few essential 

 oils containing sulphur. 



The resins are oxidation products of the terpenes. Some are 

 liquid, like Canada balsam and turpentine, while others are very 

 hard like copal and amber (a fossil resin). They are extensively 

 used in varnishes, water proofing materials, etc. Some occur as 

 mixtures with gums, of which asafetida ("fetty") is an example. 



What biological significance is possessed by these aromatic 

 compounds can only be guessed at. They are so varied in their 

 chemical nature that it is impossible that they should have the 

 same function. Those which have pleasant odors and tastes may 

 serve to attract animals which aid seed dispersal, and would thus 

 have survival value. It is interesting to note that plants growing 

 in tropical climates, where there is a keen struggle for existence, 

 are more likely to contain such materials, but this may be merely 

 a result of the environment and have no relation to " natural 

 selection." Where the odors or flavors are disagreeable, as in the 

 garlic and onion, they may be explained as protecting the plant 

 against herbivorous animals. 



Many oils and resins, such as eucalyptus oil and turpentine, 

 have antiseptic properties and are secreted especially when the 

 plant is wounded. They undoubtedly protect the plant against 

 the entrance of bacteria and other fungi. The redwoods owe their 

 longevity in part to this protective action of the secreted oils, which 

 confer upon the wood its lasting qualities. Creosoting of timber 

 does artificially what the conifers have done for themselves. 



Tannins. — The tannins, many of which are glucosides, include 

 a large group of substances that have certain common charac- 



