186 PLANT PHYSIOLOGY 



Aromatic Phenols, Alcohols, Aldehydes, and Acids. — In this 

 heterogeneous g^oup of compounds are placed those with the 

 carbons in a ring. That is, they belong to the aromatic series as 

 classified by the organic chemist. 



Phenols. — The phenols are aromatic hydroxides as shown 



OH 



by the formula for hydroquinone : which is the only one 



OH 



with two OH groups that is found in plants in the free state. 

 It is found in the leaves of the cranberry and also as a glucoside 



OH/VOH 



in many of the heaths. Phloroglucin, with three 



OH 



hydroxy Is is very widely distributed. Thymol from thyme also 

 belongs in this group. 



Aldehydes and Alcohols. — Aromatic alcohols and aldehydes 

 are very common constituents of glucosides. Saligenin or sali- 



CHOH 

 /\OH 



cylic alcohol from the glucoside, salicin, has the formula: 



CHO 

 while benzaldehyde (from amygdalin) /\ and cinnamon oil 



are examples of aromatic aldehydes. Here also might be placed the 

 sterols, but because of their relation to the fats, they were dis- 

 cussed in Chapter XIII. 



Acids. — Of the aromatic acids the best known are salicylic, 

 protocatechuic, and gallic acids: 



COOH COOH COOH 



OH 



OHx /OH 

 OH OH 



salicy'ic protocatechuic gallic 



