WATER-SOLUBLE ORGANIC ACIDS 



39 



as malic acid in green apples. Oxalic acid which seems never to be metabolized further 

 in seed plants is oxidized in mosses by a flavoprotein oxidase (1). Malic acid acts as a 

 specific attracting agent for spermatozoids of some mosses and ferns and is presumably 

 secreted by the archegonia or egg cells (2). Those acids which occur in the vacuole as 

 salts may participate in establishing a proper acid-base balance. Acids which form iron 

 chelates may aid in the vascular transport of this cation (3). 



LACTONES 



Hydroxy acids may exist as lactones or inner esters if the hydroxyl group is situated 

 so that the lactone has a 5 or 6-membered ring. Of the common plant acids isocitric is 

 often found in the lactone form: 



iso-citric lactone 



A few unusual hydroxy acids are never found in the free carboxyl form, and only the lac- 

 tones are known. Among these lactones are several compounds of interest in physiology 

 since they are often very powerful irritants of skin and mucous membranes and some are 

 fungicidal. Structures and occurrence of some simple lactones are shown below. Those 

 with hydroxyl groups may exist naturally as glycosides. Other lactones will be found de- 

 scribed in other chapters along with related compounds (e. g. , terpenoids, fatty acids, 

 etc.). 



HC^^CH 



0=C CHCH2O-&LUCOSE 



\ / ranunculin (Ranunculus bulbosus) 





 CH 



CHj'^CH 



CH3-C>I .C=0 



parasorbic acid (Sorbus aucuparia) 



HC=C0CH3 



G-methoxy-2-butenoic lactone 



vH2 -C-^O (Narthecium ossifragutn) 



