132 BIOCHEMICAL SYSTEMATICS 



leaves showed the pattern to be quite stable and characteristic for the 

 species (Eglinton, 1962). However, the techniques also served to dis- 

 tinguish species of the same genus. Four species of the genus Hebe, in 

 which hybridization frequently occurs, were compared and according 

 to Eglinton et al. the four species can be immediately distinguished 

 chemotaxonomically, e.g. Hebe odora has a major constituent of C29; 

 H. parviflora and H. diosmifolia, C31; and H. striata, C33. An entirely 

 unsuspected source of major chemical variation immediately acces- 

 sible to analysis is therefore disclosed by these investigations. 



The remainder of this section will be concerned with a few 

 selected examples of fatty acids whose distributions are restricted to 

 or characteristic of certain plant families. 



FLACOURTIACEAE 



Chaulmoogric (Cie) and hydnocarpic (Cig) acids occur in this 

 family together with lower homologues in trace amounts. The distinc- 

 tive characteristic of this group of acids is the presence of a cyclo- 

 pentene ring. Although present in a number of genera of the Flacour- 

 tiaceae they do not occur outside the family. It is therefore evident 

 that the distribution of these cyclic acids is of taxonomic interest. 



Fatty acids with cyclic groups occur also in the genus Sterculia 

 (Sterculiaceae). Thus sterculic acid (C19) possesses a three-membered 

 ring inside the chain— produced, possibly, by addition of a carbon and 

 yielding an uneven number of carbons. Subsequently, Shenstone and 

 Vickery (1961) have reported that, in addition to Sterculia and 

 Brachychiton of the Sterculiaceae, certain Malva and Gossypium 

 species (Malvaceae) produce the cyclopropene acids, sterculic acid and 

 malvalic acid. Both families are placed in the order Malvales. 



CRUCIFERAE 



In this family the unsaturated acid, erucic acid (C22), is quite 

 prominent. According to Meara (1958) it is probable that many, if not 

 most Cruciferae, contain erucic acid. Only a small proportion of the 

 total species has been subjected to detailed analysis, however, and 

 the statement is based partly on inferences derived from the low 

 saponification number of the fats from a larger number of crucifer- 

 ous species. 



Outside the Cruciferae, the nasturtium {Tropaeolum minus), 

 of the monogeneric family Tropaeolaceae, contains a large quantity of 

 erucic acid. Since the Tropaeolaceae are usually placed in the Ger- 

 aniales and show no obvious phylogenetic affinity to the Cruciferae, 

 it is likely that the high erucic acid content in the two families is 

 coincidental. It is interesting that certain relatively uncommon iso- 

 thiocyanates occur in the two families (Chapter 14). 



