^22 BIOCHEMICAL SYSTEMATICS 



acids, the families Gnetaceae, Verbenaceae, and Dipterocarpaceae 

 are among those included. Throughout the twelve groups listed, 

 families with few or no affinities are placed together, though in some 

 instances two or more families which are related on the basis of their 

 classical treatment occur together. 



Obviously, attempts to read phylogenetic implications from 

 this pattern of distribution of fatty acids will avail nothing. Yet, fatty 

 acids may prove useful to systematics since a fairly large number 

 of fatty acids are known to have limited distribution (for example, the 

 cyclic unsaturated acids of the Flacourtiaceae). However, two factors 

 must be considered to bear upon the assessment of the systematic 

 significance of the distribution of fats. There are about the same num- 

 ber of fatty acids known as there are amino acids, and many of the fatty 

 acids, like many amino acids, are widely distributed. Secondly, as 

 Hilditch has pointed out, in the formation of typical triglycerides the 

 glyceride structure tends to be dependent upon the proportions of the 

 various component acids. In other words the enzymatic esterification 

 of the fatty acids with glycerol appears to be of low specificity so that 

 the distribution of fatty acids in glycerides tends towards the maxi- 

 mum degree of heterogeneity. The effect of this is that a full range of 

 variation, for a given fatty acid complement, is permitted. If two 

 species have a similar fatty acid complement, they will both produce 

 a similar fat complement. Doubtlessly, there are exceptions to this 

 generalization, and more needs to be known about the precise control 

 of fat synthesis. 



One of the difficulties of the classification of Hilditch 

 illustrated above is that it is based upon major component acids 

 which in the majority of instances are of exceedingly wide occurrence. 

 For example, oleic acid is included in nine of the twelve groups. It is 

 not hkely that the distribution of this acid offers much to chemo- 

 systematics. Another difficulty hes in the fact that since the groups 

 are based on major components, not absolute distinctions, one is not 

 measuring the presence of a given metabolic pathway but rather cer- 

 tain favored pathways. In effect it is not always clear what is actually 

 being measured. Concentration on the unusual or the rare fatty acids 

 is likely to prove more profitable. 



Excellent treatments of the chemistry and distribution of 

 fatty acids are available in the recent hterature (Hilditch, 1956; 

 Meara, 1958). No such comprehensive account is included here and in 

 fact much of the information in these works is not strictly relevant to 

 systematics. However, certain basic considerations of the chemistry 

 of fatty acids, specifically the major variations, and a brief description 

 of certain features of their biosynthesis may be useful in providing 



