240 BIOCHEMICAL SYSTEMATICS 



ponents, however, occur in small quantities and often in only a few 

 species and these substances may provide further taxonomically useful 

 information. 



Exceptional species such as Eucalyptus macarthuri and E. 

 citriodora, in which the chief constituents are geranyl acetate and 

 citronellal, were regarded as end members of sequences in which the 

 ancestral intermediate forms have disappeared in the course of evolu- 

 tion (Read, 1944). 



McNair (1942) attempted to correlate the morphological and 

 chemical characteristics as reported by Baker and Smith and con- 

 cluded that sometimes "primitive" morphology and "advanced" oil 

 characters occurred together, or the opposite relationship occurred. 

 The extent to which this is borne out is difficult to determine from 

 the data in McNair's paper, since he presents no morphological data 

 to compare with the chemistry. Of course, some instances of more 

 rapid evolution in either morphology or oil chemistry are to be 

 expected. One point made by McNair which is noteworthy is that oil 

 constituents of the "advanced" type may appear independently in 

 groups which otherwise show no close genetic relationships. 



As noted earlier, in spite of the classic work by Baker and 

 Smith, very little work on the biological aspects of terpene chemistry 

 has been carried out. This is noted by Mirov in 1948 emphatically: 



. . . the chemistry of essential oils to the problems of biology has been 

 utterly neglected and very little organized work has been done in this 

 direction. A notable exception is, of course, the classical research on 

 the Eucalypts and their essential oils by Baker and Smith. . . . 



Mirov (1948) reported on the terpenes of the genus Pinus. He 

 included extensive tables of data arranged according to species and 

 following the classification of Shaw (1914). In the sub-groups 

 Haploxylon (having a single vascular bundle in each needle with 

 usually five needles per dwarf shoot) and Diploxylon (having a double 

 bundle with two to three needles) there did not appear to be any 

 significant general differences in their terpenes. For example, both 

 groups contained dl-a-pinene as a major constituent, and other, more 

 complex substances occurred sporadically throughout both groups.^ 

 However, Erdtman has shown distinctive differences in the heartwood 

 chemistry of the two groups. (For discussion of Erdtman's work see 

 Chapter 11.) 



2 However, in a discussion following presentation of a paper on the distribution of 

 turpentine components (1958) Mirov stated that the Haploxylon group "have decidedly 

 more sesquiterpenes" and more new substances were found in that group. Mirov beUeves 

 that the two sub-groups split very etu-ly and underwent parallel evolution. 



