284 BIOCHEMICAL SYSTEMATICS 



ratio of guaiacyl to syringyl residues was as low as 1 : 3, with monocots 

 having a shghtly higher ratio. 



Some angiosperms regarded as primitive, for example, Bellio- 

 lum haplopus and Zygogynum vieillardii (order Magnoliales) have a 

 guaiacyl-syringyl ratio as high as approximately 1:1. This ratio is 

 typical of the Hgnins of the group of gymnosperms which were noted 

 above. Casuarina stricta, discussed in a previous section as possibly 

 one of the most primitive angiosperms, has a guaiacyl-syringyl ratio 

 of 1:0.5 (Manskaja, 1959). 



It has been noted that some monocots produce lignin with 

 p-hydroxy phenyl derivatives. Furthermore, studies of lignin biosyn- 

 thesis support the presence of an enzyme mechanism in grasses which 

 utilizes tyrosine or p-hydroxyphenylpyruvic acid. (Wright et ah, 

 1958; Acerbo et al, 1958.) These facts suggest a significant difference 

 in the lignin chemistry between monocots and dicots, but as was 

 pointed out by Neish (1960), sampling is obviously inadequate at 

 this time. 



The members of other major vascular plant groups, for exam- 

 ple, ferns, lycopsids, and sphenopsids, apparently produce only the 

 guaiacyl type of lignin. Among mosses. Sphagnum contains phenolic 

 compounds in the cell walls, but Manskaja (1959) has concluded that 

 lignin itself is absent. Figure 14-3 illustrates the broad distribution of 

 lignin and the particular monomeric building units represented among 

 various plant groups in geological time. Presence of lignin in mosses, 

 shown in the figure, is questionable. Fossil lignin has undergone com- 

 plex chemical changes, and so far has not proven to be useful in provid- 

 ing insight into the actual hgnin composition (Manskaja, 1960). 



It is evident that most of the taxonomic inferences from 

 lignin chemistry are presently limited to rather broad generalizations. 

 However, Towers and Gibbs (1953) used the guaiacyl-syringyl ratio 

 together with other evidence to suggest that box elder {.Acer negundo) 

 might be separable from other maples, that is, elevated to generic 

 rank. This is one of the few examples of the actual apphcation of 

 lignin chemistry to a specific taxonomic problem. 



Isothiocyanates: Sulfur-containing secondary compounds 

 in higher plants are relatively few in number. By far the most 

 important group is the mustard oils or isothiocyanates, about thirty 

 of which have been described. The compounds occur in the living 

 plant in the following form: 



O 



OHH OH 

 S-CHC-C— C-C-CH2OH 

 Yi—c H OHH H 



^N— O-SO2— O- 



