CARBOHYDRATES 1 37 



phyletically meaningful distribution of a substance is noted, but in 

 general at this time carbohydrates have not contributed nearly as 

 much to the field of biochemical systematics as have certain other 

 classes of compounds. 



Simple sugars (a partial list only) 



The five-carbon sugar, xylose, is frequently encountered in 

 higher plants and is a constituent of polysaccharides such as xylan 

 and the hemicelluloses. Other pentoses, such as arabinose and ribose 

 are frequently found in higher plants. Ribose is associated with co- 

 enzymes and nucleoproteins, and it is an intermediate in the dark re- 

 actions of photosynthesis. It would be indeed remarkable if ribose 

 were absent from a higher plant. Free pentoses are often present in 

 detectable quantities in plant tissues. Wilhams et al. (1952), who 

 examined thirty-one different food plants, and also leaves from 

 twenty-three trees, found free pentoses in the fruits of only the lemon 

 and strawberry, but in the leaf material examined, free pentoses were 

 present in about half of the species. 



Of the hexoses, most are of little or no systematic significance. 

 Although it is true that the abihty to accumulate large amounts of 

 one or another of the common hexoses may conceivably be systemat- 

 ically meaningful. There is no evidence that such accumulation often 

 occurs. A chromatographic study of free sugars of twenty-seven 

 famihes of seed plants and ten species of algae representing three 

 phyla (Bidwell et al, 1952) showed some differences in the relative 

 concentrations of glucose and sucrose. Since sugar concentration may 

 be as sensitive to conditions of growth as amino acids, quantitative 

 differences must be evaluated conservatively. These authors reported 

 extremely low free sugar content in the algae tested, and a similar 

 statement appears in a report on carbohydrate accumulation by lower 

 plants by Young (1958). 



Free galactose, though uncommon, has been found in a variety 

 of plants, and it also occurs in the sugar component of certain complex 

 glycosides. Mannose apparently does not often occur free, yet the 

 sugar is present in polysaccharide form (mannan) in certain palms 

 and orchids. 



Although free fructose is of slight systematic importance, the 

 distribution of the fructose polysaccharide, inulin, appears to have some 

 taxonomic significance. It is quite widely known that certain Compo- 

 sitae (for example, dandelion, Jerusalem artichoke, and Dahlia) store 



